Marion Kimmel
email: marion.kimmel@meris.fr@benjamm.piel

Benjamin Piel

Hafid Baroudi

Annette Esnault-Filet

Xavier Laloum

Dominique Mazzieri

Manon Kimmel

Hydrogeologic modelling for permeable reactive barriers design

Keywords: reactive barriers, groundwater clean-up, in-situ remediation, design method, hydrogeologic modelling

est destinée au dépôt

Introduction

Perméable reactive barriers (PRB) are passive and in-situ Systems for groundwater treatment. Reactive material is placed in thé subsurface to intercept a plume of contaminated groundwater moving through thé System under its natural gradient. This créâtes a passive treatment System. Field-scale installation of a reactive barrier requires carefui design based on thé site-specific hydrogeology and on contaminant plume characteristics. Important parameters to take into account when designing reactive barriers are essentially its position with regard to thé contaminant plume, thé hydraulic site characteristics, thé characteristics of thé gâte (type of reactive material, geometry), and thé depth of thé substratum into which thé barrier is "keyed". Groundwater flow modelling is a usefui tool to understand thé hydraulic behaviour of thé site and to optimise thé reactive barrier design.

Thé study, conducted by INERIS for SOLETANCHE BACHY, aimed at understanding hydraulic mechanisms by numeric tools, for various types of contamination and site configurations. It aiso consisted of thé development of design tools. Both thé création of simplified tools such as design charts and définition of a PRB design methodology were undertaken. So-called "funnel-and-gate" configurations were studied, implemented by a spécial patented drain panel® process, with reactive-material filled filters placed in one or several gâtes, between impervious vertical walls, such as slurry walls.

Thé first part of thé study includes parametric tests. Indeed, thé System, although simple in its principle, requires considération of numeroùs parameters :

• thé flow rate to channel towards thé reactive gâtes : it dépends on thé position and thé width of thé contamination source, as well as on thé hydrogeologic parameters of thé site : hydraulic conductivity, transmissivity, pressure head, hydraulic gradient and its seasonal variations.

• thé hydraulic characteristics of the filters : thé pressure head needed to reach a sufficient flow rate dépends on thé number of filters, their section and permeability. On thé other hand, thé finer thé grain size is, thé longer thé résidence time is.

• thé life-time of thé filters : it dépends on thé flow rate through thé filter, on thé rétention capacity of thé filtering material on thé évolution of thé contaminant concentrations in thé aquifer, and therefore, on thé kinematics of exchange between thé source of contamination and thé groundwater.

• thé dimension of thé slurry walls : because of the pressure head necessary ta maintain a sufficient flow rate, thé water will be slightiy under pressure on thé upstream side of thé barrier, and some stream lines will move around thé System. Thé higher thé pressure head required, thé longer thé slurry wall must be.

Thé understanding of thèse mechanisms and their interactions is thé principal object of thé study. Numerous models using MODFLOW / MT3D / MODPATH were constructed to simulate thé behaviour of thé barriers under several configurations of contaminations and site. For each configuration, thé sensitive properties were identified and integrated into thé design method.

Thé second part is thé development of design tools, using results from thé previous simulations. A design method was set up, which allows thé détermination of both wall and filter lengths, knowing thé width of water that moves through thé gate(s) (called capture zone), thé filter section, and thé permeability of thé filters and of thé aquifer. It was shown that thé hydraulic gradient of thé site has no impact on thé width of thé capture zone (if ail thé other parameters remain equal). Thé method aiso allows prédiction of thé flow rate in thé gate(s), and thé résidence time in thé filters. Thé type of barrier studied hère is composed of two watertight walls, of equal length, placed on either side of a gâte holding thé filtering System. Thé reactive barrier is placed at 700 m from thé upstream limit of thé model, because thé impact of the barrier is higher upstream than downstream.

Characteristics of thé models

Thé boundary conditions are set equal to thé following parameters :

upstream and downstream constant hydraulic heads, respectively 20 and 15 meters, side and base limits : it corresponds to no-flow limits.

Thé hydraulic gradient, which results from thé former boundary conditions, is 5.10' 3 . A high gradient was selected so as to contrast thé flows which circulate in thé reactive gâte of the barrier. Thé height of water, at thé location of thé filters, reaches 17 m. Thé hydraulic conductivity of thé aquifer lies between 10' 6 and10' 3 m/s.

2.2. Présentation of the drain panel®, and modelling of the filter system Thé drain panel® system allows for thé installation of a battery of multiple removable filters, filled by reactive materials. This system was developed and patented by SOLETANCHE BACHY. Thé construction principle is illustrated in Figures 2 and3, « Watertight barrier : (in dark grey) Its width is 0,60 m, it is keyed in a impervious substratum.

• Slurrv wall : (in clear grey) Its width is 0,60 or 1,2 m according to thé size of thé filters, its length is 2,50 m.

• Upstream and downstream drains : (sandy colour) Their width is 0,60 m. Their length is variable. Their hydraulic conductivity ranges between 10' 2 and 1 m/s.

• Filters : (in yellow) Two diameters are used : 0,40 m (for a gâte width of 0,60 m) and 0,90 m (for a gâte width of 1,2 m). 1, 2 or 3 filters are used, of unit length 3 meters, Their permeability varies from 10' 4 to 10' 2 m/s, depending upon thé nature of thé reactive material, its state of compactness and clogging.

• Pipes : (in white when it is filled with water, transparent in thé other case)

To be modelled under MODFLOW, thé drain panel® must be compared to a simpler structure. Thé purpose of thé following calculation is to simplify thé geometry of thé device, according to thé following diagram (figure 4) :

Kdrai n K gate Watertight wall

Figure 4 -Principle adopted to model thé reactive gâte and thé drains

For thé needs of the modelling, thé gâte becomes a parallelepiped filled with reactive material.

Thé filters are thé éléments governing thé flow through thé device. Thé calculation that follows makes thé assumption that thé pressure losses in thé pipes are negligible, and oniy takes into account thé pressure losses in thé filters.

Thé System is compared to several filters, with a length L, placed end to end as shown in figure 5 :

L J

h.î

Thé models are discretized in a grid that includes between 40 000 and 47 000 cells depending on thé length of the slurry walls. Grid-cells measure 20 m as a maximum, apart from thé zone where thé barrier is set. Thé cell size decreases regularly along thé barrier, in direction of the filter gâte, from 8,4 to 0,2 m. Thé reactive gâte is discretized as shown in figure 6 : thé gâte, which measures 0,60 m or 1,20 m according to thé diameter of filter chosen, includes 4 cells; in its width. Models were carried out in order to test thé discretization sensitivity. Thé model for a gâte of 0,60 m width included 2 or 6 cells to represent thé gâte width. Thé values of flow in thé gâte were différent from 0,4 %. Thé option with 4 cells, rather than 2, was selected because it allowed better simulations of trajectories under MODPATH.

Development and présentation of thé design method of thé PRBs

Sensitivity analysis of the parameters

Works of [START_REF] Starr | In situ remediation of contaminated groundwater : thé funnel-and-gate System[END_REF] and [START_REF] Teutsch | Thé Design Of In-Situ Reactive Wall Systems -A Combined Hydraulical-Geochemical-Economical Simulation Study[END_REF] specify that, in thé case of simple reactive barriers (without drain and reagent, simply established in a trench), oniy thé width of thé gâte, thé length of thé slurry walls and thé relationship between thé transmissivity of thé gâte and thé transmissivity of thé aquifer, govern thé width of the capture zone. Thus, according to thèse authors, thé hydraulic gradient has no influence over thé width of thé capture zone. Validity of this assumption was checked when applied to drain-panel system. Lastly, according to thé works of [START_REF] Starr | In situ remediation of contaminated groundwater : thé funnel-and-gate System[END_REF], thé flow in thé gâte is maximum for an angle of barrier of 180°, and for watertight walls placed at 90° of thé direction of thé régional flow. This configuration was thé oniy one taken into account in thé models.

Steps of the design method

Thé basic idea retained for thé design method of PRB consists in determining thé capture zone width (thé width of thé water band of thé aquifer) which has to circulate through thé reactive gâte (figure 7). Then, according to thé characteristics of thé aquifer, and to thé filters chosen, thé second stage consists in determining, by thé use of design charts, thé length of watertight wall necessary for thé desired capture zone. If thé design charts do not allow it, some design methods for drains or additional gâtes are then proposed.

Thé design of a perméable reactive barrier proceeds according to thé following steps : O Calculation of the future flow rate throuqh thé gâte Thé designer must know several parameters characterising thé reactive barrier site location :

• thé local hydraulic gradient of thé site, i ;

• thé height of water in thé aquifer, h ;

• thé hydraulic conductivity of thé aquifer, K aq ;

• thé width of thé contamination plume, Z c .

Thé capture zone, Z c , concretely corresponds to thé latéral extent (in thé direction perpendicular to thé groundwater flow) of thé contamination plume, where thé contaminant concentrations are higher than thé acceptable standard, calculated either with or without a safety factor. See figure 7. Q=K.Z,.h.i

© Choice of the drain panel®equipment

Thé user has to choose thé characteristics of thé filters : diameter and hydraulic conductivity K f . Thé hydraulic conductivity of the filters will be a function of the constitutive material.

For high permeabilities of aquifer (10' 4 m/s), it is recommended to use filters of 0,9 m in diameter, which are able to accommodate larger flows.

© Calculation of thé filter length

Thé length is calculated using thé following criteria :

1. Thé life-time of thé System has to be sufficient : knowing thé quantity of contaminant which can be sorbed per g of reactive material, and thé concentration in thé inlet of thé gâte, thé designer is able to estimate thé quantity of reactive material necessary to obtain an acceptable renewai time for thé filters.

