Nicola Paltrinieri
email: nicoia.paltrinieri@mail.ina.unibo.it

Mike Wardman

Nicolas Dechy

Ernesto Salzano

Valerïo Cozzani

Proper hazard identification in safety reports has become progressiveiy more difficult to achieve. Severai major incidents in Europe in récent years, such as Buncefield and Toulouse, were not even considered by their site Seveso II Safety Case. One of the reasons is that available hazard identification (HAZID) méthodologies take no notice of least likely or unknown events. Nonidentified scénarios thus constitute a latent risk, whose management is extremely complex and open-ended. Hence this work aims to investigate thé issue of atypical scénarios and explain how they could hâve been identified.

An in-depth historical analysis of some of thèse incidents has been performed, in order to outline gênerai features of plants in which they occurred, their causes, conséquences and lessons learned. This analysis has followed a précise common scheme, which allowed a systematic approach to the problem. Based on the findings, failures connected to risk management and risk appraisal hâve been identified. Checklist questions and recommendations wîll be compiled, making efforts to improve the gênerai safety of industry. Attempts will be made to review well-known HAZID méthodologies, and thus the process of risk appraisal, with the purpose of improving the HAZID processes to identify atypical scénarios.

Introduction

In récent years, Europe has witnessed the occurrence of two major accidents that had great relevance within society, because of the deaths, injuries and damages they caused, and also because of their peculiarity and unpredictability. Indeed, the incident scénarios that actually occurred were not taken into account by their site safety reports, thus rendering the operators unprepared for the emergency. Moreover, the whole rîsk governance was based on différent kinds of incidents, which in certain ways may be considered less dangerous and easier to control.Thèse atypical major accidents are the ammonium nitrate (AN) explosion at the AZF fertiliser factory in Toulouse (France) and the Vapour Cloud Explosion (VCE) at the Buncefield oil storage dépôt in Hemel Hempstead (UK).

This work shows an in-depth analysis of some of thèse unexpected incidents, in order to outline gênerai features of plants in which they occurred, their causes, conséquences and lessons learned.

On the basis of the fîndings, well-known HAZID méthodologies hâve been studied and developed, which wïll assist future HAZID processes used to identify atypical incident scénarios.

Historical incident analysis

Past atypical scénarios

The Toulouse incident occurred on 21 st September 2001. An explosion took place in a warehouse, located among process, storage and packaging areas of the plant which mainly produced ammonium nitrate (AN), ammonium nitrate-based fertilisers and other chemicals including chlorinated compounds. Due to the vicinity of the plant to the city of Toulouse, the effects to people and the damage were catastrophic. The explosion caused 30 fatalities and damages estimated between €1,5 to €2,5 billion [START_REF] Dechy | Damages of the Toulouse disaster, 21st september 2001[END_REF]. The warehouse was used as a temporary storage of 'off-specifications'AN. This incident scénario was not considered in the safety studies or in the Land Use Planning (LUP) safety perimeters [START_REF] Salvi | Toulouse disaster prompts changes in French risk management, Environment and Poverty Times[END_REF]. In addition, the Seveso II directive did not cover the particular risk of l off-specification' AN (Directive 96/82/EC). Today this kind of material with badly defined properties is classified in (Directive 2003/105/EC) at a risk level similar to technical grade AN.

AN is a strong oxidative materîal; AN-based products hâve led to severai major accidents before 1950 (Table 1) with hundreds of fatalities and massive destruction due to the fact that the détonation properties were poorly understood. With quality driven standards, régulation frameworks and the promotion of better anti-caking agents (Marlaïr and Kordek 2005), very few explosion accidents hâve occurred since the middle of 1950's (Table 1). The major explosion in Toulouse was a severe reminder of the inhérent hazards associated with the handling and storage of AN. Table 1 shows the more serious AN major explosions previous to Toulouse incident.

Similarly, despite the warnings about the extrême reactivity of ammonium nitrate, représentée! by several incidents before the middle of 1950s (outlined in Table 1), due précautions were not taken to prevent the risk of explosion. For instance, the group of buildings where the warehouse was located was not equipped with a fire détection System (Dechy et al. 2004). Also in this case the scénario that actually occurred had been underestimated due to the fact that the hazard identification process failed to identîfy the risk.

