Elisabeth Krausmann
email: elisabeth.krausmann@jrc.it

Ana Maria Cruz

Bastion Affeltranger

Natech Accidents at Industrial Facilitiesthe Case of the Wenchuan Earthquake

Keywords: Earthquake, damage, chemical spill, hazardous materials, Natech risk

Natural disasters can trigger chemical accidents (so-called Natech accidents) with severe conséquences on man or the environment. This work highlights the main characteristics of earthquaketriggered Natechs by describing our insights from a field trip to chemical facilities in the area affected by the 12 May, 2008, Wenchuan earthquake in China. Our preliminary results indicate that damage was most severe in older facilities with masonry and un-or poorly reinforced concrète structures being the most vulnérable. Débris impact and the shaking loads caused damage to and loss of equipment and pipes where hazardousmaterials releases occurred. Emergency response on-site was hampered by a loss of lifelines.

Introduction

On 12 May, 2008, at 14:28 local time, Sichuan Province in China was hit by a major earthquake. The seismic event, which had its shallow epicenter in Wenchuan county, reached a moment magnitude of M w = 7.9 and left almost 70,000 people dead while injuring over 374,000 [1]. Over 5 million buildings are reported to hâve collapsed and many more were damaged. Direct économie losses due to the earthquake are estimated to exceed 140 billion US$ [2].

In addition to the human tragedy the Wenchuan earthquake and other récent major natural disasters Worldwide hâve highlighted how the interaction between natural disasters and technological Systems can exacerbate losses. Of particular concern is the impact of natural disasters on the chemical process industry owing to its major accident potential due to hazardous materials used or stored on site. In this case earthquake-triggered damage can lead to the release of (eco-)toxic, flammable and/or explosive substances with potentially severe off-site conséquences. Commonly this type of chemical accident is referred to as Natech accident, a term which was fîrst used by Showalter and Myers in 1994 [3] and which is derived from "natuml hazardtriggered tec/mological accident". It includes hazardous-materials releases from fîxed chemical installations, but also spills from oil and gas pipelines [4]. There are numerous examples for earthquake-triggered Natechs such as the fire in a major refmery during the 1999 Kocaeli earthquake in Turkey [5], the fîres in an oil storage dépôt after the 1994 Niigata earthquake [6], or the rupture of an oil pipeline during the Northridge earthquake in 1994 [7], to name a few.

There is very little information on the extent of the damage to and hazardousmaterials releases from industrial facilities during the Wenchuan earthquake. Some of the hardest-hit areas are home to numerous chemical companies and a few Natech accidents hâve been reported [8,9]. In order to assess the performance of the chemical industry the European Commission's Joint Research Centre in collaboration with the French National Institute for Risk and the Industrial Environment conducted a field trip in the areas affected by the earthquake in November 2008. This article describes some preliminary results of thèse site visits and briefly illustrâtes the main characteristics of earthquake-triggered Natech events using the Wenchuan earthquake as an example.

Damage analysis of chemical facilities

Prior to the Wenchuan earthquake Chinese design codes required buildings to withstand an earthquake intensity of VII on the Chinese Seismic Intensity Scale with repairable damage only [10].

In comparison, the earthquake reached intensity levels of XI in the régions near the epicenter [2]. The level of damage and destruction is therefore not surprising. The chemical industry follows the provisions of the codes but is also required to implement additional spécifie régulations in case a major accident potential exists [11].

Damage to équipaient buildings and

In total we visited 18 industrial facilities during our field trip, with only one of them not having suffered any visible damage. In most of the visited facilities people were unharmed or only slightly injured and the impact of the earthquake was restricted to structural and non-structural damage to buildings, as well as to processing and storing facilities. However, in two sites over 275 workers lost their lives and many more were injured when warehouses, office and manufacturing buildings collapsed and buried them. Given the magnitude of the earthquake and the density of chemical facilities in the affected région this number is likely to be a low estimate.

During our site visits we noticed widespread building damage and collapse, in particular in installations located in areas subject to high peak ground accélération values. Our fmdings suggest that the damage severity is directly correlated to the âge of the installation, and older facilities not built according to the latest design codes suffered more severe and extensive damage. Moreover, the construction type significantly influenced a building's résistance to the earthquake loads with masonry and un-or poorly reinforced concrète structures being the most vulnérable (Figs. 1 and2).

.*• * :

Fig. 1: Destroyed building at a chemical facility.

Falling

débris from collapsing buildings and other structures resulted in major equipment damage. Débris impact also severed and crushed pipes both inside buildings and outside. In many cases we observed bending or severing of external pipes when connected tanks or buildings were displaced or collapsed (Fig. 3 and4). Tanks and vessels were subject to a number of damaging phenomena during the earthquake. In some cases they were damaged by impacting débris or they failed under the unexpected shaking loads. Damage to or complète failure of tank and vessel foundations also resulted in toppling or tilting (Fig. 5). It is common during major earthquakes that storage tanks that are only partially filled expérience liquid sloshing, which exerts additional forces on the tank shell and may evenrually cause it to fail, releasing the tank's contents [5]. We hâve no information if this phenomenon played a rôle in damaging the tanks at the sites we visited. Pipe connections to tanks or pumps were sometimes severed. Several facilities visited experienced damage due to soil liquéfaction which caused foundation failure and the sinking or toppling of buildings and other structures (Fig. 6).

Economie losses

Direct losses due to building and equipment damage, as well as indirect costs from plant shutdown and business interruption usually weigh heavily on companies that hâve suffered one or more Natech accidents during a natural disaster. According to company managers at the facilities we visited the direct losses exceed 240 million US$. At the time of our visit 6 months affcer the earthquake repair and reconstruction was still underway. Some facilities were still shutdown or had resumed production only partially. One plant director told us that he was negotiating a cheap loan with the government as otherwise his company would hâve to cease activities altogether. Costs due to plant shutdown and business interruption add significantly to the losses the chemical industry in the earthquakeaffected areas suffered. With rebuilding times of sometimes years at the more heavily impacted facilities and fear of future severe earthquakes some affected companies hâve decided to move their production away from the vicinity of the fault zone.

Ha/ardous-maieriais releases

Many of the damaged tanks and severed pipes observed during the field trip contained substances that were toxic, flammable or explosive. However, information on hazardous-materials releases triggered by the earthquake is scarce. Releases were confirmed by résidents of two chemical facilities who reported a pungent smell shortly after the earthquake, or the appearance of burned crops and other végétation [8]. The Chinese Ministry of Environmental Protection confirmed the release of a significant amount of ammonia and sulfuric acid which polluted a river [12]. Sulfur burning and explosion was also reported [8]. With the gênerai level of destruction both on-and off-site it is likely that any releases would hâve continued for days until the tank or vessel contents were exhausted. Since Sichuan Province has a high density of chemical industry that was constructed before the coming into force of more stringent seismic design codes the overall hazardous-materials releases triggered by the earthquake might hâve been extensive.

The hazardous-materials release modes at the visited facilities were typical as research on previous earthquake-triggered Natech accidents suggests [e.g. 5,13]. Leaks and spills occurred when buildings collapsed on tanks and vessels or when pipes or pipe connections were severed due to débris impact or direct loading by the earthquake. In addition to toxic releases, fires and explosions were also triggered during the Wenchuan earthquake. A récent study indicates that the ignition probability is rather high upon release of a flammable substance during an earthquake [14].

Emergency response

Natech accidents invariably are a challenge for emergency response that has to deal with a natural disaster and a chemical accident that occur simultaneously. In addition, lifeline Systems such as water or power supply, crucial for disaster mitigation, are likely to be unavailable. An aggravating factor is that some natural disasters, such as earthquakes or floods, usually hâve an extended impact area and may therefore lead to hazardous-materials releases from multiple sources at one chemical facility or from several affected hazardous installations simultaneously. If the spécifie characteristics of Natechs hâve not been taken into considération in the emergency planning prior to the accident, crisis-management resources may hâve to be diverted from responding to the natural disaster to fight the chemical accident. In the worst case it can also mean that search and rescue opérations, e.g. for earthquake victims still buried under collapsed houses, may need to be discontinued when hazardous-materials releases are imminent. This sadly happened during the Kocaeli earthquake where a major flre in a refînery threatened a neighboring fertilizer production plant, possibly leading to toxic releases, fires and explosions. Consequently, an évacuation order was given for résidents who had to leave trapped family members and relatives behind [5].

We hâve little information on the emergency-response performance during the Wenchuan earthquake at the visited chemical facilities. There was a widespread loss of lifelines in the province and a subséquent breakdown of the electricity, gas and water supply which led to an interruption in production even in undamaged companies [8,15]. Moreover, in some cases the on-site power generators or water services were also destroyed. It is safe to assume that structural and organizational prévention and mitigation measures were not designed for an earthquake of such magnitude. In the absence of water for cooling or fire-fighting purposes or emergency power to run safety Systems we surmise that the release of hazardous materials in the hardest-hit installations would hâve been diffïcult to contain.

The level of destruction of buildings, roads and railroads was striking during the Wenchuan earthquake. This was due to the earthquake forces themselves but was aggravated by numerous landslides and rock falls that accompanied the earthquake. This highlights another problem of Natech accidents during which the applicability of standard civil protection measures cannot automatically be assumed. During a conventional technological accident with hazardousmaterials release community leaders may choose to implement protection measures such as shelter in place or évacuation of résidents. Thèse choices may not be available in the case of a natural disaster as houses may hâve collapsed or roads may be blocked by débris.

Conclusions

There is évidence from récent Natech accidents that the chemical industry is vulnérable to the impact of natural hazards and disasters. Earthquakes are of particular concern as their occurrence cannot be predicted which precludes the possibility for early warning and thus the implementation of emergency safety measures. In addition, the spatial extent of earthquakes usually leads to simultaneous multiple releases not only from one facility but possibly from several, compounding the problems arising from the impact of the earthquake on the population, the environment and the economy. Therefore, Natech risk needs to be identified and assessed, in particular in densely populated and industrialized areas, giving due considération to its multi-hazard nature.