2. Thé transit time in thé filters has to be sufficient to achieve a satisfactory treatment, even at thé end of thé life-time of filters.

© Détermination of thé length of thé barrier

Thé designer, knowing thé width of thé contamination plume (that is thé capture zone) and thé length of filter to be used, can calculate thé length of watertight barrier that is needed to obtain thé desired flow through thé gâte. To achieve that, he needs to choose and use thé design chart corresponding to thé hydraulic conductivity of the aquifer and his filters, as well as thé filter diameter. Figure 8 is an example of design chart relating thé zone of capture, thé total length of filter, and thé length of thé watertight walls. Thé design charts were drawn by simulating various aquifer and barrier configurations (watertight walls lengths, filter lengths, drains and aquifer hydraulic conductivities, and gâte widths).. Design charts are not suited for thé following conditions : if thé capture zone is too narrow (< 10 m) or if thé water flow in thé gâte is too low (< 1 m 3 /day), which constitutes an instability factor for thé simulations.

Zc design chart -linear interpolation

Général déductions were drawn, thanks to thé development of thèse design charts :

A high aquifer hydraulic conductivity causes a high water flow through thé gâte, but a narrow extended capture zone.

A small length of filter induces weak pressure losses, resulting in a high water flow through thé gâte, and a broad capture zone.

A low filter hydraulic conductivity decreases thé water flows and thé capture zones.

For K aq = 10' 4 m/s, a value of K f >. 10' 3 m/s is necessary to obtain a sufficient capture zone.

Thé design charts are identical, whatever thé drain hydraulic conductivity is (for thé same K aq and K f ).

Two aquifer hydraulic conductivities were definitely isolated from thé design method, for hydraulic reasons : thé aquifer hydraulic conductivity 10' 3 m/s générâtes too high water flows through thé gâte for very small capture zones. On thé contrary, thé aquifer hydraulic conductivity 10' 6 m/s générâtes very broad capture zones and low water flows through thé gâte. Thèse cases were not developed further. Thé design charts corresponding to thé aquifer hydraulic conductivity 10' 3 m/s are nevertheless available.

Thé design charts were calculated for a drain length of 2,8 m and a water height in thé aquifer of 17 m. If thé water height in thé aquifer is lower than 17 m, thé barrier designer can apply corrective coefficients to thé capture zone, indexed in tables such as that presented on figure 9. Thèse coefficients were aiso obtained by séries of simulations. If thé width of thé capture zone does not appear in thé available design chart, thé designer chooses a smaller capture zone and can increase it by referring to tables of increase of drain length. Those correspond to ail thé configurations of aquifer hydraulic conductivities, lengths of slurry wall and sizes of gâte. Thé designer will take care not to exceed thé maximum drain length (in red in thé table), to exploit thé gâtes at thé maximum of their capacity. Indeed, when thé équivalent hydraulic conductivity applied to thé gâte is too low to let thé water flow through thé gâte, groundwater that flows through thé drains, of high permeability, can move around thé barrier. Thé following figure is an example of table containing corrective coefficients of thé capture zones according to thé drain length, for watertight walls lengths ranging from 2x80 to 2x150 m (gâte 0,6 m, aquifer 10' 5 m/s, filter 10' 2 m/s) Thé total length of the multi-gate barrier is (n+1)L (>Z c ).

• If Zc/n > L (overlapping) : In this case, where thé capture zones of two gâtes overlap, inter-gate lengths hâve to be increased and thé multi-gate PRB design is : Thé total length of thé multi-gate barrier is 2L + (n-1).Zc/n (>Z c , since thé capture zone of a mono-gate System cannot be wider than thé barrier (2L)).

At thé conclusion of steps 4, 5, or 6, thé design of thé reactive barrier is complète. It is aiways possible to begin again designing at a former step, in order to hâve several possible configurations, and in order to make a choice according to thé technical and economical constraints of thé project.

Conclusion

Thé design method of thé PRBs equipped with drain panel® system, was conceived by INERIS for SOLETANCHE BACHY using many numerical simulations of barrier installation on simple cases of groundwater flows. Thé method is organised into 6 steps, which allows, according to thé configuration of a contamination plume in groundwater, thé design of thé reactive filters, thé length of thé watertight walls and of thé drains, and détermination of thé number of gâtes.

This method was validated, aiways using numerical simulations, by modelling real cases. In addition, a prédiction method of thé contamination migration was conceived using 1-D or 2-D analytical solutions.

Thèse solutions allow to predict thé évolution of thé contaminant concentrations upstream of thé reactive barrier, in order to evaluate thé life-time of the filters.

Figure 1 -

 1 Figure 1 -Geometry of thé models

Figure 2 -

 2 Figure 2 -Principle of the drain panel (developed and patented by SOLETANCHE BACHY)

Figure 5 -

 5 Figure5-Principle adopted to group thé filters for thé needs of calculation Thé flow that circulâtes through thé gâte is calculated by thé following formula, obtained from Darcy's law :

Figure 6 -

 6 Figure 6 -Mesh used to model thé filtering gâte

  Figure 7 -Zc définition By a calculation using Darcy's Law, thé user détermines thé water flow which circulâtes in thé portion of aquifer limited by Z c and thus through thé gâte of the future barrier :

Figure 8 -

 8 Figure 8 -Example of design chart relating thé capture zone, thé filter length, and thé watertight walls length

Figure 9 -

 9 Figure 9 -Corrective factors for a 7-m high watertable (gâte 0,6 m, aquifer 10-5 m/s) © Détermination of thé necessary drain length

Figure 12b -

 12b Figure 12b -Design of multi-gate barrier (n=3) in case of overlapping.

  can be less, either due to thé grain size distribution of thé filtering materials, or secondary chemical reactions which take place within thé filters, such as carbonate précipitation, leading ta an important réduction in K and possible clogging. Corrective coefficients of thé capture zone by decreasing thé permeability of thé filters were calculated and are proposed to thé designer in tables like figure11.If thé capture zone obtained at thé end of thé stage @ is not satisfactory, thé designer can increase thé number of gâtes and begin again thé design at thé step of his choice : 0 or ©. It should not be forgotten to re-evaluate thé necessary filter length for each gâte. One has to check that thé capture zones of thé gâtes do not overlap. If this is thé case, thé length of the wall between thé gâtes must be increased.Thé main objective of thé multi-gate System is to split up thé desired total capture zone into multiple capture zones of smaller size assigned to several gâtes. This method allows an identical capture zone to be obtained with reactive filters of lower permeability and higher length. Ail gâtes are supposed to be exactiy thé same : filter properties, drains,... In général, it appeared during thé simulations, that thé capture zone of a System with n gâtes is n times bigger than thé capture zone of a System with one gâte (same equipment of thé barrier). To design thé reactive barrier, thé capture zone to be assigned to each filter gâte must be equal to Z c i gate = Z c /n, with n thé number of gâtes of thé system. Thé multi-gate design begins with thé design of a barrier intercepting Z c /n with a given filter length. Thé designer uses thé adéquate design chart and calculâtes thé length of slurry wall necessary to intercept Z Design of multi-gate barrier (n=3) in case of no overlapping.

	1,14 1,19 1,42 1,68 1,79 2,19 2,47 Figure 10 -Z c corrective coefficients if drains are added to thé System Filter length : 4 m 1,07 1,12 1,16 1,35 1,55 1,64 1,91 1,93 Filter length : 9 m 1,06 1,09 1,14 1,29 1,43 1,49 1,66 1.60 Thé tables mentioned above are adapted to filters of hydraulic conductivity 10' 2 m/s. However, hydraulic Z c corrective coefficient 1 m 0,60 4m 0,35 9 m 0,25 Figure 11 -Example of table for thé estimation of thé capture zone for a filter permeability of 10' 3 m/s, comparée! to a filter of 10' 2 m/s Thé tables allow thé designer to evaluate thé possibilities for contaminated water to move around thé reactive barrier, for example in thé event of filters clogging. © Multi-qate System design haïf barrier length) : • If Zc/n < L (no overlapping) : In this case, thé multi-gate PRB design is : H h Z c (<nL) H h L L L L conductivities of filters Filter length Figure 12a -

c /n. Let L be thé half-length of barrier. Then, thé designer compares Zc/n to L (mono-gate

Acknowledgments

Funding for this research was provided by thé French agency ADEME (Agence de l'Environnement et de la Maîtrise de l'Energie). Thé authors would like to thank particularly Mrs Petra Bajeat, for her technical input throughout thé project.