In both the cases the incident scénarios were not deemed sufficiently crédible to be captured by current HAZID méthodologies, but with hindsight it can be stated that they represent a real and impending risk to industry and society. Thus, the current contribution aims to outline lessons and recommendations, and to define a différent approach to the hazard identification process that should help in identifyïng little known incident scénarios such as those which occurred at Buncefield and Toulouse.

Scénarios of this kind, normally excluded by risk assessments, from this point on will be named as atypical scénarios.

Methodology approach of historïcal incident anaiysis

The methodology of historical incident anaiysis is a way to better understand the actual mechanisms of past accidents in order to avoid them in the future. The atypical incidents mentioned (Buncefield and Toulouse) had some similar précédents, knowledge of which might hâve helped in the safety assessment process, in order to foresee the atypical scénarios that occurred. Hence, the development of a method for analysing past incidents may allow better knowledge management and horizon scanning, which will lay the foundations for a more gênerai methodology of hazard identification.

In order to agrée a systematic anaiysis of incidents, a common template for historical anaiysis has been outlined. It is divided into five parts concerning différent aspects of possible incidents (Table 3). A référence to thèse frameworks is a way to better organise and understand the root and direct causes of the incident. In this way, aspects of risk management that are lacking may be detected early and included in any subséquent analysis.

Table 3: Historical incident anaiysis template

Conséquences, damages, effects to system, people and environment are the other main topics of this section that also aims to describe the emergency response measures taken during the incident event and the actions to restore, repair and reclaim after the event.

Lessons Iearned from officiai investigations (foliowing corrective action) are reported in part D in order to evaluate which aspects should be broadened and how they should be treated. Finally, part E reports sources of information and références used.

First results

Up to now, five major incidents hâve been analysed by means of the template described (see Table 4). Plants involved in the incidents are characterised by the important fact that the dangerous substances stored could represent a threat to the nearby population (see Table 4). In fact in some cases thèse plants are located close to the urbanised area, as shown in Figure 3. The need to properly identify hazards in thèse cases is even more important.

ERMF faiilures -Techm'cal, technological dimension

A clear example of failures of Technical, Technological' dimension of the ERMF is the event that triggered the incident of St Herblain. The direct cause to the incident was attributed to the failure of a rubber joint at the pipefitting (Figure 4) [START_REF] Lechaudel | Accident investigation report, Analyse de l'accident du 7 Octobre 1991 à Saint-Herblain dans l'enceinte du dépôt pétrolier de la société GPN[END_REF]. The joint was guaranteed by the manufacturer to last with a maximum aromatic concentration of 30%. Instead, the incident occurred on the first day of opérations with a 98-octane gasoline with an aromatic concentration of 55%. Gasoline released from the conséquent leak poured into a rétention basin, where it vaporised and led to a mist formation (enhanced by 100% humidity in the vicinity of the site).

Figure 4: Location of the failing rubber joint at the pipe fitting

Another faîlure of this dimension of risk management can be identified in the design of Buncefield tanks, which may hâve contributed to the vapour/mist formation. Tests demonstrated (MHB 2008) that a deflector plate on the top of the tank, like in Buncefield, channels only a part of the overfilling fuel onto the tank wall but the rest runs over the top of the plate, fragmenting into dropiets. Moreover, fuel running down the wall could hit structural stiffening rings and become detached from the tank wall, creating another cascade of dropiets (Figure 5). Thèse conditions would promote the evaporation of the lighter components of petrol, such as butanes, pentanes and hexanes. The freefalli of dropiets Ieads to entrainment of air and mîxing between the air and fuel vapour, and the formation of a rich fuel/air mixture.

Ullage ventilation hole

Fuel spilling over deflector plate

Structural stiffening ring

Figure 5: Pattern of fuel dispersion

four dimensions of ERMF framework. Thèse recommendations emphasised the need to increase the technical protection provided by primary, secondary and tertiary containment Systems in oil dépôts, without disregarding human and organisational factors. Recommendations address, amongst others, compétent authority and central government leadership, in order to ensure thèse improvements and an early response to a major incident. Finally, as the case of the incident at Toulouse, it is suggested that a revïew of COMAH régulations (UK implementation of Seveso II) and LUP processes be carried out, ensuring that they are based on a simplified, generic approach to risk assessment for flammable storage sites (MIIB 2008).