The fîndings from our fleld visit to industrial facilities affected by the Wenchuan earthquake confirm the observations made during other earthquakes in terms of typical Natech damage and failure modes, as well as hazardous-materials release potential and mechanisms. They also highlight the potential for a loss of lifeline Systems to cause interruption of production, as well as the inability to prevent and/or mitigate a chemical accident. The Wenchuan case shows that a realistic assessment of the expected earthquake severity and the resulting forces on buildings and other structures is required to evaluate and if necessary improve the seismic résistance of industrial facilities to avoid building collapse, as well as débris formation. One already positive move in this direction is the updating of the seismic design code after the Wenchuan earthquake while some companies hâve opted for design event magnitudes higher than those requested by law.

Fig. 2 :

 2 Fig. 2: Building collapse due to insufficient reinforcement.

Fig. 4 :

 4 Fig. 4: Pipe crushing due to a fallen concrète fence.

Fig. 5 :

 5 Fig. 5: Toppled vessel.

Fig. 6 :

 6 Fig. 6: Structural damage to a silo due to soil liquéfaction.

Acknowledgements

We thank Zhou Ling (Tsinghua Univ.), Braven Zhang and Xiulan Zhang (Beijing Normal Univ.), the Deyang government and the French consulate in Chengdu for kindly providing support in organizing the site visits.

Références

[1] US Geological Survey, "Magnitude 7.9 -Eastern Sichuan, China", http : //earthquake.usgs.gOv/eqcenter/e qinthenews/2008/us2008ryan/#sumrn ary, 2008.

[2] P. Shi, "China Wenchuan earthquake disaster (May 12, 2008)


I Zdanevitch

O Bour

L Llinas

S Lejal°

BP2 F-60550 Ineris

Verneuil-En-Halatte

COMPARISON OF POLLUTING POTENTIALS OF LIQUID EMISSIONS FROM MBT PLANTS

leachates from two waste management plants which include MBT and an associated landfill have been studied. Leachates have been sampled on different parts of the landfill, and, when possible, process waters have also been sampled. Classical parameters have been measured on all samples and values have been compared: between leachates of the same site, between sites, and to literature values. Leachates show different behaviours depending on the compound: some values are typical of young waste, while others are more representative of old, stabilized waste.

INTRODUCTION

Mechanical biological treatment (MBT) of municipal solid waste (MSW) is presently largely developing in France, with different purposes: • reducing the volume and stabilizing the waste prior to landfilling, • recovering of organic matter through compost which can be used in agriculture, • generating renewable energy (biogas) with anaerobic digestion. The two first objectives can be achieved either by anaerobic or aerobic biodegradation (composting). An important advantage of MBT is -in theory-the reduction of the gaseous and liquid emissions from the storage of stabilized waste. Evaluation of this reduction can be related to a given stage of stabilization of waste.

Since 2006, INERIS conducted several measurement campaigns on different aerobic MBT plants and landfills in France in order to assess the environmental benefits obtained by treating the waste, compared to classical landfilling of untreated waste.

MATERIALS AND METHODS

Description of the sites

Both plants undergo an aerobic stabilization of the waste prior to landfilling, and both plants have their own landfill. Meanwhile, there are differences in both the mechanical and biological treatment. Leachates are not rejected to surface waters but treated, either on-site or off-site.

Site A

Site A is situated in a rather rural area of France, and receives the waste from approximately 40,000 people. The plant is surrounded by fields and woods.

On this site, there is no grinding of the waste at the entrance of the plant. Waste undergo a first sorting: the large fraction (>450 mm) goes directly to the landfill (cell 1), the fine fraction (<70 mm) is sent to the composting area. The intermediary fraction goes to a rotating tube where it is humidified, and stays for 2 to 3 days. There, aerobic degradation starts. This fraction is then sorted. The larger fraction is mainly constituted of plastics: it is baled and sent to cell 2. The finer fraction (<50 mm) goes to the composting windrows but separately from the first fine fraction.

Windrows are aerated by forced air blowing under the material. The controlled composting process generally lasts 4 weeks. After this first step, waste is deposited in windrows for a maturation step of several months. The two composted fractions are then deposited together into the cell n° 2.

Therefore, this site has four leachate ponds : • pond n° 1 corresponds to the larger fraction), which contains very little organic matter, • pond n° 2 receives leachates from the stabilized fine fraction, • pond n° 3 receives leachates from the intermediary fraction, constituted mainly by plastics, which can contain a little degradable matter, • pond n° 4 receives the waters from the different steps of the process (pre-degradation, composting, maturation...) Leachates from all four ponds have been sampled.

Site B

Site B is located in a mixed area (rural and urban parts), not far from the sea. On this area, there are separate collections of packing and biowaste. Residual waste which enters the stabilization plant therefore contains less organic matter than typical municipal waste. The plant treats the waste from around 180,000 people. The plant is located in the suburb of the main town, and has close neighbours.

When arriving in the plant, residual waste is first grinded and iron is separated. The waste is then homogenised and humidified, and goes to composting tunnels for (3 + 2) weeks. Waste is automatically transported and turned in the tunnels, and conditions (aeration, temperature, humidity) are controlled. After 5 weeks, stabilized waste goes to the landfill which is situated at a few kilometres from the plant.

The landfill has two different areas: the first one is constituted by a single large cell, which received mixed untreated waste until 2006. The second area is constituted of smaller independent cells which receive only stabilized waste since 2006. Leachates are collected separately from the two zones, but goes to the same pond. There is a water treatment device on the landfill. Samples are labelled as follows:

• "Leach. K1 " samples were taken at the outlet tube from the old cell called K1, • "Leach. K2" samples were taken at the common outlet of the new cells of the K2 area.

Sampling and analysis

Different leachates were sampled on the two sites, in order to compare the pollution potential between different situations: • on the first site, both the process waters and leachates coming from the 3 sections containing the different sorted waste were studied,

• on the second site, leachates from the two different sections, corresponding to the older untreated waste, and the younger stabilized waste were sampled separately. Samples have been taken on each pond, directly at the collector outlet, stabilized if needed, kept at low temperature and sent to the analytical laboratory within 2 days.

On all leachate samples, several analyses were run: • the classical parameters (pH, conductivity, organic contents, salts and metals) were measured on either unfiltered or filtered samples, depending on the standardized method, • suspended matter was examined by scanning electronic microscopy (SEM), and X-ray microprobe. Suspended matter content of young leachates can be very high and the study of this specific fraction can bring useful information as it represents the finest fraction of the solid phase, though it is not often reported (see an example in [START_REF] Ziyang | Size-fractionation and characterization of refuse landfill leachate by sequential filtration using membranes with varied porosity[END_REF]. In addition to the "classical" parameters given previously, we have studied the refuse phase of leachate filtration by electronic microscopy. Two different preparation processes were run: filtration on cellulose membranes of porosity 0.45 |j,m, or deposition of 5 drops of leachates on a glass slide and slow drying at ambient temperature. Samples were then examined in the chamber of the SEM and elementary analysis was performed with the micro-X-ray probe.

RESULTS AND DISCUSSION

Variation in the values of the different parameters depend on the treatment process, on the sorting which can be made, on the type and age of landfilling, and eventually on the season during which waste has been collected and treated. The complete result set is given in Table 1. 1 000-5 000 20-200 50-1000 4 000-8 000 500-2 000 1-15 (4) 100-800 1 000-5 000 10-100 5-100 0.1-0.5 0.2-0.5 5-20 <10 (5) <700 <400 0.5-3 (1) Laboratory measurements (2) Adsorbable halogens (3) fjg/1 unless stated (4) as total phosphorous (5) only maximum values are reported Interpretation of the data can be made by: • comparison of behaviours between samples from the same site, e.g. between waste of different nature (site A) or between untreated and treated waste (site B), • comparison of data between the two sites, the MBT process and the landfilling being different between the two plants, • comparison with data from literature. There are several references in the literature which give typical value ranges for the classical parameters on leachate coming from landfills of different ages, to which we can compare our results. We have focused on studies which report a large number of values, on a great number of parameters and a large number of sites, as for instance [START_REF] Kjeldsen | Present and long-term composition of MSW landfill leachates: a review[END_REF], [START_REF] Oman | Chemical characterization of landfill leachates -400 parameters and compounds[END_REF], or results obtained on sites which receive MBT waste [START_REF] Robinson | Leachate quality from landfilled MBT waste[END_REF]. Some of these values are reported in the table for comparison.

Values cited by Kjeldsen et al were compiled from 15 studies published between 1979 and 1997. Since then, composition landfilled of waste has changed, especially in Europe where separated collections of different fractions are now made. Therefore, the polluting potential has decreased, for instance for chlorinated compounds. But ranges of values cited by Kjeldsen are very large and cover a lot of different situations. A study realised in Sweden covered a lot of parameters (400) measured on the leachates of 12 sites, receiving different types of waste [START_REF] Oman | Chemical characterization of landfill leachates -400 parameters and compounds[END_REF]. Robinson et al published in 2005 a study realized on leachates coming from different MBT sites. The basis is the sorting of organic matter from Municipal Solid Waste (MSW), which leads to MSOR (Mechanically sorted organic residues). This fraction of waste, when landfilled without treatment, gives rise to high organic, ammonia and chloride content of the leachates. On the contrary, when this fraction of waste undergoes stabilization process, the corresponding leachates from the landfilling are generally weaker than those of a classical waste deposit without treatment. The stabilization processes described by Robinson et al ranged from a "low" to "high" degree of composting. Because of the MBT processes which are used on site A and B, we have selected for comparison the values given by Robinson on leachates from landfilled waste which had a "low" to "moderate" stabilization process.

3.1 pH pH of leachates are different for all the samples, but are never really acidic, comparatively to the first biodegradation phases on classical landfills without MBT (Kjeldsen). Values range from 7.2 on the large refuse cell of site A, to 7.95 on the composted cell of site A, and from 8 to 8.6 on the two cells of site B. The lower pH value is found on the leachate from pond n°4 of site A, receiving the process waters, which can correspond, to a certain extent, to an "acidic" biodegradation phase. As Robinson pointed out, composting the organic matter before landfilling avoids the acetogenic degradation process in the landfill cell. pH are even less acids on site B than on site A, maybe because there is a part of selective collection of biowaste in this area, so the waste which goes to the MBT plant has less organic residues than on site B, and acidogenic/acetogenic phenomena maybe lowered.