Annelise Emonet

Michel A Bues

Constantin Oltean

Olivier Bour

Pierre Le Thiez

M A B Uès

C Oitean

Bour

Experimental and numerical studies of trichloroethylene migration

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Chlorinated solvents represent one category of thé most widespread pollutants in soils and aquifers. Among them, thé Tricholoroethylene (TCE) is a DNAPL [Pankow and Cherry, 1996]. Its industrial use as a dry cleaner leads to contaminate plenty of industrial sites and justifies a spécifie study on its behaviour. In porous média, TCE can be found under différent forms: pure, volatile and dissolved in water and adsorbed on thé solid matrix. Each form is a risk of pollution for soils and aquifers, at long and short terms. In order to focus our study on thé phenomena of volatilisation and dissolution, we will not take into account thé adsorbed form of thé pollutant. In fact, we try to understand and explain thé behaviour of TCE in a soil, in order to détermine and apply thé most appropriate remediation technique for a contaminated site. Thé migration of a pollutant through a porous médium is controlled by its physical and chemical parameters but aiso by thé characteristics of thé porous médium. For example, thèse characteristics can be thé capillary pressure, hydraulic conductivity, effective porosity ... Moreover, thé behaviour of TCE can be différent whether it is in thé unsaturated zone (at irreducibie water saturation) where itc an volatilise in contact with gas phase or in thé saturated zone, where it can dissolve in water, Several studies realised since 1980 [START_REF] Abriola | Modeling multiphase migration of organic chemicals in groundwater Systems. A review and assessment[END_REF][START_REF] Miller | Dissolution of trapped non-aqueous phase liquids : mass transfer characteristics[END_REF]Powers et al., 1991;[START_REF] Powers | An expérimental investigation of non-aqueous phase liquid dissolution in saturated subsurface Systems : steady state mass transfer rates[END_REF]Geller and Hunt, 1993 and Powers et al., 19 94]ha ve been interested in mass transfer during dissolution. Other authors [SIeep and Sykes, 1989;[START_REF] Griffol | Contaminant migration in thé unsaturated soil zone: thé effect of rainfall and evapotranspiration[END_REF][START_REF] Cho | Simulating thé volatilisation of solvents in unsaturated soils during laboratory and field infiltration experiments[END_REF]Jellali et al., 2001] hâve focused on thé volatilisation of NAPL and especially on vapourm ovements and on thé influence of a water infiltration on their dissolution. This study tries to understand and measure thé process ofv olatilisation and dissolution ofa DNAPL injected in a homogeneous porous médium. Thé aims oft his study are:

(i) to dimension, using thé numerical results, a 2D expérimental set-up allowing to follow thé migration ofT CE in a variably water saturated sand, (H) to compare thé expérimental and numerical results obtained during an injection of TCE.

MATER1AL AND METHODS

Simulations with SIMUSCOPP

SIMUSCOPP is a numerical modelling software package developed by the Petroleum French Institute (IFP). It permits to détermine thé fluid velocities, pressures and concentrations of chemical species in multiphase flow by solving a set of coupled flow and transport équations [START_REF] Tardy | A numerical model of hydrocarbon contaminant flow in soil and aquifers. Development and Application of Computer Techniques to Environmental Studies VI[END_REF]. Thé chemical species can be found in one, two or three fluid phases. Thé mass transfer between chemical species can be taken into account or not. In our study, thé chemical species is represented by thé trichloroethylene (TCE). In a first time, thé numerical code was used in order to détermine thé optimal dimensions of a laboratory model represented by a column. Then, it was used in order to simulate an experiment involving thé injection, in thé upper part of thé expérimental set-up, of 400 ml of TCE for2 hours. Thé change of thé concentration of vapours and dissolved TCE are followed for one day. Thé comparison between numerical and expérimental data is presented in thé last section.

Experimentalse t-up

Thé expérimental set-up (figure 1) is represented by a glass column. Its optimal dimensions, chosen according to numerical simulations, are: 0.2m in diameter and 1.0m inl ength. Thé columni s progressively filled with water (each time 6 cm of water are maintained above thé ground surface) and sand is added by 800 g fractions. Thé column is struck several times with a hammer to allow thé optimisation of grain sand distribution and to avoid compressive effects between each sand level. Once thé column is completely filled with water and sand, it is drained by its base. Thé drainage is stopped when water level is about 0.2 m. So, thé saturated zone is located between 0.0 and 0.2 m, thé capillary fringe between 0.2 m and 0.5 m approximately and finally, thé unsaturated zone between 0.5 m and 1.0 m. We note that thé main characteristics of thé sand, ;'.e -, hydraulic conductivity, effective porosity, capillary pressure and relative permeability were experimentally determined. Thèse experiments are presented in thé third part oft he paper. Thé column is equipped with 3 pressure gauges and 1 température probe. In ordert o allow sampling of gas and water with syringes, 18 holes are located on thé column's side. Thé syringe dimensions permitted sampling at 5.0 cm inside thé sand. Thé samples, mixed with 1 mL of Methanol, which reacts as a trap for pure, volatile and dissolved TCE, are analysed by a gas chromatograph equipped with a flame ionisation détecter (FID) and a capillary column. Thé gases used in thé chromatograph are H z , N 2 and air. Thé flow rate of N 2 inside thé capillary column is about 5 mL/min and its température is constant and fixed at 70° C. Thé températures of FID and injecter are respectively 250 and 200° C.

CHARACTERISATION OF THE POROUS MEDIUM

One sand has been used in our experiments. It is coming from a river located in eastern France. Mainly composed of quartz, thé properties of this sand are presented in Table 1. Thé capillary pressure -water saturation or rétention curve is one of thé most significant data for SIMUSCOPP package. Consequently, it has been determined using an expérimental device (Figure 2). Represented by a column of dimensions: 0.55 m in length and 0.09 m in diameter, this device is made up of a stack of 8 plexiglas cylinders (9.0 cm in diameter and 7.0 cm in height). Thé column is completely filled with water and thé sand isad ded by fractionso f 400 g. Thé optimal grain sand distribution is obtained in thé same way as for thé expérimental set-up. Once thé column is completely filled with water and sand, iti s drained by its base until thé imposed water level is reached.

. ; r In order to measure thé pressure évolution in thé porous médium, a tensiometer probe is attached to each cylinder. One of its end isbu ried into thé porousm edium and thé other one isli nked to a pressure gauge. At thé end of thé experiment, thé sand is sampled and water saturation is measured.

Consequently, for each level corresponding to tensiometerp robes, thé stabilised recorded pressure in relation to water saturation can be graphically represented (Figure 3). Six experimentsha ve been carried outan d ail expérimental points are plotted on Figure 3. Thé interprétation of this figure permits to obtain Van Genuchten model parameters, /.e., a and n [START_REF] Van Genuchten | A closed-form équation for predicting thé hydraulic conductivity of unsaturated soils[END_REF] that are listed in table 2.

COMPARISON 0F NUMERICAL AND EXPERIMENTAL RESULTS

Thé expérimental data are numericaliy analysed using SIMUSCOPP package. Thé simulated domain, similar to thé expérimental set-up, is divided into 400 bilinear regular éléments of dimensions: 0.005 m in X direction (radius) and 0.01 m in Z direction (length). As in expérimental data, 400 ml of TCE are injected for2 hours. Thé pollutant injection is realised below thé upper limit of thé domain into an area of 1.5 cm 2 corresponding to 3 elementsi n X direction and one élément in Z direction. Thé TCE migration is followed for one day. Thé comparison between thé expérimental and numerical results is presented by a spatial and temporal concentration évolution ofv apours and dissolved TCE.

Evolution of TCE vapours

Indicated times on thé figures correspond, as far as thé experiment is concerned, at thé time of thé first sample taken of a sampling séries. For thé simulation it is linked to thé state of thé mode! at this spécifie time. It takes approximately halfan hour to sample an entire séries. On thèse graphs, thé squares represent thé mean value of at least 4 expérimental values. Thèse experiments hâve been conducted according to thé same procédure. One uncertainty, due to sampling and analyse, is associated with one experiment. Thé envelope curves around thé mean expérimental value resuit from thé mean of thé overvaluation (or thé undervaluation) of each expérimental value by its uncertainty. It gives yellow triangles for thé high trend and blue triangles for thé low trend. Lozengesr epresent numerical results. Thé blue ones are related to thé simulation that takes into account thé sand properties: especially thé hydraulic conductivity of about 15 10' 4 m/s. Thé red lozenges correspond to a simulation realised with a hydraulic conductivity of 75 10' 4 m/s. Thèse hydraulic conductivities are respectively assimilated to thé intrinsic permeabilities: 15 Darcy and 75 Darcy.

Wec an notice (Figure 4) thé increasing évolution of thé concentration of TCE vapours, for thé experiment as well as fort he simulations, in thé column with time. This évolution of vapours is related to thé migration ofpu re TCE in thé column. Thé évolutions of vapours concentration, given by thé simulations, are différent at thé beginning. Thé blue curve that corresponds to thé simulation with thé intrinsic permeability experimentally determined présents a slower progression compared with thé other and with experiments. On thé otherha nd, thé red curve that has been realised with anotheri ntrinsic permeability (75 Darcy), has an évolution faster and doser to expérimental results. Moreover, for simulations, once thé maximal concentration of TCE vapours is reached, thé unsaturated zone of thé column is completely saturated by vapours. After 3 hours, one can notice thé decreasing of vapeurs concentrations in thé upper part of thé column. This phenomenon reproduces thé expérimental trend that can be seen at each time, for thé upper level.

Although thé numerical results are quickly stabilised in thé column, this equilibrium state is not so évident for thé expérimental ones.

Evolution of dissolved TCE

Another graphie représentation has been adopted to illustrate thé évolution of dissolved TCE in thé water of thé column. For three levels situated at z= 35, 25 and 15 cm from thé base of thé column thé évolution of dissolved TCE concentration isf ollowed versust ime. Asf or as thé évolution of TCE vapours, blue circles correspond to thé mean of 4 ta 6 expérimental results. Two trends are defined considering thé mean of thé addition or thé removal of thé uncertainty related to each experiment (yellow and blue triangles).

Thé numerical results are represented with lozenges: blue ones for thé simulation with an intrinsic permeability of15 Darcy and red ones for thé simulation with intrinsic permeability of75 Darcy.

TOf solubility

Z=35 cm Vapour concentration (ppm v/v) For each level, thé concentration of dissolved TCE in thé water of thé column increases with time. Thé maximum concentration reached corresponds to thé solubility ofT CE. For thé experiment, at level z=35 cm, this value is quickly reached (3 hours) and even exceeded it. This is due to thé fact that during water sampling, some TCE can be taken with water in thé syringe and be analysed. For thé other ievels, concentrations evolve more slowly. For level z=15 cm, thé maximum concentration is reached after 14 hours. Thé numerical results are différent in thé first time of thé simulation. Thé simulation with a higher intrinsic permeability reaches thé maximum concentration faster (less than 2 hours), for each level. For thé other (intrinsic permeability of 15 Darcy), it takes more time (6 hours) to be completely saturated.