Further development of work

By means of an hîstorical analysis of atypical incidents, direct and root causes hâve been identified, Thèse causes can be attributed to common events such as improper rîsk management or to a more gênerai failure of ineffective hazard identification. The combination of thèse causes has led to the incident, as effectively explained by the 'Swiss cheese model' [START_REF] Reason | The contribution of latent human failures to the breakdown of complex Systems[END_REF]) [START_REF] Hudson | Application of TRIPOD to measure latent errors in North Sea gas platforms: validity of failure state profiles. First international Conférence on health, safety and environment[END_REF].

In this case the différent slides represent the four dimensions of ERMF and the characteristic holes of the Swiss cheese symbolise the failures. Figure 6 shows the application of this model to the Buncefield case.

TECHINICAU TECHNOLOGICAL

The design of the tank itself hâve contributed to ttie vapour/mist formation.

HUMAN/ MANAGEMENT

Rôle and responsibilities of contrai room operators not clearly défined and front Une staff not properiy trained to be able to reliably detect, diagnose and respond to potential incidents.

GOVERNANCE/ COMMUNICATION

Lack of a formai methodology for industry to détermine safety integrity levels required for overfill protection at dépôts that store and fransfer petroleum products on a large scale

POLICIES/ REGULATIONS/ STANDARDS

At a site like Buncefield, land use planning considered as worst crédible scénario a major liquid fuel pool fire. With the purpose of irnproving risk management for thèse type of industrial plants and, thus, to close some of the holes in the cheese slices, checklist questions will be formulated, based on the lessons learned from past failures.

Moreover, to address the incapability of HAZID methods to capture atypical scénarios, a bow-tie methodology is being applied to plants involved in the accidents considered hère. Figure 7 shows an example of an event tree referring to oil storage tanks of Buncefield oil dépôt.

-Figure 2 :

 2 Figure 2: Risk governance framework (ilMTeg-Risk Description of work 2009)

Figure 6 :

 6 Figure 6: Swiss cheese model applied to ERMF Buncefield case.

Table 4 : Summary of atypiçal incidents analysed (Dechy ét al. 2004) (MIIB 2008)

 4 Toulouse and Buncefield hâve been previously described and used as examples of atypiçal incident scénarios. St Herblain, Naples and Newark are quoted in Table2as vapour cloud explosions in oil dépôts caused by LOC of gasoline. Analysis has highlighted important aspects of the incident event and several failures of risk management, which would need corrective and resolving actions.

		Toulouse	Buncefield	St Herblain	Naples	Newark
		(2001)	(2005)	(1991)	(1985)	(1983)
	Plant typology	Chemical and plant fertiliser	Oil storage dépôt
		Off-
	Substances involved	specification ammonium	Gasoline and fuel oils
		nitrate
	Main incident scénario	Reactive explosion chemical	Vapour cloud explosion
	APmax (kPa)	150	200	50	80	/


M Caudron

F Emeriault

M Al Heib

A large 3D physical model: a tool to investigate the consequences of ground movements on the surface structures

Soil subsidence of various extend and amplitude can result from the failure of underground cavities, whether natural (for example caused by the dissolution of rocks by underground water flow) or man-made (such as mines). The impact of the ground movements on existing structures (houses, buildings, bridges, etc…) is generally dramatic. A large small-scale physical model is developed in order to improve our understanding of the behaviour of the building subjected to ground subsidence or the collapse of cavities. We focus on the soil-structure interaction and on the mitigation techniques allowing reducing the vulnerability of the buildings (structures).

Introduction

INERIS has been working for many years on the "ground movements" risk caused by old mines. Several research tasks are followed concerning the stability of the cavities, the prediction of the ground movements, the behaviour of the building subjected to this hazard, mitigation techniques... A large small-scale physical model is developed in order to improve our understanding of the behaviour of the building due to ground subsidence or collapse. This paper focuses on the soilstructure interaction and on the mitigation techniques allowing reducing the vulnerability of structures.