Conductivity

Conductivity of leachates from all our samples (from 8 to 30 mS/cm) are rather high : they still fall in the ranges given by Kjeldsen, but the value measured on the cell of composted fine waste of site A is higher than the range cited by Robinson. Conductivity is mostly due to the salt content of leachates, and little to the organic matter, which is not charged (except the acids): the stabilization given by the biodegradation of the organic matter does not change much the electrical charge of the liquid phase.

Suspended matter

We have measured the suspended matter content of the leachates, as we have started specific studies on this phase. This parameter is not often reported in the litterature. [START_REF] Oman | Chemical characterization of landfill leachates -400 parameters and compounds[END_REF] cite a range from 8 to 2100 mg/l, with an average of 196 and a median value of 32.

Obviously, leachates content a lot of suspended matter, but in our samples values are rather low, especially for the high fraction cell on site A and for the old cell on site B. The higher content is found, with no surprise, on the process water on site A, and on the leachate from the young cell (receiving MBT waste) on site B.

Organic content

Due to the biodegradation processes, organic content of leachates generated by mechanicalbiological treated waste is lowered compared to the content of leachates from untreated waste. BOD is the parameter which decreases the most on treated waste. COD also decreases, but to a lesser extent, so the ratio BOC/COD falls below 0.1, which normally characterizes "stabilized" waste. COD diminishes less than BOD because it also relates to non degradable organic compounds and to inorganic matter, which are not treated by the biodegradation processes.

It is considered that leachates from stabilized waste should have values of BOD < 100 mg/l, and COD < 1000 mg/l, beside the value of BOD/COD of 0,1 [START_REF] Ravelojaona | Définition de l'état éco-compatible des installations de stockage de déchets ménagers et assimilés[END_REF]. It is not the case here: only leachate from pond n°1 of site A (corresponding to large refuse) falls within these values.

BOD values are generally low on all our samples, except on the process waters on site A: see Figure 1. Considering that all the cells of site A and cell K2 of site B are young, this confirms that composting waste prior to landfilling lowers significantly the biodegradable content of the waste.

Nevertheless, composting has less action on COD, which is confirmed here: COD of leachates from cell 2 on site A, and from cell K2 on site B, which correspond to the composted waste, are the highest (see Figure 2). This shows that these cells still have a polluting potential, which is confirmed by the biogas emissions, still high enough to be burnt in flares. COD is relatively low (value around 1000 mg/l) only for the leachate from cell 2 of site A, which corresponds to the large refuse, the most inert fraction of waste. Plastics in cell n° 3 probably still contain a little organic matter and salts, which go to the leachates of this cell. With low BOD values (except for the process waters on site A) and relatively high COD values, the ratio BOD/COD falls well below 0.1 for all the leachate samples: see Figure 3. The lower value is encountered on the leachate of the composted waste of site A. For site B, this ratio is the same for the leachates from the zones K1 and K2, though BOD and COD values are a little higher on cell K2, which is younger. This shows that, due to composting prior to landfilling, cells K1 and K2 have reached the same proportion of biodegradation in relation to the polluting potential of the waste.

AOX (adsorbable organic halogens) is a global parameter. It does not give information about organic species, but there is a mandatory value of this parameter for liquid release in surface waters (Arrêté du 09/09/1997): concentration should be lower than 1 mg/l when the total daily flux is higher than 30 g/d (which corresponds to a liquid flux of 30 m 3 /d).

Here, leachates from site A, pond 1 (large refuse) and 4 (process waters) fulfil this limit value, and leachate from cell K1 of site B nearly fulfils it: see Figure 4. As seen previously, leachates from pond n°2 on site A and from cell K2 on site B (both receiving composted waste) have nearly the same value, 2.3 and 1.9 mg/l respectively, which is only twice the limit value for environmental release (though limit values are exceeded for other parameters such as BOD and COD).

The highest AOX value is shown by the leachates from pond n°3 of site A (plastics), which may come from chlorinated organic compounds traces in the plastic bottles (such as cleaning products) and by possible degradation of chlorinated plastics such as PVC.

Salts

Ammonia and chloride are compounds of a big concern in landfill leachates : there are generally present at high concentration, they are poorly degraded, and their release by leaching of the waste can last a long time. There is not a large release of these compounds in the process waters of site A, but they are mainly emitted by the stored waste.

Once again, leachates from the two cells receiving composted waste (cell 2 -site A and cell K2 -site B) have nearly the same concentration in ammonia and chloride.

There are also two remarkable values in ammonia concentrations: • leachate from pond n° 3 (plastics) of site A has quite the same value as pond n°2 (composted waste), • on site B, leachate from cell K1 (old waste) is more concentrated than leachate from K2 (young composted waste): composting the waste lowers a little ammonia concentrations. The mandatory limit value for environmental release (Arrêté du 09/09/1997) for total nitrogen is 30 mg/l. All leachates largely exceed this value, imposing the treatment of these waters before any environmental release.

There is a higher concentration of chloride in leachate from cell K2 than K1 on site B. This may indicate either a slow decrease in the emission of chloride by the untreated waste, or a higher release due to composting processes prior to landfilling (giving access to chlorinated salts). Meanwhile, this phenomenon does not occur for ammonia. More measurements will be necessary in order to clarify this point.

Metals

Aluminium and iron are the two metals which are the most concentrate on leachates from both sites (ranges of mg/l). They are considered as non toxic (iron) or weakly toxic (aluminium) and therefore, not often measured or published in the litterature. Toxic heavy metals are all found in very low concentrations, and as low (except for arsenic in leachate K2) as the limit values for release in the surface waters in the French Arrêté of 1997. We cannot conclude on chromium, as only total chromium was analysed, and the mandatory limit values are for Cr 6+ . Yet, all landfill leachates that we have sampled are treated, either onsite or off-site. Therefore, metals in these leachates will not induce a health or environmental problem.

Metals generally show low concentrations because a large fraction of metals are immobilized in waste by complexation phenomena. Then, a change in the conditions prevailing in waste (pH, redox) could free a large amount of metals [START_REF] Kjeldsen | Present and long-term composition of MSW landfill leachates: a review[END_REF].

Micro-analysis of the suspended matter

On the cellulose filter, carbon cannot be distinguished between the leachate solids and the cellulose support. Therefore, only the minerals can be studied. On the glass deposit, the slow drying the formation of crystals, mainly sodium and calcium chlorides, which hide the metals. Examples of the SEM pictures are given in Figure 8.

On site A, filtration of leachate from pond 3 (plastics) did not give a good sample, and could not be studied by SEM. On each sample filtrated on the cellulose membrane, at least 1,000 particles were analysed automatically: dimensions and occurrence of elements are given in Table 2 andTable 3.

The smaller dimension of the particles analysed on all samples is related to the porosity of the cellulose membrane: the minimum size is 0.5 |j,m. The maximum size of the particles can vary a lot from a sample to another, so for comparison we have considered the size in which 95% of the particles fall: see in Table 2. Particles from leachates of site B are smaller, and there are the smaller on leachates from cell K2 : 40 % of the particles have a dimension of 1 |j,m. We have not found references from the litterature to compare our data: this parameter will be measured on other leachates in order to find tendancies.

N. D.: not determined

Occurrence of the elements is the number of times each specific element is detected by the X-ray probe on the surface of a particle. They are reported for the leachates of the two sites in Table 3.

Analysis were performed at different times, so we have extended the number of elements which could be determined, as they bring additional information to classical salts and metals analysis. X-ray microanalysis on the suspended matter show the same tendency between different leachates, as metals measured by ICP on unfiltered leachates. They also reveal the presence of oxygen (when determined) and chlorides on all the particles which were automatically analyzed by the microscope for the waste deposits which contain organic matter.

CONCLUSIONS

Leachates from MBT waste show a different behaviour, depending on the pollutant: some values are similar to those of young, untreated waste (COD, chloride) whereas others represent a certain degree of stabilization (pH, BOD, ammonia, AOX...). Biodegradable organics are the compounds which decrease the most during pre-treatment. Salts (and especially chlorides) are the compounds which diminish the less in leachates from treated waste, compared to untreated waste. Ammonia seems to decrease a little. Biological treatments do not degrade (or little) these compounds, which will remain the bigger after-care problems in landfills management.

Figure 1 .

 1 Figure 1. Biological oxygen demand of the various leachates of the two sites A and B

Figure 4 .AmmoniaFigure 5 .ChlorideFigure 6 .

 456 Figure 4. adsorbable organic halogens compounds on the various leachates from sites A and B

Figure 8 .

 8 Figure 8. SEM of leachate solids deposited by a: filtration on cellulose membrane, and b: drying on glass

Table 1 .

 1 Values of the parameters measured on the different leachate ponds of the two sites, compared to literature values

				Site A		SiteB	Typical values
			Pond1	Pond 2	Pond 3	Pond 4	LeachK1	Leach K2	Kjeldsen	Robinson
		pH(1)	7.2	7.95	7.6	7.1	8.05	8.6	4.5-9	7.5-8.5
	Conductivity mS/cm (1)	9.14	30.4	18.8	18.5	18	7.98	2.5-35	10-20
	Suspended matter, mg/l	12	44	68	300	14	110
		COD, mg O2/l	1240	12 800	5 680	7 200	3 010	8 940	140-152 000
		BOD5, mg O2/l	69	230	210	4 590	120	390	20-57 000
		BOD/COD	0.06	0.02	0.04	0.64	0.04	0.04	0.02-0.8
		Ammonia, mg/l	633	1 787	1 530	379	2 070	1 690	50-2200
		Chloride, mg/l	610	3 270	1430	620	1 630	2 870	45004 000
		TOC, mg/l	440	3 700	1 700	2 200	1040	2 700	30-29 000
		PO4, mg/l	12.6	86.9	76.8	50.6	25.5	19.6	0.1-23 (4)
		Ca, mg/l	275	375	340	1 730	51	220	10-7 200
		SO4, mg/l	20	9	<5	<5	29	500	8-7 750
		AOX(2),mg/l	0.72	2.3	5.2	0.32	1.1	1.9
	Metals (3) :	Al (mg/l)	0.6	20	6	3.9	0.91	11
		As	<2	50	58	9.8	110	430	<1000 (5)
		Cd	<10	<20	<10	<10	<10	<10	<400
		Cr (mg/l)	0.39	2.2	3.6	0.1	0.62	4	<1.5
		Cu (mg/l)	<0.2	0.3	0.14	0.14	<0.1	0.17	<10
		Fe (mg/l)	3.2	76	21	25	12	14
		Hg	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<160
		Ni	<100	810	270	250	170	430	<13000
		Pb	<100	220	<100	<100	<100	<100	<5 000
		Se	<200	<400	<200	<200	<100	<100
		Zn (mg/l)	0.14	1.7	0.88	2.5	0.19	0.71	<1000

Table 2 .