And one can notice that thé expérimental increase of concentration for each level is between thé two simulations.

Discussion

Thé fact that an equilibrium state is not reached for thé vapours and that concentrations in vapours decrease in thé upperpa rt of thé column may be explained by losses of vapours into thé atmosphère.

Anothere xplanation can be thé évolution (increasing and decreasing of 1°C) of the température in thé laboratory. Indeed, thé physico-chemical properties of thé TCE and especially thé tension vapour pressure, are very reactive according to température. A slight modification of température may lead to an increase or decrease of the maximum vapour concentration and may aiso explain thé decreasing of concentration between 4 to 14 hours and thé increasing trend around 24 hours.

Thé expérimental évolution of TCE vapours as well as dissolved TCE follows thé same trend as thé numerical results. However, thé simulation with an intrinsic permeability of 75 Darcy seems to be doser to thèse expérimental évolutions (especially for thé vapours) than thé other simulation, whose data correspond to thé experiments. This may be explained, as it was evocated by Pankow and Cherry [1996], by thé fact that thé hydraulic conductivity is experimentally determined for water in a water saturated porous médium and not according to thé physical properties of thé TCE (density and dynamic viscosity). Thèse properties may increase thé TCE speed migration in thé porous médium.

That can be a reason why thé simulation with a higher intrinsic permeability fits better expérimental results than thé other one.

CONCLUSION

This study présents an expérimental device devoted to thé study of thé migration of a DNAPL in a homogeneous porous médium. This set-up allows to follow thé volatilisation and dissolution of TCE in thé air or water of thé column. Thèse phenomena can be numerically reproduced with SIMUSCOPP. Thé évolutions of thé expérimental and numerical results for each phase are similar. But, thé simulation that seems to fit better thé expérimental results, takes into account a higher intrinsic permeability than thé expérimental value. That brings into relief thé fact that thé assimilation of thé hydraulic conductivity with thé intrinsic permeability for a porous médium may be used carefully by taking into account thé differentf luids involved. Another noticing results are thé development of a sampling procédure for différent form of TCE and their analyses by gas chromatography. Thé method that has been adjusted for thèse experiments allows to: (i) trap vapours of TCE and dissolved TCE in a same solvent (methanol) and (ii) détermine thé level ofc oncentration of TCE in each phase. This expérimental device and thé analytical procédure will be used with différent porous média and pollutants. After those studies, heterogeneities might be introduced. Another application of this expérimental set-up may be thé test of différent techniques of remediation (injection of surfactant in thé médium,i njection ofa ir...). Thé numerical code is aiready used at thé site scale in order to evaluate thé migration of a pollutant and dimension remediation techniques.

Figure 1 :

 1 Figure 1: Experimentalse t-up of TCE injection.

Figure 2 :

 2 Figure 2: Expérimental set-up for détermination of rétention curve.

Figure 3 :

 3 Figure 3: Rétention curve for thé sand.

  Figure 4: Calculated and expérimentale volution of dissolved TCE concentration.

Figure 5 :

 5 Figure 5: Calculated and experimental evolution of TCE vapours concentration.

Table 1 : Main characteristics of the sand. Property Value

 1

	Mean grain diameter (mm)	0.56
	Minimal density (g/cm3)	1.43
	Maximald ensity (g/cm3)	1.63
	Grain density (g/cm3)	2.635
	Hydraulic conductivity(m/s)	1.50E-04

Table 2 : Van Genuchten parameters for thé sand.

 2

	m
	0.70	3.36	0,084
	They enable to estimate thé relative permeabilities for each two-phase fluid couple in a three-phase
	System [Van Genuchten,19 80;P arker eta /., 1987].
	Thèse expérimental and analytical data hâve been implemented in SIMUSCOPP.


Introduction

For the last ten years and more, INERIS has acquired a practical experience on hydrogen safety. Among others, the following experimental topics have been dealt with : confined hydrogen explosion, liquid hydrogen leak and subsequent dispersion in air phenomenon, hydrogen jet release,d ispersion and inflammation, measurement of hydrogen concentration in air-hydrogen atmospheres, pressurised hydrogen tank testing.

In parallel risk assessment have been carried out on hydrogen-related equipment (reformers, electrolysers ... ).

This long lasting experience is now invested in hydrogen technologies. In this field, INERIS is involved in several European and national projects as well as in standardisation working groups focussing on the safe handling of hydrogen 2 New energy 2.1 Hydrogen as a new "energy carrier" ?

Hydrogen based economy and associated energy converters fuel cell systems are said to be a part of the response to increasingly worrying ecological and economical issues embodied by either global warming or the depletion of fossil energy sources.

Indeed, hydrogen shows many benefits in the field of air pollutant as its combustion reduces pollutant and greenhouse gas emissions (carbon dioxide...) 1 . Besides, energy conversion yields seem to be better and finally it allows fossil fuels dependant economy to rely on other energy loops. Hydrogen can virtually be produced from a wide variety of domestic sources. It can also provide a storage medium for intermittent and seasonal peak energy needs.

Hydrogen can be used as an energy carrier for many traditional technologies such as cars (direct combustion engines using hydrogen or hydrogen based mixtures, fuel cell systems), electrical plants, systems to provide heat and electricity for buildings, remote power unit systems, backup systems... . Hydrogen technologies are under test across Europe (stationary power units, boats, city buses, cars...). Different challenges are being faced : technological, hydrogen production, hydrogen storage,... . Along with public acceptance, authorities agreement for demonstration projects is sometimes difficult to obtain for these new technologies (imported technologies are not CE marked, no straight forward applicable regulation...). Regulation and standardisation will therefore have to make their way to ease hydrogen technologies implementation.

To answer some of potential public fears, this paper will compare hydrogen hazards with more conventional fuels. We will then discuss hazards related to fuel cell systems. All together, we will try to evaluate if there is any new hazard to be expected along with hydrogen technologies development. Finally, we will present existing regulations and standardisation expectations prior to briefly discuss social acceptance aspects.

What is a fuel cell ?

Fuel cells are the core of the hydrogen economy. A fuel cell is an electrochemical energy converter in which hydrogen is combined with oxygen (from air supplied to the system) to produce water and electricity, emitting neither pollutants nor excessive noise. The expected theoretical efficiencies are varying according to the technology used but are higher than those of conventional combustion processes. This means that in the long term,fuel cells will probably be an alternative energy converter.

Any fuel containing an atom of hydrogen (methanol, gasoline, natural gas, biogas, and so on) can be used as fuel in a cell.

Different types of fuel cell have been develop d so far. All of them can run with hydrogen. Proton exchange membrane fuel cells (PEMFC) are more than any other technologies expected to be commonly found (cars,h ome generator..).

Figure 1 gives a schematic view of flows within a fuel cell as well as its working conditions.

Figure l.S chematic view offlows and working conditions ofa fuel cell.

Hydrogen is supplied either from a storage system where it may be in the form ofa cryogenic liquid, hydrides or compressed gas, or from a production system within the installation itself: for example by electrolysis of water, hydrocarbon cracking,v apour reforming of methanol, and so on. The operating principle is simple: the cell consists of an assembly of elementary units each constituted by a cathode chamber and an anode chamber separated by two electrodes that channel the electrons, with an intermediate electrolyte which may be solid or liquid according to the type of cell. The atom of hydrogen, under the effect of a catalyst, breaks down into a proton and an electron. Protons enter the electrolyte, while the electrons are collected and passed through the external electric circuit before returning to the cathode, where they recombine with protons and oxygen to form a molecule of water. Besides water (by-product of the reaction), electric current and heat are collected at the output, in varying proportions according to the chosen operating point, which is particularly valuable in fixed combined generation applications.

We have seen that hydrogen goes along with fuel cells. In the following paragraphs we will try to identify specific safety issues related to its use in a gaseous form.

General considerations

Gaseous hydrogen is a highly flammable, colourless, odourless and non toxic gas. It is the lightest of all gases.

The following table 1 offers a comparison of hydrogen safety characteristics with the ones ofp ropane, methane and gasoline. We will now discuss these safety characteristics in the light of accidental phenomenon.

Hydrogen dispersion

In similar release conditions (diameter and pressure), hydrogen is expected to mix with air and disperse more rapidly than other fuels, driving down more quickly its concentration in air in unconfmed space. Its low molecular weight drives this property : higher jet speed and higher diffusion coefficient. sources : "Investigation on safety, regulations and acceptability ofhydrogen", Euro-Quebec Hydro-Hydrogen Pilot Project (EQHHPP), November 1993.

C.E. Thomas, "Direct Hydrogen fueled proton exchange membrane fuel cell system for transportation applications -Hydrogen Vehicle Safety Report", Direct Technologies Inc., prepared by Ford Motor Company for DOE, May 1997.

A source : J.L. Alcock, "Compilation of existing safety data on hydrogen and comparative fuels", EIHP 2 Project, Shell Global Solutions, May 2001.

On the other hand, its higher leak flow (3 times the volumetric one of propane or methane in turbulent conditions) 5 ' its diffusion in air ability (4 times the one of propane or methane) and its buoyancy tends to make hydrogen more hazardous in confined spaces than other fuels.

Flammability

Lower flammability limit is a key safety factor for vapour and gases knowing how easy it is to ignite an explosive atmosphere. Hydrogen MIE is 10 times lower than conventional fuels (17uJ 6 ). At stoechiometric conditions the weakest static electricity discharge (brush discharge) is entitled to ignite the mixture. At LFL,M IE of methane is comparable to the one of hydrogen.

Experience shows that hydrogen air-mixture ignites most of the time 7 Ability to ignite makes hydrogen very large flammability range less of interest since we would expect it to ignite before reaching high concentration values. Ability to ignite would also tend to minimise explosive atmosphere volume and therefore associated effects. However, if hydrogen accumulates above LFL in a confined space, it is more likely to ignite.