This paper is composed in the first part of the presentation of the physical model, the measurement facilities and the basic hypothesis. The second part will detail the building model from the prototype considerations to the simplification done for the small-scale model. We developed then two similar tests, the first one presents the case of a ground subsidence in greenfield condition (no surface structure) while the second one presents the case with the building model: a loaded slab. Three positions of the structure with respect to the cavity position are considered in order to determine the effect of these positions. The effects are detailed for the behaviour of the soil and the building in terms of displacements, strains and loss of support.

EPJ Web of Conferences

Mining subsidence & soil-structure interaction

The subsidence is caused by the collapse of anthropic or natural underground cavities. This phenomenon can be very prejudicial for the structures and infrastructures on the surface and for the population. Empirical rules exist for the estimation of the building damages caused by such ground movements (…). But they are limited by the context of their definition. There is very few relationships to determine the intensity of the building damages caused by ground movements and taking into account the soil-structure interactions.

Several research projects are focused on the study of the ground-structure interactions phenomena under the effect of the soil movements ( [START_REF] Standing | Impact of underground works on existing infrastructure[END_REF][START_REF] Lee | Influence zones for 2D pile-soil-tunnelling interaction based on model test and numerical analysis[END_REF][START_REF] Sung | Ground behavior due to tunnel excavation with existing foundation[END_REF][START_REF] Abbass-Fayad | Modélisation numérique et analytique de la montée de cloche des carrières à faible profondeur. Etude de l'interaction sol-structure due aux mouvements du terrain induits par des fontis[END_REF][START_REF] Deck | Taking the soil-structure interaction into account in assessing the loading of a structure in a mining area[END_REF][START_REF] Burd | Modelling tunneling-induced settlement of masonry building[END_REF][START_REF] Nakai | 3D and 2D model tests and numerical analyses of settlements and earth pressures due to tunnel excavation[END_REF]).

While previous works [START_REF] Caudron | Contribution of the experimental and numerical modeling to the understanding of the soil-structure interaction during the event of a sinkhole[END_REF] have allowed shedding some light on the interest of taking into account the soil-structure interaction. This paper will focus on the conception of a new small-scale physical model intended for the study of mining subsidence and the consequences on surface building. Previously, a 2D small-scale physical model was used [START_REF] Caudron | Sinkhole and soil-structure interactions: Development of an experimental model[END_REF]. However, the 2D nature of this model was a too important limit for the expected results preventing any reliable interpretation to the reality.

Physical model for mining subsidence

Very few physical models have been used to study the effects of ground movements caused by the mining process or by the collapse of old mine. We can mention block caving apparatus from [START_REF] Castro | A study of isolated draw zones in block caving mines by means of a large 3D physical model[END_REF][START_REF] Trueman | Study of multiple draw-zone interaction in block caving mines y means of a large 3D physical model[END_REF]. This model is dedicated to the study the block caving exploitation scheme and not to the ground movement rising to the surface. Few models are focusing on the phenomena corresponding to the formation of a subsidence trough. Genis [START_REF] Genis | Assessment of dynamic response and stability of an abandoned room and pillar underground lignite mine. 12 th IACMAG[END_REF] studied the effect of an earthquake on the stability of an old coalmine located in Japan. They showed that depending on the geometry of the cavity, the failure may take place in the pillars or at the mine ceiling. Dyne (1998) designed a model, trap door type, in order to represent the formation of sinkholes in an ancient coalmine in Pennsylvania. The model was a very simple one with a trap door of four different widths and a single layer of sand as overburden.

For these different works, the potential damages caused by the subsidence on the surrounding buildings were not taken into account.

Ren [START_REF] Ren | Research on deformation and subsidence characters of ground and wall rock due to underground mining by model testing[END_REF] achieved several tests in centrifuge concerning subsidence features of ground and wall rock due to underground mining. Few details are mentioned in the different papers published on the parameters and the hypothesis made to this model.