 2 size of 95% of the leachate particles from the sites A and B

					Site A				SiteB
			Pond	1	Pond 2	Pond	4	Leach.	K1 Leach.	K2
	Max	size |j,m,	11		6.6	7		3	<4
	95%	particles

ACKNOWLEDGEMENTS

This work was financed by the French Ministry of Environment and ADEME. The Authors wish to thank Guillemette Jantolek and Francis Guillot for undertaking the field campaigns.


O Bour

I Zdanevitch

M Briand

L Llinas °

ESTIMATING METHANE EMISSION AND OXIDATION FROM TWO TEMPORARY COVERS ON LANDFILLED MBT TREATED WASTE

Surface gaseous emissions and upper layer waste gas composition were measured on two French MBT plants with aerobic pre-treatment process. The goals were to characterize the gaseous emissions, and to assess the efficiency of the upper layer to oxidize the methane flux coming from the residual organic fraction. The first plant was operated without recovery of organic fraction and with concentration of the fine fraction in a cell. The methane fluxes were high and the oxidized methane fraction was moderate (40%) for selected point measurements. On the contrary, the second plant organic fraction was recovered. The residual methane emissions were low with a high fraction of oxidized methane (up to 80%). These results were obtained with favourable humidity and temperature conditions.

INTRODUCTION

Landfill methane emissions must be reduced in order to limit the global warming impact of a sustainable landfill. The decline of the quantity and quality of the landfill biogas (LFG) produced after the final phase of landfill capping needs to consider natural oxidation processes in order to mitigate the residual methane emissions. The results presented in the past Sardinia Symposia showed that mature compost could be used for the building of the biocover. Moreover, high methane oxidation efficiency (nearly 100% for a methane supply of 4 -4.5 l/m 2 /h) was measured with column tests using old municipal solid waste material [START_REF] Huber-Humer | Improvement of the upper waste layer to foster methane oxidation[END_REF][START_REF] Huber-Humer | Biotic systems to mitigate landfill methane emissions[END_REF].

Another result of these studies is the possible improvement of the methane oxidation efficiency with the addition of structural compounds and waste sieving/homogenisation. MBT processes produce waste with high porosity homogeneity and therefore the MBT processed wastes are good candidate for the building of a biocover.

Due to the lack of data concerning the real fugitive methane emissions of this type of waste in the windrows and on the temporary and final covers, INERIS conducted from 2006 several surveys on different French landfills. The first part (2006 -2007) of this study focused on the temporary covers, which generally produce a significant amount of fugitive methane emissions for landfills without waste pre-treatment (Tarimini & al, 2003;Ackermann & al, 2006Ackermann & al, , 2007)).

The second part on this program deals more specifically about the efficiency and building materials of biowindows and is represented by on-going research.

The results presented in this paper are a part of a research program dealing with the comparison of gaseous emissions [START_REF] Zdanevitch | Gaseous emissions reduction from aerobic MBT of municipal solid waste[END_REF] and assessment of methane oxidizing potential of different types of covers and waste storage.

The results concern two MBT plants without methane recovery (plant A and plant B). The surveys were realized during the end of spring of 2006 and 2007.

MATERIAL AND METHODS

Methodology

The surveys conducted on the two MBT plants combine methane and carbon dioxide surface fluxes, methane and dioxide concentration measurements of gas collected in the shallow waste layer, and humidity measurements. The assessments of the oxidized methane fluxes used a LFG mass balance method considering the surface fluxes and the ratio between the methane and LFG concentrations measured in the waste [START_REF] Christophersen | Field investigations of methane oxidation in soil adjacent to an old landfill[END_REF].

The waste LFG concentration measurements were realized every two surface fluxes measurements in a second step, after the surface flux mesurement. The gas probe was inserted in the waste and allowed to sample gas at a depth of approximately 75 cm (Figure 1). The methane load before oxidation was determined by the methane fraction in the gas sample pumped from the gas probe and by the LFG (methane and carbon dioxide) surface fluxes and can be described by the equation (1) from [START_REF] Christophersen | Field investigations of methane oxidation in soil adjacent to an old landfill[END_REF] :

T CH4inf
where JCH4inf and JCO2inf are the methane and carbon dioxide loads before oxidation, respectively, and JCH4SU P and JCO2su p are the methane and carbon dioxide surface fluxes, respectively. From the equation (1), methane oxidized flux J ox is calculated by the difference between JCH4inf and JCH4SU P .

This method supposes that the ratio between the surface gas concentration in the chamber and the gas pumped from the probe is only influenced by the methane oxidation. The repartition of the carbon dioxide fluxes must be checked in order to verify that the major source of carbon dioxide is linked with the LFG sources in the shallow waste deposit located near the surface.

In order to maximize the accuracy of this method, a selection of the measurements points based on the assessment of the relative error was made. The filtering of the data provided by this method reduces the local and global uncertainty of the methane oxidized surfaces fluxes determination.

The assessment of the relative error concerning the oxidized methane flux was determined by the equation ( 2)

ÔJox/Jox = ((5JcH4inf/JcH4inf) * JCH4inf+ (5JcH4sup/JcH4sup) * JCH4sup)/ (JCH4inf-JCH4sup) (2)
where 5J OX /Jox is the relative error of methane oxidized fluxe J ox , and 5JcH4in/JcH4mf and 5JcH4sup/JcH4su P are the relative errors of the methane load charge and methane surface flux, respectively.

Surface fluxes measurements and gas sampling

The surface fluxes measurements were performed with a static chamber with recirculation. The upper part of the chamber has a fan to mix the gas, and two sampling tubes. The base of the chamber was sealed into the waste. The sampling gas was pumped into two analysers, a portable flame ionization detector (Telegan FID, AutoFIM II) and an infra red (IR) absorption detector (RSDynamics, Ecoprobe 5). The probe was hammered to the depth of 750 mm, in order to avoid the dilution with atmospheric gas. Then, the gas was pumped and analysed by the IR detector.

1)

Static chain h ei with («ciicuUiii»n Figure 1. Two steps of the measurements of the methane oxidation in the upper layer of the MBT processed waste storages

EXPERIMENTAL RESULTS

MBT plant without organic fraction recovery

For the MBT plant A, the fine fraction which was sieved at a 70 mm mesh and the processed fraction which was sieved after pretreatment at a 50 mm mesh join aerated windrows for 6 weeks. After a passive maturation of 3 -7 months, the compost was stored in the final storage cells. Each 6 months, a new 3 meters layer of composted waste was deposited on the storage cell.

Methane and carbon dioxide surface flux and concentration measurements

Two surveys were conducted on the first MBT plant A, totalizing 71 surface fluxes measurements. Ordinary kriging was used to interpolate the global flux from the punctual measurements. The global methane flux observed on the surface was high (18 m 3 CH4/h) during the first survey and could be explained by the storage of a fraction which contains more organic matter than raw waste, and because aerobic treatment of waste leads to a more rapid onset of methanogenic conditions [START_REF] Robinson | leachate quality from landfilled MBT Waste[END_REF].

The global carbon dioxide flux density was lower than the global methane surface fluxes (12 m 3 CH 4 /h). This high ratio (1.5) between the methane and carbon dioxide global fluxes was even higher in the locality of the more recent deposit where this ratio reached 1.67. The more recent deposit locality (delineated by the isovalues 200 -500 ml methane/min/m 2 in Figure 2) concentrates the methane fluxes (approximately 14 m 3 CH 4 /h). There is a clear relation between the locality of the high methane and high carbon dioxide surface fluxes.

This relation between methane and carbon dioxide emission location could also be observed on the samples gas from the probe (Figure 3). For each gaseous compound (carbon dioxide and methane) the concentrations generally reach 30% -40% at a depth of 750 mm. The low concentration levels were essentially observed on the perimeter of the storage cell and could also be linked with local sealing problems of the probe in the waste and dilution of LFG collected from the gas probe.

The meteorological conditions of the first survey (June 2006) were rather good during the surface emissions measurements, on contrary of the second survey in May 2007. For the first survey, there was little rainfall before the surveys. Surface fluxes were measured on a regular grid with a grid spacing of approximately 5 m. Measurements of moisture content could be done by the sampling of only 3 liters of waste in the shallow layer on 3 different locations, due to the good homogeneity of the MBT processed fine fraction. The average moisture content of this superficial layer collected from 0-100 mm depth, 16% (v/v), was sufficient to allow favourable conditions for methane oxidation and was also not too high to promote gas migration only in preferential flow paths.

The barometric pressure gradient was limited to 0.5 -1 hPa/h and therefore did not interfere with the surface flux measurements. During the second survey (May 2007), the porosity of the waste fine fraction was almost saturated due to the rainy periods before the survey.

Assessment of the methane oxidized fraction

An average oxidative rate of 30 -40% for the selected measurements points (18 points) was reached for the two surveys. The selected points allowed the observation of an oxidation rate for rather high (more than 200 L CH 4 /m 2 d 4 ) to low methane charge. This method cannot be applied for the high methane charges values of the hotspots and obviously for the near zero methane fluxes. Comparisons with similar MBT treated wastes [START_REF] Juha-Kalle | Mechanically-biologically treated municipal solid waste as a support for microbial methane oxidation to mitigate landfill greenhouses emissions[END_REF] show that the oxidizing rate could reach 80% with the same methane charge for columns laboratory tests. The observed in situ methane oxidation potential seems to be limited to a value of approximately 70 L CH 4 /m 2 d 4 on this site for favourable temperature (25°C) and humidity conditions (Figure 4).

MBT site B with organic fraction recovery

The second MBT plant stores only the refuses of sorting before the process, and therefore the organic content is strongly reduced in the landfill cell. The treatment combines an aerobic pretreatment (3-4 days) and a sorting process, with a final separation done at a 10 mm mesh. The fine biodegradable fraction is separated and goes to composting windrows.