Hydrogen auto-ignition temperature (858°K) is slightly higher than the one of other fuels. Autoignition is not a usual cause of hydrogen ignition8

Formation of an explosive atmosphere

We have seen that hydrogen disperses more rapidly in air. This propriety enhances formation of large explosive atmosphere in confined spaces. We will now consider high pressure leaks 9 in open spaces without obstacle.

The following table 3 compares explosive atmospheres volumes for different gases at different leaking pressure and diameter.

5 See note 3 One has to bear in mind that spark discharges of the human body has an energy level of about 10 mJ.

Reference : C.E. Thomas, "Direct Hydrogen fueled proton exchange membrane fuel cell system for transportation applications -Hydrogen Vehicle Safety Report", Direct Technologies Inc., prepared by Ford Motor Company for DOE, May 1997. The following observations can be made: at a given pressure, whatever the gaz, distance to LFL doubles when leaking diamater also doubles. At the same time, explosible mass or explosible volume are multiplied about 10 times, in any situations, methane leads to smaller explosive atmospheres volumes, in any situations, hydrogen leads to much larger explosible volumes, whatever the pressure, hydrogen leads to explosible volumes about 40 times higher than the ones with methane, at the same leaking pressure, hydrogen and propane lead to the same explosible mass. Explosible volume is somehow very different due to hydrogen low molecular weight. For this specific case, figures given by PHAST 6.1 (DNV) are half of those given by Explojet.

Explosible volume is given between LFL and UFL Reference : L.C. Cadwallader and J.S. Herring , "Safety issues with hydrogen as a vehicle fuel", Idaho National Engineering and Environmental Laboratory, September 1999.

Pressure effects

Damageable pressure effects accompany explosion phenomenon. Because of its speed of pressure rise hydrogen explosion induced pressure effects are expected to be more severe than for other gases even though volumetric energy release is lower than the one of other studied fuels. This quick evaluation shows that hydrogen has a different behaviour than conventional fuels. Hazardous or safe potential of hydrogen is to be revealed by the technology and the context of use.

Therefore, we now propose a risk evaluation overview of a PEMFC.

4 Risk analysis of Proton Exchange Membrane Fuel Cell systems

Critical events related to the core of the cell

Whatever technology is used, the core of the cell consists of an assembly of elementary units in which an electrochemical reaction takes place between a fuel (hydrogen) and an oxydant (usually oxygen from the air). The core of the cell remains unchanged over time. In non degraded mode, hydrogen and oxygen never come into direct contact.

Hydrogen leakage and release : formation of an explosive atmosphere

Leakage (accidental situation) can be distinguished from normal release. Normal release is caused by chronic purging 13 of the core (PEMFC). Purged hydrogen is then released into air. In confined spaces, one has to pay attention nott o progressively enrich the air with hydrogen until it reaches its LFL (high speed release and homogeneous mixing or stratification and further mixing phenomenon). In unconfrned spaces no explosive atmosphere is to be expected unless at the point of hydrogen release. The volume of this explosive atmosphere is driven by the flow and the release diameter at operating pressure. Premixing below LFL prior to release is a possible safety solution.

Along with chronic purging, chronic leakage is to be evaluated. Indeed, hydrogen diffusion ability makes it difficult to confine. As a consequence, minor chronic leaks have to be expected.

Hydrogen accidental leakage in PEMFC encompasses a wide range of situations:

purge malfunction : purging becomes permanent leading to more hydrogen to be released into the outside air, membrane puncture can lead to an equivalent situation as well as to form an explosible mixture within the stack. This latest situation is not critical mainly because of reduced volume and the presence of catalyst that would favour moderated hydrogen combustion, core, manifold and piping leakage, reverse electrolysis of the cells... .

In PEMFC, membrane rupture is a noticeable situation with the potential to lead to large leaks. Membrane rupture can be induced by : inappropriate pressure balance between each side of the membrane side, excessive local temperature (lack of humidification, inhomogeneous or insufficient cooling ...), membrane ageing.

Hydrogen purging drives away excessive nitrogen (nitrogen from the air side permeates through the membrane to the hydrogen side) and water on the cathode side.

Any of the above situations can lead to the formation of local (open spaces) and extensive explosive atmosphere (confined spaces). Severity of the above situations depends on the studied fuel cell. Possible consequences whatever minor or severe the leak is have to be quantified. As such, for each system, one should put figures on : the maximum leakage rate and associated maximum explosive volume, chronic purge induced explosive atmosphere in open air and ventilated confined space, the chronic leakage flow of hydrogen.

Electrical hazards

Beside hydrogen leaks and associated explosive atmosphere, electrical hazard is a prevalent risk due to the presence of high voltage.

Basic safety principles

General safety principles

Enforcement of safety principles requires first of all to avoid the hazard (replacement), then to reduce/control its potential and finally to set safety distances with potential targets. Users should also be informed about risks of the equipment.

Control of hydrogen risk

Above principles apply to fuel cell systems. Control of hydrogen hazards goes through:

• reduction/avoidance of chronic release of hydrogen by appropriate design, • minimising the size of explosive atmosphere in case of leakage or release (operating pressure, pipe size, maximum hydrogen flow14 ...), • avoidance / limitation of confinement in design and use of the system (open space or controlled ventilation), • control of ignition sources (appropriate electrical equipment, physical segregation between electrical and hydrogen parts,...), • early detection of leak and leak interruption before reaching a hazardous situation, • enforcement of state of the art technical safety principles (redundancy of critical equipment, fail safe system,...) • regular checks and maintenance, • end-user information.

Complete control of hydrogen leakage is not realistic. However driving down leakage situation to acceptable level of risk should be the main objective.

Under construction IEC standards (IEC TC 105) suggest among others leaking tests to be undertaken.

This brief evaluation shows that this new technology brings along risks that can be overcome with traditional approaches and safety measures. systems It goes without saying that since fuel cell systems are made up of a variety of equipment, such as compressor, storage tanks, electrical systems,... that many existing regulations and standards can be applied.

Specific regulations and standards are under construction in Europe and at the international level. On legal and standardisation point of view, stationary applications have to be distinguished from mobile applications.

Standardisation

An ISO 15 technical committee -TC 197 -"Hydrogen Technologies" was set up in 1990 to prepare standards for systems and devices involved in producing, storing, transporting, measuring and using hydrogen. Its 7 th working group has prepared a standard on the "Basic requirements for the safety of hydrogen systems" An IEC16 technical committee -TC 105 -"Technologies of fuel cells" was set up in 1996 to prepare standards for fuel cell technologies for both stationary and mobile applications. Its 3 rd group focusses on the safety of stationnary fuel cell systems.

In North America, organisations such as the National Hydrogen Association (NHA), the Canadian Hydrogen Association, the National Fire Protection Association (NFPA), the American National Standards Institute (ANSI), and the American Society for Mechanical Engineers (ASME) are working to draw up codes,s tandards and guides concerning the construction and use of fuel cell systems.

In Europe, there is no specific standard for stationary fuel cell systems. A technical committee has been created within the CEN 17 /CENELEC 18 and covers residential fuel cells with a power up to 70 kW.

Regulations

In Europe existing directives among others can be applied to stationnary fuel cell systems for CE marking.

Three of these directives appear to be applicable in every cases :

- To overcome this barrier, UNECE 20 rules relative to onboard hydrogen storage are under construction.H owever,t hese later rules do not cover the use of a fuel cell in a vehicle.

Meanwhile future dedicated regulations, a manufacturer can apply for a prototype approval of its vehicle.

Social acceptance of hydrogen

For lay people, the use of hydrogen induces hazardous meanings (explosion).

However, development of hydrogen use will not take off without the public (potential users) onboard.

Social brake is not a dead end. Studies conducted in Germany came to the conclusion that social ac-21 ceptance of hydrogen can be overcome by raising public awareness2 . This communication action could consist in:

1. offering to the public information about hydrogen technologies (through schools and the mass media) ; 2. developing public tests, pilot projects and demonstrations, and these should be accompanied with explanatory measures in order to introduce the information ; 3. advertising upon safety approaches ; 4.

stressing environmental advantages of hydrogen.

Conclusion

Hydrogen economy adresses some of today's environmental issues. As such, it is entitled to become one of tomorrow's new energy carrier. If so, hydrogen is supposed to offer the same services at an equivalent price as today's fuel with equivalent level of safety.

Comparative safety studies require not only to focus on the product itself, but also on supporting technologies as well as on context of use (confined environnement).

It is obvious that hydrogen has different properties than today's fuel. Some of them tend to make it safer (it disperses more rapidly in open space) and some other more hazardous (potentially strong pressure effects) in foreseen context of use. However, it does not bring along risks than can be overcome with traditional approaches and safety measures.

Fuel cell systems do not appear to put public safety at stakes. High pressure storage could be more critical and require special attention from industrial, standardisation and regulatory bodies.

Hydrogen technologies developers seem to have learned from other energy carriers errors in the sense that safety studies are intrinsequely part of the development. On going standardisation work pushes towards an appropriate and homogeneous level of safety in order to avoid any preliminary accident that could geopardise hydrogen future use in everyday life. However, the use of hydrogen will inevitably lead to accidental situation as other fuels would.

Thus, in comparing hydrogen with current energy technologies, one should not be blind on hazards related to the use of gasoline for instance. Gasoline has not the same propension as hydrogen to 20 United Nations Economic Commission for Europe 21 Ludwig-Bôlkow-Systemtechnik GmbH and Ludwig-Maximilians-Universitàt Miinchen, "The acceptance of hydrogen technologies ", http ://www. hydrogen. org/accepth2/. explode but it takes fire. Statistics show that gazoline fire sets by car accidents counts for 1%22 of fatalities on the road. Would hydrogen be responsible for 1% fatalities in case of car accidents ? One has to come to turn with the reality of today's fuel in their context of use. Comparative studies should pay attention not to underestimate risks of usual fuels because of their everyday usage and overestimate risks of hydrogen. Hydrogen requires to be better known through experimental / demonstration projects.