Methane and carbon dioxide surface flux and concentration measurements

Only one survey could be conducted on the MBT plant B, before the completion of the final cover of the refuses storage. Surface flux measurements were done on the surface of the refuses which received also crushed wood coming from the refuses of the green waste composting process. The global methane flux observed on the surface was low (0.5 m 3 /h) and was only observed in a few measurements points, on contrary of the carbon dioxide surfaces fluxes which were observed on a large set of the measurements points (Figure 5). Global carbon dioxide surface flux was much higher than global methane surface flux and reached 4.5 m 3 /h. We could see a link between the locality of the high methane and carbon dioxide surfaces fluxes, and the methane surfaces fluxes was only observed in the center of the large area delimited by the isovalues 50-100 ml CO 2 /min/m 2 . Gas probe concentrations measurements showed also a correlation between the locality of the high levels of methane and carbon dioxide concentrations at the depth of 750 mm. Gas migration could also occur in this shallow waste layer, with high content of plastic bags. The high spot of methane surface flux observed on the surface was at a minimal distance of 5 to 10 meters far from the methane isovalue of 5 ml CO2/min/m 2 . Due to the recovery of the fine fraction (organic matter) the carbon dioxide surface fluxes coming from the aerobic degradation were expected to be low. For the plant B, crushed wood was used for the completion of the final waste surface layer. Also the moisture content was only checked by a water balance calculation with the use of daily rainfall and evaporation instead of in situ or laboratory moisture measurements. Water reserves of 100 and 150 mm were used in order to estimate the free porosity for the LFG migration and diffusion in the shallow waste layer of the refuses deposit (Figure 7). The survey was done during the last week of May in 2007. The water reserve seemed to be almost full in this period but we did not observe any indications of water saturation during the survey.

Assessment of the methane oxidized fraction

The same point selection procedure as for plant A was applied to the measurements points (32 points). This method allowed estimating an average oxidizing rate of more than 80% for the temporary cover, but with average oxidized methane fluxes of only 35 L CH4 /m 2 d 4 on this site. The lower load of methane and the presence of crushed wood (which seems to have similar structural properties as wood chips which are generally used in laboratory columns) probably explained this good oxidizing rate.

The filtration of the methane oxidized fluxes by the relative error calculation limits the dispersion of the results but also eliminates the low oxidized fluxes coming from larger methane loads. The selection method shifts the average of the methane and carbon dioxide fluxes observed on the whole data set. The average methane oxidized fluxes coming from the punctual measurements (Figure 8) and selection method is similar to the average oxidation rate estimation coming from an assumption of a LFG load with 60% methane and 40% carbon dioxide. Also this shift causes a slight overestimate of CH4 oxidation.

RESULTS AND DISCUSSION

Different behaviours (Figure 9) can be observed on the two data sets provided by the methane oxidized fluxes measurements and data filtration. For the plant A, the methane load limits the methane oxidizing fraction. An asymptotic tendency observed with only 3 measurements points must be confirmed (asymptotic value of approximately 60 ml/min/m 2 of oxidized methane for the higher methane load). We do not observe the same limitation of the punctual methane oxidized fluxes for the plant B. Plant B cell, receiving refuses, may produce a particular longer channelling flow path in the macro pores which could explain a higher length of gas migration. This migration seems to be observed between the centre of the areas of high methane concentration in the waste at a 750 mm depth and the surfaces fluxes hot spots. The temporary layer of the plant B was modified in the late 2007 by a final cover with a clay layer and a compost layer. Improvements of the methods could be done by the measurements of the gas concentration at different depths but the survey period was limited by the time period of stable weather conditions. The requirements of the surface fluxes measurements limit the survey weather conditions (no rainfall during the survey and the days before, limited barometric gradient pressure) and therefore also the possibility to obtain new data set.

CONCLUSIONS

Oxidized methane fluxes measured on the site B were lower than those coming from site A, and the oxidating rate reached 80% and 40 % for a selection of the measurement points of the site B and respectively the site A. The methane load, higher for the site A, was the principal factor of the oxidation rate difference. These oxidation rates measured on the temporary cover highlight the role of methane oxidation, even for the landfilling phase before the final capping. The combination of waste aerobic pre-treatment, organic fraction recovery and methane fluxes oxidation produces very low residual methane fluxes. These results obtained for only three surveys need to be confirmed but allow extrapolation of a residual methane emission level below the value of 5 m 3 /ha/h for the MBT plant refuses storages with improved cover and MBT plants with organic fraction recovery. The methane emission levels of 5 and 10 m 3 /ha/h were proposed as guidance values for the use of biotic systems [START_REF] Stegmann R; | Discussion of criteria for the completion of landfill aftercare[END_REF][START_REF] Bour | Determination of guidance values for closed landfill gas emissions[END_REF]. These results have been obtained in favourable conditions (medium temperature and humidity) and need to be checked for the winter weather conditions and especially different moisture conditions. The limitations of the methane oxidation were observed for humid conditions then the porosity of the superficial layer was filled with water. The very punctual LFG emissions under these conditions could hardly be measured. The accumulation of LFG below this water saturated layer probably leads to a drastic reduction of the oxidation potential. The ongoing research will focus on the optimizing of the bio covers and biowindows for the different climatic conditions with the participation of INERIS in the METHALIX program funded by the French agency for the energy and the environment.

Figure 2 .Figure 3 .

 23 Figure 2. Methane and carbon dioxide surface fluxes for the site A

Figure 4 .

 4 Figure 4. Comparison of oxidized methane fluxes for MBT processed waste from laboratory (Einola & al., 2007) and in situ measurements with relative errors bars

Figure 6 .

 6 Figure 5. Methane and carbon dioxide surface fluxes for the site B

Figure 7 .

 7 Figure 7. Water reserve assessment for the upper layer of the site B

Figure 8 .

 8 Figure 8. Oxidized methane fluxes versus methane loads for the site B with relative errors bars

Figure 9 .

 9 Figure 9. Comparison of the oxidized methane fluxes for the sites A and B

ACKNOWLEDGEMENTS

The work presented here in this paper is supported by the French Ministry of Environment and sustainable development and ADEME.

Views expressed in this paper are those of the authors and do not necessarily represent those of any organisation.


Jacques Chaineaux

Sébastien Evanno

Bernard Gautier
email: bernard.eautier@edf.fr

Study of the dimensioning of ATEX formed in the case of leakage of H 2 -N 2 mixtures from pressurized ducts

Keywords: ATEX, EXPLOJET software, Explosion, H 2, LEL, PHAST software, V ATE X, XLEL

teaching and research institutions in France or abroad, or from public or private research centers.

Study of the dimensioning of ATEX formed in the case

of leakage of H2-N2 mixtures from pressurized ducts

Introduction

EDF-SEPTEN is confronted with the problem of assessing the risks of ATEX formation during exploitation of various networks of ducts, which convey a pressurized gas (pure hydrogen or hydrogen-nitrogen mixture with known composition) and which are set up in ventilated locals with various volumes of a few hundred cubic meters. Within the scope of studies on safety considering major aggressions (e.g.: earthquake), major leaks of the guillotine rupture type are taken as a hypothesis on these networks, with the goal of controlling their consequences [START_REF] Gautier | Reference Base for safety requirements concerning protection against the risk of internal explosion for the EPR[END_REF].

Under the assumption of loss of the seal of a member of the network, the gas contained in the network would be discharged into the locals through a leak orifice and would mix with air in order to form an ATEX [2], at least in the proximity of this orifice.

EDF-SEPTEN requested INERIS to evaluate the volume of ATEX which would be formed inside these locals [START_REF]Studies of hydrogen leaks in large locals (INERIS -DRA -06 -P71922)[END_REF], [START_REF]Study of dimensioning of the formed ATEX in the case of H2-N2 mixture leaks from pressurized ducts (INERIS DRA-07-89258-08949A)[END_REF].

Our study is an integral part of a request from the French Nuclear Safety Authority (NSA) which dealt with various cases of hydrogen leaks, supposed to occur in certain locals and originating from various pressurized ducts, bringing into play pure hydrogen or H2-N2 mixtures with a defined H2 content, CH2 -N2 (vol. %), in order to evaluate the ATEX volume (VATEX) and the distance to the LEL (XLEL) • In this study, a simplified model may be established in order to determine for defined H2-N2 mixture leaks, the following quantities which characterize the dimensions of the ATEX resulting from the leak :

• the distance to the LEL, XLEL (m),

• the volume of formed ATEX, VATEX (m 3 ), from the following parameters defining a leak :

• the data relating to the leaking duct:

• its nominal diameter, D (m),

• its outer diameter, D ext (m),

• its thickness, e (mm),

• the data relating to the leak orifice supposed to be circular :

• the diameter of the leak orifice d (m) ; under the assumption of a guillotine rupture of this duct, d is related to D ex t and to e by the relationship d = (D ext -2e).

• the diameter of the leak orifice, d (m); under the assumption of a half-guillotine rupture of this duct, d is related to D ext and to e by the

relationship d = (D ext -2e)/ v 2 .
the H2 concentration of the H2-N2 mixture, CH2 -N2 K7°) > the absolute pressure in the duct, P (absolute bars).

Methodology used

Phenomenology of gas leaks

During a leak on a pressurized network, the compressed gas will mix with ambient air through a most often turbulent jet. The conditions under which the discharged gas and air mix depend on many parameters.

However, if the leak occurs in free air and while it is delivering, it is possible to state that :

• it is always pure gas which is present in the plane of the leak orifice,

• on the contrary, there always exists an area of the space sufficiently away from the leak orifice where the air concentration of discharged gas remains very low or even zero. The leak therefore generates a concentration field. For this reason and under the assumption that the discharged gas is flammable, there always exists an area of the space where the gas concentration in the air belongs to the explosivity domain and where the air-gas mixture therefore forms an ATEX. Beyond this area, there no longer exists any possibility of ATEX.

If, on the contrary, the leak occurs in a confined medium (for example in locals), a concentration field also appears within the locals by accumulation. Its development over time, as well as the possible increase in the formed ATEX volume depend :

• on the volume of the locals as compared with the gas volume (a priori limited) which is able to be discharged through the leak orifice,

• on the air renewal rate of the locals, regardless of whether it is equipped with mechanical or natural ventilation.

Thus, under the assumption of a hydrogen-nitrogen mixture leak in locals, the problem is therefore not of knowing whether an ATEX will be formed, but rather of knowing whether the volume of this ATEX will be such that its explosion would cause significant damages.