Still, acceptance relies on expected users : the public. Pragmatic safety studies are one thing public acceptance is another one. If hydrogen is an appropriate solution, it has to take the public onboard to erase the fear of the unknown.

Information given by Laboratoire d'Accidentologie et de Biomécanique PSA Peugeot Citroen Renault -France

Table 1 .

 1 Comparison ofhydrogen safety characteristics with more conventional fuels ..

	Property	Unit	Hydrogen	Methane	Propane	Gasoline
	Molecular weight	kg/kmol	2.016	16.043	44.10
	Heat of combustion (high)	kJ/g	141.86	55.53	50.41
	Limits of flamability in air
	(LFL -UFL) Downward	vol%	4.0-75.0	5.3-15.0	2.1-9.5	1-7.8
	propagating
	Limits of detonability in air (LDL-UDL)	vol%	18.3-59.0	6.3-13.5	3.1-7 4	1.1-3.3
	Stochiometric composition in air	vol%	29.53	9.48	4.03	1.8
	Minimum Ignition Energy in air (MIE)	ml	0.02	0.29	0.26	0.24
	Maximal Experimental Safety Gap at NTP (MESG)	cm	0.008	0.12	NA	0.074
	Auto-Ignition temperature	°K	858	813	760	228-471
	Buoyency	W/r to air	0.07	0.55	1.52	3.4-4.0
	Diffusion coefficient in NTP-air	cm 2 /s	0.61	0.16	0.12	0.12
	Diffusion velocity in NTP-air	cm/s	<2.0	<0.51	N.A.
	Burning velocity in NTP air Energy of explosion NTP	cm/s g TNT/m 3	265-325 2.02	37-45 7.03	N.A. 20.3	0.42 44.22
	Toxicity		non toxic (asphyxiant)	non toxic (asphyxiant)	non toxic (asphyxiant)	toxic

Table 3 .

 3 Comparison of explosive volumes at similar leaking conditions for hydrogen, methane & propane (Explojet/INERISsoftware).

	Pressure
		Distance t	0
		Ï.FL (m)
	10 bar	Flammable mass of fuel (kg)
		Explosible vol-
		ume (m 3 ) 11
		Distance to
		LFL (m)
	250 bar

Table 4 .

 4 Explosible volumes of hydrogen / air formed for different leaking diameters at 700 bar (Explojet/INERIS software).Hydrogen burning velocity is far greater than the one of other fuels. Generally speaking, likelihood to transit from deflagration (DDT) to detonation regime is in relation with this burning velocity.However, past accidents have shown that few DDT have been observed. In open spaces, deflagration is the combustion regime to be expected unless a very energetic source (explosives) ignites the detonable mixture 12 .

			Leak diameter (mm)
	Pressure (bar)		1	2	4
		Distance to LFL (m)	9,1	18,2	36,4
	700	Flammable mass of fuel (kg)	0,0318	0,255	2,04
		Explosible volume (m 3 )	6,39	51,1	409
	Combustion regime
	10
	11
	12

  Its worth indicating that ATEX 94/9/EC is applied as part of the machine directive expectations

	Machine Directive 98/37/EC 19
	Low voltage Directive 73/23/EEC
	-Electromagnetic Compatibility Directive 89/336/EEC, 92/3 I/EC, 98/13/EC.
	Today, there is no regulation, neither for fuel cell vehicles nor for mobile hydrogen storage.
	European Directive 70/156/CE "type-approval of motor vehicles and their trailers" does not include
	hydrogen vehicle type-approval.
	ISO: International Standards Organisation
	16 IEC: International Electrotechnical Commission
	17 CEN : Comité européen de normalisation
	1 8 CENELEC : Comité européen de normalisation électronique
	19

Source : R. GALLEY & C. GATIGNOL, "Les perspectives offertes par la technologie de la pile à combustible", Report of the "Office Parlementaire des choix scientifiques et technologiques", France, p.

30,2 001.

See note 7.Current hydrogen mobile technologies favour high pressure storage of hydrogen.

Maximum hydrogen flow is to be balanced with maximum fuel cell stack hydrogen consumption.


D Hourtolou

O Salvi

ARAMIS Project: development of an integrated Accidental Risk Assessment Methodology for Industries in the framework of SEVESO II directive

The ESREL conference welcomes a special session on ARAMIS European project. This session represents halfway workshop of the project, which started in January 2002. The aim is to disseminate first results and collect comments from the public. This article is one of the five papers constituting the session and it presents the overall frame of ARAMIS project. ARAMIS objective is to build up a new integrated risk assessment method that will be used as a supportive tool to speed up the harmonized implementation of SEVESO II Directive. The proposed method results in an integrated risk index composed itself of three independent indexes. Index 1 assesses the consequence severity of first defined reference scenarios. Index 2 evaluates Safety Management effectiveness and accounts thus for the scenario probability. Index 3 estimates the environment vulnerability. Efforts have been made to disseminate work progress and results from the start. A dedicated w eb-site h as b een c reated a nd a r eview c ommittee g athering i ndustry r isk e xperts a nd d ecisionmakers from EU competent authorities periodically monitors the project.

INTRODUCTION

ARAMIS project was accepted for funding in February 2001 by the European Commission, in the 5 th Framework Programme for Research and Technological Development, in the field of "Energy, Environment and Sustainable Development", chapter entitled "Fight against major natural and technological hazards". This three-year project started in January 2002. The project has been built in particular on the conclusions and results of ASSURANCE and I-RISK, two other European projects funded in the 4 th Framework Programme.

ASSURANCE stands for ASSessment of the Uncertainties i n R isk A nalysis o f C hemical Establishments. This project was a benchmark exercise, which aimed at improving the understanding of the sources and types of the uncertainties connected with risk analyses. As a rough conclusion, the project stated that the benchmark exercise revealed noteworthy variation in the final results. Discrepancies were p resent b oth i n t he a ssessment o f frequencies and in the assessment of consequences. The different results would have obviously affected the relevant risk-informed decisions, mainly land use planning, emergency planning and acceptability of risk.

The initial statement of I-RISK project -I for integrated-was the idea that Quantitative Risk Assessments (QRA) and safety management audits were so far two separate tools and that would be valuable to integrate both to address major hazard management. In this respect, the main objective was first to develop a management model for risk control and monitoring, then to implement this model into a dynamic QRA tool. Conclusions of the project point out that the integrated technical and management model was very robust and helped audit organizations in a new way. However it also turned out that a full-scale site integrated QRA was too time and detail demanding, so not currently practical or relevant.

From both projects, but also from everyone's experience in his own country, it emerges the need for a methodology giving consistent rules to identify accident scenarios and taking into account both prevention and mitigation measures peculiar to each plant operator. Those safety measures are obviously controlled in a safety management system.

There is also an underlying need for a risk assessment method that could reach a consensus amongst risk experts from b oth Industry and C ompetent Authorities and then be used with reduced uncertainty to make risk-informed decisions. The ARAMIS project has been set up to propose solutions to both latter requirements.

The paper first recalls the context of major accident hazards prevention in the EU. ARAMIS overall objectives are then expressed and the work contents are described in details. As a conclusion, the expected impacts of such a methodology are addressed.

The annual report from the European Environment Agency (1999) indicates, among others, that the trend in notified accidents has been constant over the last twenty years. This statement alone shows that many of the often seemingly simple "lessons learned" from accidents have not been yet enough implemented in industry's standards. There is unfortunately no doubt that disasters will continue to occur throughout the EU. Some will be due to technology, some to the hazards of nature. The problem of low-probability, high-consequence accidents is likely to remain a key issue in terms of risk management. Nevertheless, hazards have to be managed and risks can be reduced.

The most significant EU Directive to help protect people and the environment from major accident hazards is the SEVESO II Directive. This applies to industries that use "significant amounts of hazardous substances". Their operators must demonstrate in particular they apply a major accident prevention policy and they implemented appropriate prevention and mitigation measures controlled and monitored in a safety management system.

The SEVESO II Directive sets very clear objectives relating to major hazard management, but the remaining question is: how to reach them and to control they are reached? For instance, there is no harmonized definition of the scenarios to be considered for risk assessment. The ASSURANCE project showed in this way that the chosen scenarios (BLEVE, full bore rupture or small leakage, amount of substance caught in an explosion, etc.) are different according to the experts judgement and experience, and according to the deterministic or riskbased approach of the Member State applying the Directive. Moreover, constraints in land-use planning [START_REF] Cassidy | Special issue : The SEVESO II Directive (96/82/EC) on the control of major accident hazards involving dangerous substances[END_REF] sometimes urge the operators to consider reduction of the safety zones by choosing "realistic" scenarios and accounting for the effectiveness of dedicated safety devices. Actually the lack of rules for identifying scenarios and carrying out risk assessment makes often the expert's job tricky and largely too subjective to base transparent risk-informed decisions on it.

In addition to uncertainties in risk analyses, differences of culture among the Member States result in a multiplicity of methods and approaches [START_REF] Kirchsteiger | Special Issue on International Trends in Major Accidents and Activities by the European Commission towards Accident Prevention[END_REF]. At a recent JRC International Workshop [START_REF] Kirchsteiger | Promotion of technical harmonization on risk-based decision-making[END_REF], most participants agreed that comparative risk assessment along harmonized procedures would significantly help the decision understanding. A harmonized risk assessment methodology would thus ensure that risk-based decision making provides the necessary transparency and the right balance between scientific understanding and principle of precaution.