An explosive atmosphere of more than ten liters present in a constant amount in closed locals is always considered as dangerous independently of the dimensions of the locals [START_REF]Guide of good practices with no restriction for implementing the ATEX 1999/92/EC Directive[END_REF]. It is possible to proceed with an approximate evaluation by applying the empirical rule according to which explosive atmospheres present in closed locals are dangerous when the amounts are greater than one ten thousandth (1/10 000) of its volume. However this does not mean that the whole space should be described as a dangerous location; only the portion of the locals where a dangerous atmosphere is likely to form should be considered as a dangerous location.

Under the assumption of sufficiently ventilated locals where accumulation remains low, the evaluation of the risks of explosion related to the formed ATEX in the case of a leak is therefore directly related to that of the maximum volume of formed ATEX on the jet.

Approach for evaluating the formed ATEX volume: identification of the influent parameters

In order to evaluate the maximum volume of formed ATEX, all the parameters which have an influence on this volume should be identified first.

Parameters related to the leak conditions

Among the parameters which have a determining influence on the maximum volume of formed ATEX, the conditions of the leak are first found, and in particular the leak flow rate, representative of the rate with which the gas will be discharged into the locals.

For a given gas, the leak flow rate itself only depends on the following parameters :

• the section of the leak orifice,

• the orifice coefficient,

• the internal pressure of the network.

• Assumption on the section of the leak orifice

The leak rate first depends on the section of the leak orifice: considering that the question is to evaluate the risks of explosion of an ATEX following a safety approach, EDF-SEPTEN has defined the section of the leak orifice as the one which corresponds to the guillotine rupture of the duct installed in the locals.

Let us note that, for nominal diameters of more than 50 mm, EDF-SEPTEN considers under certain assumptions leak diameters equal to the nominal diameter divided by2 , which amounts to stating that the leak section is half of the one which corresponds to the nominal diameter.

In certain locals, it is not only a duct which is installed, but also the capacity which is connected to this duct ; as each capacity is equipped with a manhole, EDF-SEPTEN has contemplated the guillotine rupture of tapping of the manhole and the consecutive discharge of the gas through the totality of the section of the manhole. This is why the diameter of the tapping of the manhole, the thickness of the metal sheet of this tapping and the diameter of the leak orifice appear for these leak conditions.

• Influence of the network pressure on the leak rate

For a leak orifice with a given section, the leak rate depends on the value of the pressure of the network relatively to the critical pressure which has the value of 1.89 bars absolute :

• if the pressure of the network is greater than the critical pressure, the leak occurs under supercritical flow conditions,

• if the pressure of the network is less than the critical pressure, the leak occurs under subsonic flow conditions.

INERIS has several leak calculation models for characterizing explosive atmosphere. Taking into account the leak conditions retained in the scenarios considered here, INERIS has envisioned to perform calculations with the computational code of INERIS EXPLOJET 4.0 [START_REF]Explojet: Code for calculating explosions in turbulent flammable gas jets[END_REF] derived from fluid mechanics and the commercial software package PHAST in its version 6.4 [START_REF]Evaluation of PHAST versions 6.0 and 6.1, summary report[END_REF].

> Case of supercritical flow

In order to deal with the cases corresponding to supercritical flow, we used the EXPLOJET software package [START_REF]Explojet: Code for calculating explosions in turbulent flammable gas jets[END_REF] which was developed by INERIS during the nineties, with which supercritical gas jets with densities different from that of air may be simulated.

Developed within the scope of European experimental projects and compared with tests, it is more particularly suitable for simulating supercritical jets. The output data (concentration, rate, flammable mass, distance to LEL) are defined by the dynamics of the jet.

With this software, air overpressures likely to be generated consecutively to the inflammation of free jets of flammable gases may be calculated (pseudo-similarity theory applicable to turbulent jets with variable density).

In its version 4.0, this code consists of three computational modules which allow :

-calculation of the concentration, rate and turbulence fields prevailing in the formed explosible atmosphere (based on the applications of similarity laws, the existence of which has been proved in certain cases during experiments conducted by INERIS), -calculation of the flame velocities likely to be observed, as a function of the position of the inflammation point (a module designed to be applied to the case of supersonic jets with variable density, however, its design is such that this module may also be applied to the case of subsonic jets with variable density), -calculation of the air overpressure field generated by the propagation of flames [START_REF] Deshaies | Dynamic effects generated by a spherical flame at constant velocity[END_REF],

essentially, it should be recalled that this is not strictly applicable if the flame velocities exceed 250 m/s.

Assuming that the gas is discharged in the form of a free leak jet and without any obstacle, and also supposing that the jet is stationary, i.e. fed under the initial pressure of the network, assumed to be constant, with EXPLOJET, it is possible to calculate the following quantities :

the volume flow rate of the discharged gas (in Nm 3 /h), the distance from the point of the axis of the jet where the discharged gas content exactly reaches the LEL, (XLEL (m)), the volume of the formed ATEX, VATEX (m 3 ), the mass of hydrogen contained in the formed ATEX (rriH2 (kg)).

In order to reach these results, the EXPLOJET software mainly needs the following data :

• Data relating to the ambient air : its temperature, the atmospheric pressure.

• Data relating to the discharge gas : its pressure in the network, the kinematics viscosity and the y ratio of the specific heats, its temperature, its LEL in air.

• Data relating to the leak orifice supposed to be circular : its diameter, the orifice coefficient CD (for all the cases, the assumption of a guillotine duct rupture has led us to consider a single value equal to 0.77). Generally, the influence of the shape of the orifice in a capacity is expressed by a discharge coefficient (0 < CD < 1).

In order to provide assistance in determining the value of the discharge coefficient CD, an INERIS report [START_REF]INERIS Q-19 Report « Determination of characteristic quantities of the source term required for using a model for atmospheric dispersion of accidental emissions[END_REF] specifies values of CD, depending on the shape of the orifice for turbulent flows of homogeneous fluids (either totally liquid or totally gaseous).

The assumption of a stationary jet is of course not verified in the investigated cases here and it amounts to increasing the maximum volume of the ATEX likely to be formed; indeed the diameters of the leak orifices are often large and would allow rapid decrease of the network pressure, so that the maximum volume of the formed ATEX would necessarily be less than the volume calculated by EXPLOJET.

Further, in order to express the final result, in reality, the maximum amount of H2-N2 mixture likely to be contained in the formed ATEX should be taken into account. Of course, we must ensured that the volume provided by EXPLOJET does not exceed the amount of H2-N2 mixture initially contained in the network.

> Case of subsonic flow

In order to deal with the cases corresponding to subsonic flow, we used the software package PHAST [START_REF]Evaluation of PHAST versions 6.0 and 6.1, summary report[END_REF] version 6.4 developed by DNV Software. The versions 6.0 and 6.1 of the software package were subject to full evaluation by INERIS, the version 6.4 being placed in the continuity of the evaluated versions.

The PHAST software is based on the UDM (Unified Dispersion Model) method which combines models of jets, of dispersion of dense or lightweight gases and of passive dispersion. PHAST in particular allows simulation of gas jets, with variable density in their inertial phase and beyond in the entrainment phase in which inertial forces are dominated by atmospheric dispersion and gravity forces.

The complexity of this type of software is intermediate between Gaussian type dispersion calculations and three-dimensional software packages proceeding with finite volumes or elements.

The version 6.1 of the PHAST software was the subject of an evaluation of its capability of determining the consequences of accidents according to a strict methodology. An evaluation report has been established [START_REF] Duplantier | Evaluation of PHAST versions 6.0 and 6.1[END_REF] and is available on the INERIS internet site (http : //www. iner is. fr).

Like EXPLOJET, this software leads to the mass leakage flow rate and to the distance to the LEL on the axis of the jet, but it does not allow direct access to the volume of the ATEX or to the hydrogen mass contained in the ATEX.

It essentially requires the same data as EXPLOJET; let us note however that it has several limitations as to the amounts of discharged gas; moreover, it is basically reserved for calculations of atmospheric dispersion of gases, i.e. outdoors (in particular, stability characteristics of the atmosphere and wind speed are required), and dispersion in confined locals is treated with significant uncertainty. To be more specific, in order to limit this uncertainty, the software should be used with the maximum stability index of the atmosphere.

Parameters related to the amount of discharged hydrogen-nitrogen mixture

The maximum volume of ATEX which would be formed in the case of a leak, first depends on the total amount of H2-N2 mixture (insofar that it is limited) which is contained in the network and which is therefore capable of being discharged into the locals through the leak orifice; this amount itself depends on the following parameters :

• the proportion of H2 in the H2-N2 mixture contained in the network, as well as the explosivity limits in the air of this mixture,

• the internal volume of the network,

• the pressure of the H2-N2 mixture contained in the network.

• Flammability characteristics and H 2 content of the H 2 -N 2 mixture contained in the network

In the case when the gas of the network is not pure hydrogen, this is a hydrogen-nitrogen mixture, the hydrogen volume content of which is comprised between 3.2 and 43.1 %.

For having a hydrogen-nitrogen mixture form an ATEX in the presence of air, it is required that the content in the hydrogen-nitrogen mixture belong to the explosivity domain of this mixture, comprised between the lower and upper explosivity limits [LEL and UEL (Upper Explosive limit) respectively] :

• in the case of a mixture without any nitrogen (pure hydrogen case), the LEL of hydrogen is equal to 4 %,

• in the case of ternary air-hydrogen-nitrogen mixtures, the LEL of each mixture may be obtained from a ternary diagram such as the one in the following Fig. 1. The result of this diagram is that a hydrogen-nitrogen mixture, for which the hydrogen content is less than or equal to 5.2 % by Vol. is not flammable in air at ambient pressure and temperature and therefore cannot form any ATEX. The curve of Fig. 2 allows determination of the value of the LEL of an H2-N2 mixture depending on its H2 content CH2 N2-

Lires mixt H o -' LEL 20- 15 - It' '• J :. -- 1 I 20 A H2 content (C H '• ^_ i i i J b'J %'J j.
) of H2-N2 mixtures (vol. %)

1 100
Fig. 2 : LEL variation of H2-N2 mixtures versus their H2 content Cm-N2

• Internal volume of the networks Each network consists of several duct sections defined by their inner diameter and their length, and the internal volume of which may be calculated; however this volume is always much smaller than the internal volume of the capacity(ies) to which the relevant networks here are connected, so that the internal volume of the network may be considered as equal to the sole volume of this or these capacities.