Proposing a harmonized methodology for risk assessment is a tough and tricky task. However, deterministic and probabilistic approaches should not be opposed since they are often complementary [START_REF] Libmann | MIRIAM: an integrated approach to organize major risk control in hazardous chemical establishments[END_REF]. From a historical point of view, deterministic methods first allow to check the safe design of an installation. Probabilistic methods help then evaluate the residual risk of the installation. Both approaches have their strengths and weaknesses [START_REF] Hourtolou | ASSURANCE -Assessment of the Uncertainties in Risk ANalysis of Chemical Establishments[END_REF]. A first basic idea of ARAMIS is to take advantage of each approach's strengths and to develop upon it an alternative semi-quantitative method based on the evaluation of the safety barriers -lines of defense-peculiar to each site.

OBJECTIVES

The main objective of the ARAMIS project is to create a new integrated risk assessment methodology by combining the strengths of different methods currently used in European Countries. Accordingly, the method should be flexible enough to account for different national cultures like deterministic or riskbased approaches, in order to become a recommended tool used by risk experts and endorsed by risk decision makers in the whole EU.

The proposed method in ARAMIS should allow to characterize an integrated risk index composed itself of three distinct and independent indexes. Index 1 is to assess the consequence severity of first defined reference scenarios. Index 2 is to evaluate prevention management effectiveness, which allows thus to account for the reference scenarios probability in a semi-quantitative manner. Index 3 is to estimate the environment vulnerability by evaluating the sensitivity of potential targets located in the vicinity of a SEVESO plant. The project has been set up (Figure 1) to reflect the logical construction of the risk index and has been divided accordingly into work packages: 1 First goal is then to develop a method to identify "reference" accident scenarios. These scenarios are consensual "realistic" scenarios to be used in SEVESO II safety report and taking account of some prevention and mitigation measures of the site according to their effectiveness.

2 Second task is to build up the integrated risk index made up of the three distinct indexes, i.e.:

-consequence severity evaluation, -prevention management effectiveness, -environment vulnerability estimation.

DESCRIPTION OF WORK

The work plan of the project has been built according to the logical construction of the final resulting risk index and it has been presented the same way in this article.

Halfway through the life of the project, the newly built methodology is to be tested on three SEVESO industrial sites in Europe. At this stage, two new partners from eastern-European countries will join the consortium and will also test the full method as totally unbiased end-users, each in one additional test site of their own country.

Moreover and since ARAMIS is intended to be a supportive tool to promote harmonized risk assessments throughout Europe, the project leaves from the start an extensive part to large exchanges with potential end-users of the method. Identified endusers are both industrial companies and Competent Authorities in charge of enforcing SEVESO II Directive. For that purpose, a dedicated Work Package deals with valorization and dissemination of project progress and results. Industrial partners have also been directly included in the consortium and a Parallel Review Team gathering potential end-users has been constituted.

WP1 : Scenario identification

Identification of the possible accident scenarios is a key-point in risk assessment [START_REF] Lauridsen | The ASSURANCE project[END_REF]. However, especially in a deterministic approach, worst case scenarios are considered, often without taking into account existing safety devices and implemented safety policy. This approach can lead to an overestimation of the risk level and does not promote the implementation of safety systems.

The aim of this Work Package (Delvosalle & al., 2003) is first to identify major accidents without considering safety systems. A second step is then to study in depth safety device effectiveness and safety management efficiency, which will allow to assessqualitative-probabilities, in order to identify finally Reference Accident Scenarios taking into account some of the implemented safety systems.

The first objective is to define a Methodology for the Identification of Major Accident Hazards (MI-MAH). On the basis of considered equipment and properties of handled chemicals, the methodology must be able to predict which major accidents are likely to occur. Properties of substances are found out thanks to Directive 67/548/EEC (substance clas-sification and labeling) and their own conditions of use (pressure, temperature, flow, etc.).

The work has been divided in several parts. Firstly, it was necessary to select a general approach. The bow-tie method was chosen [START_REF] Bellamy | Major Hazard Management : Technical-Management Links andtheAVRIM2 Method, in Seveso 2000 -Risk Management in the[END_REF] because it is a highly structured tool and it is considered as a very good way to establish links with other parts of the project and especially Management Efficiency (Figure 2). Secondly, a special effort was made to develop a common typology of equipment and hazardous substances. Thirdly, event trees and fault trees centered on critical events have been built, and above all a methodology able to build generic trees was created. Critical events are defined as "Loss of Containment" or "Loss of Physical Integrity" event.

PREVENTION

Safety Barriers MITIGATION

ORGANISATIONAL ACTIVITIES FOR BARRIER ASSURANCE

Figure 2. Bow-tie approach for scenario identification At this stage, the MEVIAH methodology is able to predict which major accidents are likely to occur on a given equipment. With the help of a deep study of safety systems, causes of accidents and a historical analysis of known accidents, the objective of the work to be done is now to place lines of defensesafety functions, safety barriers-on the different branches of the trees. This will lead to a second Methodology of Identification of Reference Accident Scenarios (MIRAS) which has to take into account some of the safety systems according to their effectiveness. Therefore, the Reference Accident Scenarios will use results from the work performed on the prevention management effectiveness (Figure 3). These scenarios are used afterwards as an input to evaluate the severity index, i.e. the hazard potential. The objective of this task is to define a severity index S characterizing the possible consequences of accident scenarios (Casal & al., 2003). hi this respect, only the physical characteristics of the phenomena involved in accidents are studied in order to evaluate the severity of both major scenarios and reference scenarios identified in WP1. First task of WP2 was the selection of the most suitable models for the calculation of the effects of the various dangerous accidental phenomena. Thus, a survey of the existing models for the calculation of the effects due to explosions (overpressure and missiles), fires (radiation), toxic clouds (concentration), BLEVE-Fireballs (overpressure, missiles and radiation), pollutant plumes into the water (concentration), soil pollution and domino effects is now available at this stage of the project.

The Severity Index must be independent of the other two indexes. It is thus constructed in such a way that every dangerous phenomenon has a corresponding specific sub-index. The contribution of each dangerous phenomenon to the global index S is strongly related to the probability of occurrence of the phenomenon associated to each critical event (e.g. probability of ignition) and identified in the WP1 event trees (Figure 2).

Each specific sub-index associated to the various physical phenomena takes into account in its construction the following parameters: -the effect area concerned with the phenomenon, e.g. a disc in case of an explosion, a plume sur-_ face for gas dispersion ; -the kinetic of the phenomenon: rapid for an explosion, much slower for a fire ; -the potential of generating domino effect: fragment emission, delayed phenomena triggered off.

The severity index S is therefore a function of parameters only associated with physical phenomena. All the identified scenarios should then be evaluated and ranked in this way according to the calculation of S o for Major Accident Hazards and S re f for Reference Accident Scenarios.

WP3 : Prevention management effectiveness

This work package deals with the assessment of safety management efficiency and its effect on the calculation of external risks for SEVESO plants [START_REF] Duijm | ARAMIS project : Assessing the effect of safety management efficiency on industrial risk[END_REF].

The methodology is based on the identification of initiating events and direct causes of the accident scenarios (bow-tie approach). Safety barriers are then related to generic fault and event trees representing all possible accident scenarios leading to critical events (Figure 2). The safety organization includes both the adequacy and completeness of technical and managerial barriers (lines of defense) that are implemented to prevent these accidents and the management system that ensures that these barriers are maintained and adjusted properly.

Major hazard Identification

Major accident hazards

Generic lines J of defense (LOD) "i

M sito -F ( M LOD1 • M LOD2 . M SMS> Figure 4. Structure of index M
The methodology recognizes a number of dimensions of safety management ( delivery s ystems), d erived from previous work on safety management modeling, notably the I-Risk [START_REF] Hale | An audit method for the modification of technical risk assessment with management weighting factors[END_REF] and MIRIAM (Plot, 2002) models. These are made explicit in specific functions that need to be executed to maintain a safety barrier. Examples of these delivery systems are: ensuring good competence and commitment of employees, manpower availability, communication, procedures, plans, hardware and human-machine interfaces.

Currently, the focus is on developing instruments to measure the set of dimensions, using a combination of audit, questionnaire, interview and o bservation techniques. The combination of measurements ensures that not only the implementation of functions, but also its conditions and outcome (e.g. good safety commitment of the employees) are taken into account.

The measurement techniques address in particular the specific safety functions (MLODI and MLOD2, Figure 4) found in a given establishment. However measurement will also be carried out in a generic way onsite (MSMS, Figure 4), assuming then the quality of the dimensions represents a common mode for the quality of safety barriers maintenance. The efficiency of the barriers can then be adjusted according to the measurement scores to select the final set of Reference Accident Scenarios.

The assessment of technical barriers effectiveness follows the principles described in the norms IEC61508 and IEC61511 (Functional safety : safety instrumented systems for the process sector). Among these principles, effectiveness is analyzed through the definition of "Safety Integrity Levels" linked to device characteristics (design, reliability, maintainability, testability...) and also through criteria upon the activities in charge to maintain them.

Challenges in the development of the methodology include the need to calibrate the. scores in the measurement of dimensions, as well as the need to determine the efficiency of technical barriers as a function of these scores.

WP4 : Environment vulnerability

An installation handling dangerous substances is hazardous only according to the potential of vulnerable targets liable to be affected. In assessing the overall risk level of a plant, it is therefore quite relevant to characterize the spatial vulnerability of the environment surrounding the plant. Such is the aim of this work package [START_REF] Tixier | ARAMIS project : Assessment of the environment vulnerability in the surroundings of an industrial site[END_REF].