The value of the internal volume of the networks considered here (inclusive capacity(ies)) is comprised between 1 m and 60 m . The networks connected to the compressed gas supply frames are generally assumed to have infinite capacity (permanent operating conditions) in the case of the absence of any cut-off system.

• Internal pressure of the gas of the networks

In the present report, all the internal pressure values are expressed in bars absolute. For a few locals, this pressure has a value of 155 bars absolute; for the whole of the other locals, it is comprised between 1.3 and 7 bars absolute.

• Calculation of the maximum amount of discharged hydrogen

The amount of H2-N2 mixture which would be discharged may be calculated by performing the product of the 2 following parameters :

• the volume of the capacity connected to the faulty member of the network,

• the relative gas pressure in this network,

As for the amount of hydrogen contained in the network, it may be obtained by multiplying the amount of H2-N2 mixture contained in the network by the proportion of H2 in this mixture.

• Parameters related to the ventilation conditions of the locals In the case of leakage on a network, it is conceivable that the maximum volume of the formed ATEX as well as the period during which this ATEX would persist, depend on the relative extent of the ventilation flow rate of the locals (i.e. the rate with which the atmosphere of the locals is renewed with fresh air) with respect to the leak rate (i.e. relatively to the rate with which the air of the locals is loaded with hydrogen).

Consequently, the volume of the ATEX is calculated in a relatively inaccurate way by assimilating it to that of an ellipsoid for which the major axis and the minor axis are estimated from the value of XLEL-This is why the relationship (4) shows that the quantity VATEX varies as to the power 3 of XLEL-

Conclusion

The INERIS study for EDF-SEPTEN basically consisted of establishing a simplified computational model for determining the parameters VATEX and XLEL, for defined H2-N2 mixture leaks.

From the moment that a flammable H2-N2 mixture is discharged into the air of locals, through a leak orifice, it is established that :

• an ATEX is always present at the leak orifice,

• the volume of this ATEX depends on the section of the leak orifice and on the network pressure upstream from the leak,

• depending on whether this pressure is greater or less than the critical pressure, i.e. a little less than 2 bars absolute, the leak will occur under the conditions of a respectively supercritical or subsonic jet,

• regardless of the flow conditions, the leak is characterized by its flow rate,

• depending on the values of this flow rate and of the ventilation flow rate of the locals, an ATEX may form in the locals and may accumulate there, further away from the leak orifice,

• the dangerousness of each of the ATEX which form in the locals is all the more significant since their volume is greater.

Let us add that as the present evaluation is conducted within the scope of a safety step, the dangerousness of an ATEX is appreciated with regard to the effects which its explosion would produce on sensitive pieces of equipment. The tolerance on this account is greater than in terms of protecting workers for example.

In the case of a leak under supercritical flow conditions, the formed volume of ATEX at the leak orifice may be calculated by means of the EXPLOJET software developed by INERIS, assuming the jet to be stationary, free and without any obstacle, which amounts to obtaining an upper bound value of the actual volume.

In the case of a leak under subsonic flow conditions, we evaluated the volume of the ATEX formed at the leak orifice by using the PHAST software but the obtained results are marred with significant uncertainty, because above all this is an atmospheric dispersion software which we therefore used at the limits of its capabilities.

Finally, we compared the results obtained with those resulting from the application of the homogeneous approach, a method which consists of :

• assuming that the discharged H2-N2 mixture homogeneously mixes with the atmosphere of the locals,

• comparing the content of this mixture with its LEL,

• deducing the possibility of formation of a dangerous ATEX in the locals.

In the case of freely developing jets, the evaluation of the ATEX volumes and distances on the jet is a useful addition to the approach by homogeneous dilution. In the first case, the instantaneous ATEX is characterized, in the second case the ATEX is characterized by accumulation. In the investigations conducted by EDF-SEPTEN, they were able to ascertain that the first type may become sensitive in the case of large size locals where accumulation does not necessarily lead to a notable rise in the average values.

Fig. 1 :

 1 Fig. 1 : Ternary hydrogen-nitrogen-air diagram

Table 1

 1 hereafter indicates the LEL value of the other hydrogen-nitrogen mixtures considered here.

	Hydrogen content of the	LEL
	H2-N2 mixture	(% vol.)
	(% vol.)
	43.1	9.3
	42.6	9.3
	23.5	16.4
	18.8	20.0

Table 1 :

 1 Values of LEL for the applied hydrogen-nitrogen mixtures

Results and interpretation

We indicated earlier that the case of leaks under supercritical flow conditions which correspond to P > 1.9 bars absolute, and the case of leaks under subsonic flow conditions, which correspond to P < 1.9 bars absolute, should be considered separately.

Case of leaks under supercritical flow conditions

Establishing the simplified model consisted of clarifying the functions which each relate the following quantities :

• VATEX, as a function of the following parameters :

In order to specify the quantities XLEL and VATEX, with the EXPLOJET model, we calculated the values of each quantity for various pairs of d and P values.

For each pair, we imposed fictitious values of the LEL of hydrogen, which amounts to studying the influence of the H2 content of the H2-N2 mixtures, designated as CH2-N2-

The values of the LEL of the H2-N2 mixture versus their H2 content (CH2 N2) are grouped in Table 1 and shown in Fig. 2.

Let us add that in order to take into account the assumption of a guillotine rupture of the duct, the orifice coefficient d was taken in every case to be equal to the value proposed by the PHAST model, i.e. 0.77, as well as for the calculations performed with EXPLOJET.

The calculations led to the following relationships :

and

wherein the quantities and parameters have the definitions and units as indicated above.

As regards the leaks under supercritical flow conditions, the relationships (1) and (2) show :

• the exponent 0.5 in how the quantity XLEL varies with P,

• the exponent 1 in how the quantity XLEL varies with d,

• the exponent 1.5 in how the quantity VATEX varies with P,

• the exponent 3 in how the quantity VATEX varies with d.

Case of leaks under subsonic flow conditions

As in the case of leaks under supercritical flow conditions, establishing the simplified model consisted of specifying the functions which relate XLEL and VATEX to P, d and CH2-N2-But this time, we calculated the values of the first two quantities for different pairs of d and P values with the PHAST model.

In order to study the influence of the H2 content of H2-N2 mixtures (CH2-N2). we again imposed here fictitious LEL values for hydrogen, by using the values of Table 1 and as shown in Fig. 2.

The calculations led to the following relationships :

and V ATEX = 0.004 x X\EL = 0.004 x P 114 xrf 3 x C 3 H2-NZ [START_REF]Study of dimensioning of the formed ATEX in the case of H2-N2 mixture leaks from pressurized ducts (INERIS DRA-07-89258-08949A)[END_REF] wherein the quantities and parameters have the definitions and units as indicated above.

As regards the leaks under subsonic flow conditions, the relationship (3) shows :

• the exponent 0.38 in how the quantity XLEL varies with P,

• the exponent 1 in how the quantity XLEL varies with d,

• the exponent 1 in how the quantity VATEX varies with CH2-N2-Unlike the EXPLOJET model which directly provides the volume of the ATEX by integrating the function (integral of (c(x, y, z)dxdydz), the PHAST model only considers the extension of the ATEX on the axis of the jet, i.e. XLEL.


Stéphane ; Lafortune
email: stephane.lafortune@ineris.fr

Jean-Claude Pinte

NORM in the petroleum and geothermal industries: evolution of the French radioprotection legislation

Fluids produced from oil or gasfields and aquifers are naturally radioactive due to the presence of potassium-40 and isotopes from decay chains of uranium-238 and thorium-232 in these reservoirs. Thus, natural radionuclides can be carried away towards surface when petroleum or geothermal reservoirs are drilled and produced. These radionuclides can precipitate in deposits forming in process equipment, which can then be unusually radioactive, expose workers to hazardous materials and create waste disposal problems. Workers can also be exposed to high concentrations of radon. Due to their natural origin, these accumulations of radionuclides in equipment are called Naturally Occurring Radioactive Materials (NORM). They are also known as TENORM, because they are "technologically enhanced".

In closed systems, exposition to gamma radiations penetrating equipment to the external surface can be reduced by restricting and controlling access to appropriate areas. But personnel may come into direct contact with NORM during maintenance and cleaning operations, when opening equipment and vessels.

In France, hygiene and safety rules concerning the protection of workers in the petroleum and geothermal industries are formulated in a specific regulation related to extraction and mining. Presently, both petroleum and geothermal industries are not included in the list of activities which have to follow radiation protection measures concerning exposition to NORM. French authorities are currently planning to update the list by including these two activities. There is a real need to realize a state-of-the art of the exposition level of workers and to standardize the dose limits with those given by the French Labour and Public Health Codes. This paper will present the origin of NORM and radon contamination in the petroleum and geothermal industries and discuss the French legislation evolution for the radiation protection of personnel working in these two industries.

What are NORM and where do they come from? NORM (acronym for Naturally Occurring Radioactive Materials) refer to materials containing radionuclides (or radioactive materials) from natural sources (Californian DHS 1996).

NORM can be found in underground oil and gas reservoirs or aquifers. They are present in rocks as uranium-238 ( U), thorium-232 ( Th), potassium-40 ( K) and their radioactive progeny.

Whereas potassium-40 has only two stable daughter isotopes: calcium-40 ( 40 Ca) and argon-40 ( 40 Ar), uranium-238 and thorium-232 have longer decay chains before reaching final stable isotopes: lead-206 ( 206 Pb) and lead-208 ( 208 Pb) respectively. The main decay chains of uranium-23 8 and thorium-232 are shown on the figure below (see fig. 1). If the less soluble isotopes, such as uranium and thorium, remain bound to the rock matrix, soluble and gaseous elements produced by radioactive decays can accumulate in fluid phases and migrate out of the reservoir [START_REF] Paschoa | NORM from the monazite cycle and from the oil and gas industry: problems and tentative solutions[END_REF].

Thus, NORM can be found in different phases. As for radon (Rn), they can be in gaseous phase. They can also be dissolved in fluids (oil, water) and reprecipitate as carbonates and sulphates of calcium, barium and strontium... if there is any change in temperature or pressure conditions of the fluid phase.

NORM in the petroleum and geothermal industries

A technologically enhanced occurrence In oil and gas fields or in the geothermal industry, NORM may occur as radioactive materials that have been concentrated through production processes. Thus, some people also call them TENORM because they are "technologically enhanced". Concentration processes are different and depend on the nature of the radionuclide.

Depending its solubility and its ability to precipitate as a mineral phase, a radionuclide can (1) stay trapped in the rock matrix within the reservoir, (2) precipitate along the pipes of the industrial installations, (3) be present in residues or (4) in produced fluids (oil, gas, water) [START_REF] Gray | NORM contamination in the petroleum industry[END_REF][START_REF] Sniffer | Identification and assessment of alternative disposal options for radioactive oilfield wastes. Summary guidance[END_REF].

During extraction processes, produced fluids are submitted to temperature and pressure changes: equilibrium with reservoir conditions is no more ensured and deposits can precipitate onto the internal surface of pipes or as particulates which will be accumulated in residues and sludges. These deposits can contain radionuclides and be considered as NORM [START_REF] Betts | NORM management and disposal -Options, risks, issues and decision making[END_REF].

Considering the half-life times of the different radionuclides from uranium-238 and thorium-232 decay chains, radium ( 224 Ra, 226 Ra and 228 Ra), radon ( 220 Rn and 222 Rn), lead-210 and polonium-210 are the main radionuclides that could be found in oilfield, gasfield and geothermal facilities.

Risks induced to workers

The enhanced levels of radionuclides in industrial facilities can potentially raise a problem to operating and cleaning personnel. The radioactive decay of radionuclides from chains of uranium-238 and thorium-232 takes place by emission of either alpha or beta particles and by the emission of gamma rays. Exposition to these ionising radiations can cause damage to the body tissues.

Whereas alpha particles are stopped after a penetration of about 50 micrometers in a human tissue, and beta particles go no further than 1 centimeter, gamma rays are highly penetrating. One must be aware that even if they do not penetrate far the human body, alpha particles are particularly hazardous when they are inhaled or ingested, because the energy released by the radioactive decay is expended over a very short distance [START_REF] Reed | Evaluating the real risks of radioactive scale in oil and gas production[END_REF].

Personnel working near closed systems where NORM accumulations occur could then be exposed to gamma rays. For personnel working near open systems or opening closed systems for cleaning operations, the risk of direct contact with radioactive materials by inhalation of gaseous NORM and ingestion of solid NORM is real [START_REF] Gray | NORM contamination in the petroleum industry[END_REF][START_REF] Reed | Evaluating the real risks of radioactive scale in oil and gas production[END_REF][START_REF] Sh | Influence of radon and decay products on fiel equipment service personnel[END_REF].

NORM issues have been well studied in different states of the USA or in countries as Canada, Norway, United Kingdom and by other oil-producing countries. Several studies have been performed and have permitted to detect equipments where NORM accumulations can preferentially occur (see fig. 3 and4) [START_REF] Gray | NORM contamination in the petroleum industry[END_REF]). Taking into account experts' conclusions and experience feedbacks, stakeholders have to define hygiene and safety rules concerning the radiation protection of workers that are appropriate to risks incurred by the personnel of the petroleum and geothermal industries.

French regulation for radiation protection of workers

Dose limits and categories of workers

In France, hygiene and safety rules concerning the protection of workers are formulated in the Labour Code.

But, hygiene and safety rules specific to extractive and mining industries (including petroleum and geothermal industries) are an exception and are formulated in a dedicated regulation called Règlement Général des Industries Extractives (RGIE or "French specific regulation related to extractive and mining industries").

Concerning radiation protection aspects, the two legislative texts divide workers in three categories referring the level of exposition that they can encounter: "A category" workers, "B category" workers and other workers who do not belong to one of the two previous categories.

Each category is defined by specific levels of exposition: the classification of a worker in one category depends on the "effective dose" and/or the "equivalent dose" (see glossary for definitions) received by this worker.

Pregnant women and under-18-year-old workers cannot work in environments that expose them to radiation levels which could justify considering them as "A category" workers.

Each category also implies specific constraints for the employers concerning risks prevention in terms of information, formation, medical care of their employees... Dose limits given by the two texts to define the categories of workers are different. The dose limits defining categories of workers in the French Labour Code are below the values given by RGIE. A standardization is currently under study. The limit values are given in the two tables below (see tables 1 and 2).

Table 2. Dose limits for each category of workers defining by the French specific regulation related to extractive and mining industries (RGIE).

All workers

A category workers B category workers

Effective dose limit

< 50 mSv per 12-month period < 30 mSv per 3-month period > 15 mSv/year > 5 mSv/year Specific effective dose limit < 12.5 mSv per 3-month period for pregnant women < 10 mSv/year for pregnant women

Each legislative text defines the list of activities where radiation protection aspects have to be considered by the employers.

French law requires employers from extractive and mining industries to implement measures to prevent their workers from exposure to radiation while (1) mining radioactive substances, (2) when using a radioactive source, or (3) working underground. Petroleum and geothermal industries are possibly concerned by items (2) and (3).

Exposition to NORM

Presently, the French legislation does not regulate the risks of exposition to NORM for workers from petroleum and geothermal industries.

But exposition to NORM is regulated for industries whose hygiene and safety rules are formulated in the French Labour Code. Indeed, the French authorities have drawn for these industries a list of professional activities or classes of professional activities, where exposition to NORM has to be taken into account for the radiation protection of workers [START_REF] Cazala | NORM industrial plants with potential radiological impact on workers and local population: first lessons from the review of available risk assessment studies in France[END_REF]). The following diagram (see fig. 5) shows the provisions of the French Labour Code for the listed professional activities.

The employers must determine the doses absorded by their workers and send the results of the dosimetry studies to the French authorities Individual effective dose < 1 mSv/year

The employers do not have to take new specific measures to protect their workers from exposition to NORM Individual effective dose > 1 mSv/year

The employers take measures to avoid the exposition of their workers to NORM or to reduce the individual effective dose below the limit value If the individual effective dose cannot be reduced under the limit value, the employers have to respects provisions of the French Labor Code concerning radiation protection measures for"B category" and "A category" workers The principles presented in the previous diagram could be used as a working basis for the updating of the French legislation, in the case where the exposition to NORM for workers from extractive and mining industries would be regulated.

Conclusions

The French law regulates the risks of exposition to NORM (Naturally Occurring Radioactive Materials) for people working in industries whose hygiene and safety rules are formulated in the French Labour Code.

But these risks are not presently taken into account in the French specific regulation related to extractive and mining industries, and more precisely for people working in the petroleum and geothermal industries.

Considering experience and feedback from other countries and from industrial companies, French authorities are currently discussing the updating of the specific regulation related to the protection of workers from extractive and mining industries to consider exposition to NORM. This will mainly concern the petroleum and geothermal industries.

Because a good knowledge of the real exposition to NORM of the French workers in the petroleum and geothermal industries is a fundamental basis for discussions, dosimetry studies will certainly be launched in a next future.

Glossary Equivalent dose

The equivalent dose takes into account the fact that for the same quantity of energy deposited in an organ or a body by radiation, the biological effects will depend on the nature of the radiation received (alpha/beta particles or gamma rays). It is expressed in sievert (Sv) and is determined by multiplying the absorbed quantity of energy by a weighting factor depending on the nature of the radiation.

Effective dose

The effective dose takes into account the fact that at a same equivalent dose value, biological effects will depend on the irradiated organ. It is expressed in sievert (Sv) and is the product of the equivalent dose with a sensitivity factor depending on the organ which has been irradiated.

Fig. 1 .

 1 Fig. 1. Decay chains of uranium-238 (left-hand) and thorium-232 (right-hand). Half-life times are given in years, days, hours, minutes or seconds. Mother isotopes are shown by a red background, daughter radioactive isotopes by a blue background and daughter stable isotopes by a white background. Red frames show the main radionuclides present in NORM and found in oil and gas production facilities.

Fig. 2

 2 draws the list of possible NORM occurrences in these industrial facilities.

Fig. 2 .

 2 Fig. 2. Schematic cross section of an oil/gas reservoir or an aquifer with migration pathways and phases of the main radionuclides which can induce occurrences of NORM accumulations in petroleum and geothermal industrial facilities.

Fig. 3 .Fig. 4 .

 34 Fig. 3. Simplified design of an offshore oil production unit. Equipments where NORM accumulations are mainly found are shown by red frames. Equipments where radon or polonium are mainly found are shown by a red background (Reed et al. 1991).

Fig. 5 .

 5 Fig. 5. Measures for the radiation protection of personnel exposed to NORM and working in industries whose hygiene and safety rules are formulated in the French Labour Code.

surrounding formations Cap rock Reservoir rock J ^ Plating out of 222 Rn daughters: 210 Pb and 210 Po ^ + Co-precipitation of Ra isotopes with Ba 2+ , Sr 2+ and Ca 2+ B 222 Rn migrates with gas and condensate - 22S Ra, 226 Ra, 224 Ra, 222 Rn ""••"""' dissolved in produced water - Additional unsupported 210 Pb 2 * (\ from 222 Rn decay transported in solution v_-/ - WELL Rock: 238 U and 232 Th remain bound to the rock matrix Fluid phases: 238 U and 232 Th daughters transported with the different fluid phases

  Produced water: 226 Ra, 228 Ra, 210 Pb, 222 Rn

	Gas: 222 Rn
	Condensate: 222 Rn, particulate scale
	Oil: particulate scale, 222 Rn
	°-Upper

Table 1 . Dose limits for each category of workers defining by the French Labour Code.

 1

		Effective dose limit	Equivalent dose limit for the hands, the forearms, the feet and the ankles	Equivalent dose limit for the skin (for a 1 cm 2 surface)	Equivalent dose limit for the crystallnie lens	Articles in the French Labour Code
	All workers	< 20 mSv per 12-month period	< 500 mSv per 12-month period	< 500 mSv per 12-month period	< 150 mSvper 12-month period	R4451-12 R4451-13
	A category workers	> 6 mSv/year	> 150 mSv /year	> 150 mSv /year	> 45 mSv/year	R4453-1 R4453-2
	B category workers	> 1 mSv/year	no value	> 50 mSv/year	> 15 mSv/year	R4453-3
	Population	< 1 mSv/year	no value	< 50 mSv/year	< 15 mSv/year	French Public Health Code