Vulnerability of the surroundings depends on the features of the environment that are potential targets (human, environmental and material) and on the type of impact due to hazards (fire, explosion, and toxic release). It also strongly depends on the considered target area.

To address this issue, the area of interest in the vicinity of a plant has been first divided into meshes and the potential targets have been identified and localized with the support of Geographic Information Systems (GIS). The major difficulty is then to rank and prioritize the various sensitivities of the targets according to the various expected impacts.

A suitable solution has been found out by applying a multi-criteria ranking approach, such as SA-ATY methodology which allows to define priorities from complex situations. At this stage of the project, SAATY method has been extensively applied to the concern of vulnerability estimation.

First step was to describe and classify potential targets, hazards and impacts in adequate typologies. Following step involved expert judgement. Through experts' answers to specific questionnaires, SAATY method helped build up mathematically the generic coefficients of target vulnerability.

Final step will be to test and validate the calculated coefficients and resulting index through fullscale case studies.

Thanks to the combined use of SAATY method and GIS, index V should be represented as vulnerability maps of a plant surroundings, and should become in this way a powerful tool for risk-informed decision making.

WP5; Risk level integration and validation

Characterization of final Risk Level RL

The risk level RL of an installation in its environment is a function depending on the severity index S, the vulnerability index V and management effectiveness index M:

RL=Ax M x ()
The objective of this phase is to study the relation between S, M and V to characterize final risk level.

It will be decided at this stage whether the risk level should remain characterized by 3 separate indexes or whether the 3 indexes could be aggregated into one single index.

Case studies

Halfway through the life of the project, five case studies will be carried out with the collaboration of five different SEVESO establishments throughout Europe in order to test and validate or improve the new methodology.

To select the test sites, it has been assured that both countries with consequence-based and probabilistic approaches would be represented. Moreover two case studies out of five will take place in Slovenia and Czech Republic. Two institutes from these countries will indeed join the consortium at this stage and test the full method with the test sites. Both of them will thus act as totally unbiased endusers since they were not involved in the method development.

After these full-scale exercises, the ARAMIS methodology will be improved again and give rise to its last version in the project.

WP6-7: VALORIZATION, DISSEMINATION

Since ARAMIS is intended to be a supportive tool to promote harmonized risk assessments throughout Europe, the project leaves from the start an extensive part to large exchanges with potential end-users. Identified end-users are both industrial companies and Competent Authorities in charge of enforcing SEVESO H Directive.

hi the valorization process, industrial end-users are directly represented in the consortium through an association of European industrial companies. This helps the consortium to relay information about the project progress and results, and to convince plants for the case studies.

A Parallel Review Team gathering risk experts from industry and EU competent authorities has also been constituted. This review team gathers every six months to monitor the project and thus to ensure needs from end-users are indeed fulfilled and the final approach will be widely accepted, ha this respect their main comments concern the applicability and usefulness of achieved results.

In the dissemination process, a dedicated web site has been set up first: please visit http://aramis.jrc.it. The web-site aims at promoting the project towards the public and also works as a quick communication tool among the partners. An electronic newsletter is also released every six month on the web-site, in order to get the public informed of work progress.

Two workshops were also planned during the project. This article is part of the halfway workshop held at the ESREL conference. A final workshop will also take place at the end of the project to disseminate main achievements to all relevant stakeholders. Proceedings of the workshop will be made available on the web-site. INVOLVEMENT

The consortium consists of twelve organizations expert in the field of risk analysis (Table 1). Nine partners represent mostly research centers throughout Europe. The last three institutes represent Newly Associates States from Eastern Europe. EC-JRC-IPSC and especially MAHB has a recognized international expertise in the field of major accident prevention. It animated EU Working Groups dealing with the application of SEVESO Directives and is also experienced with accident databases and GIS tools at a European level. MAHB is WP leader of the dissemination activities and also coordinates the Parallel Review Team.

FPMs-MRRC has a great experience in the application of SEVESO Directives, and already developed methodologies and tools in the field of domino effects and source term / consequence modeling. MRRC is WP1 leader concerning scenario identification and also brings its experience to WP2.

UPC-CERTEC has a recognized expertise in the evaluation of accident consequences: dispersion, explosion, fire modeling. UPC is WP2 leader.

ARMINES is a consortium of research centers from French school of mines. Three different research teams take part in the ARAMIS project.

"Pôle Cindyniques" from Mines de Paris has built up a debriefing and interview methodology to learn better from both technical and organizational incidents. By this means, Mines de Paris contributes to develop ARAMIS organization model and management effectiveness index.

"SITE" Center from Mines de Saint-Etienne has a long experience in risk analysis and environmental management system. In ARAMIS, this team mainly focuses on developing generic "bow-ties" and an assessment method for technical barrier effectiveness.

"

LGEI" Laboratory from Mines d'Alès has developed a methodology based on GIS and multi-criteria SAATY approach to study risks in transportation of hazardous substances. This partner is WP4 leader concerning environment vulnerability estimation, which intends to use the same competence.

RIS0 i s experienced with both SEVESO Directives. It also coordinated the ASSURANCE project. Furthermore, it is experienced in applying functionoriented modeling to analyze the effectiveness of an organization and its safety culture. RIS0 is WP3 leader dealing with Prevention management.

UROM is experienced in risk analyses and area risk studies, linked in particular with the use of GIS. Its activities in ARAMIS mainly concern the i nterfaces to build up between the model developed for V index and the use of GIS to represent this index.

CMI has a long experience both in fire and explosion modeling and in the use of safety management standards. In this respect, CMI works in WP2 to list and select appropriate models for consequence modeling, and in WP3 to analyze how the common management standards fit in the ARAMIS model.

TUD was a major partner of I-RISK project. It brings its expertise to the project in safety management modeling and auditing, and also in scenario identification with the use of bow-tie approach.

IChemE-EPSC only participates in the dissemination process to other member companies or associates from EPSC. It also cares about Industry participation into the Parallel Review Team.

US is the largest Slovenian research organization. VSB-TUO has been involved for long in Industrial Environment Research, and has enlarged since 1995 its focus towards technological and natural risks. Both institutes have been chosen to test the full ARAMIS method in companies of their own country and also to compare it to commonly used approaches in their respective country. 9 REFERENCES ARAMIS project supports the European Research Area concerning knowledge improvement, encouragement of the Science-Industry dialogue and harmonization in decision-making process related to hazardous establishments.

The resulting method should indeed be proposed as a recommended and harmonized tool used by risk experts and recognized by risk-informed decisionmakers of EU competent authorities. Harmonizing industrial risk assessment in Europe would significantly contribute to the European Commission's overall efforts to establish harmonized policies following the SEVESOII Directive.

For both Competent Authorities and Industry, such a harmonized risk assessment procedure would constitute first a useful comparison tool for industrial sites, which integrates the strengths of both probabilistic and deterministic approaches. The risk assessment procedure would at last be linked to the setup of progress plans within the framework of a safety management system. It would also allow to reach a consensus in the selection of accident scenarios that takes into account plant-specific safety devices and safety management effectiveness, i.e. suitable frequencies for the scenarios as required in a safety report for risk control demonstration.

The participation of potential end-users in the project from the start, in particular through the Parallel Review Team ensures that the ARAMIS project will contribute on a very practical level to EC research objectives and to consistent implementation of European policies in major hazard prevention.

  Figure 1. ARAMIS methodology representation

OFigure 3 .

 3 Figure 3. Scenarios identification process

Table 1

 1

	. Description of partner organization
	Organization full name	Short name	Country
	1. Institut National de l'Environ-	INERIS	France
	nement Industriel et des Risques
	Accidental Risk Division
	2. European Commission -Joint	EC-JRC-	Italy
	Research Centre -Institute for the	IPSC-
	Protection and Security of the	MAHB
	Citizen -Major Accident Hazard
	Bureau
	3. Faculté Polytechnique de Mons	FPMs-	Belgium
	-Major Risk Research Center	MRRC
	4. Universitat Politecnica de	UPC	Spain
	Catalunya -Centre for Studies on
	Technological Risk (CERTEC)
	5. Association pour la Recherche	ARMINES	France
	et le Développement des Métho-
	des et Processus Industriels
	6. Ris0 National Laboratory	RIS0	Denmark
	System Analysis Department
	7. Universita di Roma	UROM	Italy
	Dipartimento Ingegneria Chimica
	8. Central Mining Institute	CMI	Poland
	Safety Management and Techni-
	cal Hazards
	9. Delft University of Technology	TUD	Nether-
	-Safety Science Group		lands
	10. Institution of Chemical Engi-	IChemE-	U.K.
	neers European Process Safety	EPSC
	Centre
	11. Jozef Stefan Institute -De-	US	Slovenia
	partment of Inorganic Chemistry
	and Technology
	12. Technical University of Os-	VSB-TUO	Czech
	trava -Energy Research Centre		Republic
	MERIS is the coordinator of the project. It has a
	European expertise in the field of major accident
	prevention. It works as technical support for the
	French Ministry of Ecology in charge of SEVES O II
	Directive application. In particular, it manages with
	a steering committee the risk index aggregation and
	validation phase. INERIS is also deeply involved in
	the valorization and dissemination process.

ACKNOWLEDGEMENTS

The ideas presented in this publication are developed in the frame of EU project ARAMIS "Accidental Risk Assessment Methodology for Industries", contract n°EVGl-CT-2001-00036 funded by the Energy, Environment and Sustainable Development Program in t he 5 th F ramework P rogramme for S cience Research and Technological Development of the European Commission.

The project is coordinated by ESfERIS but also includes a consortium of fifteen efficient institutions representing ten countries which all contributed to the results presented during this workshop.

The Parallel Review Team represents also fifteen organizations either from Industry or from EU competent authorities, which are also kindly acknowledged for their advice and contribution during the periodic reviews.