On this subject, several small-scale physical models have been realized in the field of tunnel. For example, Nakai [START_REF] Nakai | 3D and 2D model tests and numerical analyses of settlements and earth pressures due to tunnel excavation[END_REF] and Shahin [START_REF] Shahin | Influence of surface loads and construction sequence on ground response due to tunnelling[END_REF] use a 2D physical model for studying the influence of the position and nature of the foundation on a building in response to tunnelling-induced settlements.

Model tests have been performed in the context of EU research project "QUAKER" [START_REF] Bransby | Centrifuge modeling of normal fault foundation interaction[END_REF] to study the influence of several buildings, considering different geometries, weight and foundation systems, on the path followed by a fault activated by the Kocaeli earthquake. Very interesting results are obtained, showing a more or less important bifurcation of the fault when a building is located close the path obtained in greenfield condition. The bifurcation can even become very important in the case of heavy building or when a deep foundation system is used.

The Apparatus

Hypothesis and conception

Physical model must respect similitude laws; the first part of the hypothesis comes from the laws of similitude. It is very difficult, if not impossible to achieve a perfect similitude with a small-scale model. Since our physical model is designed to be used in a 1g environment (earth gravity) and not 14th International Conference on Experimental Mechanics in a centrifuge, it will be difficult for quantitative interpretations of the experimental data to be made.

The geometry scale factor may range up to 1/50. With a size of 2m by 3m in the horizontal plan for a height up to 1m, we would be able to represent a soil block as large as 100m by 150m with a height of 50m. A key limitation due to the 1g environment is that for high scale factor values, the difference between the achieved values of stresses and the normally scaled ones become very large and might cause some important differences.

Considering underground mines as cavities at the origin of the ground movements, caused by their collapse, the maximum depth of 50m may appear like an important limit to this model. However, we chose not to model the cavity itself but only the equivalent void raising the ground surface. This is achieved by the use of an electric jack with a vertical moving trap-door. The physical model is designed to be equipped with 48 jacks at most, in a configuration of 6 rows of 8 columns.

The model is still in an early stage of development, only one jack is installed. The cross-section of the actuator is limited to 250x250mm², corresponding up to 12.5x12.5m² in the prototype scale. The apparatus is indeed limited to localized phenomena: sinkhole or collapse/subsidence of smallextent. When more jacks would be installed, various shapes of collapse/subsidence would be able to be modeled..

The soil

The sand of Fontainebleau, well-known by the actors of physical modeling in geotechnics, is used. Category NE 34 is chosen whose granulometry and characteristics are presented in figure 1 and table 1.

Monitoring

Measurement is achieved by the mean of stereo digital imagery. This allows monitoring the whole top surface of the ground and more especially where ground movements happen. A commercial product is chosen, Vic3D from Limess Gmbh, being very effective in the task of computing the movements with the grains of sand showing very low contrast on the photos.

Two cameras, 4MP each, are used. They allow a maximal frequency of capture of 8 images/second at full resolution, with the possibility to reach 30 images/s with a 1MP resolution. They have to be calibrated before starting a test by the use of a test pattern. A good calibration allow obtaining very precise measures with an error of 1/100 of a pixel in good conditions: that's 10µm when 1 pixel is equal to 1mm. In the tests presented later in this paper, we have a ratio close to 2 pixels per millimeter. But, due to the nature of the sand, constituted of small particles and so not a continuous media, the corresponding error is around 0.13 pixel giving 0.065mm: this is quite a good performance [START_REF] White | Soil de formation measurement using particle image velocimetry (PIV) and photogrammetry[END_REF].

One disadvantage of this method of monitoring is the huge volume of data created by a single test. With a volume of 8Mo per capture (two images of 4Mo each) and considering the full speed EPJ Web of Conferences frequency of capture, it represents near 2Go of raw data to be stored each minute. For a full test and with the exploitation files for the digital correlation process, this corresponds to a total between 30 and 40Go.

The building model

A building model was created to investigate the effects of the soil-structure interactions during the formation of a subsidence trough and the effects of this ground movement on the structure.

The geometry chosen for the building was inspired from the database existing on the building damaged by mine subsidence in the east of France, for instance the Lorraine basin [START_REF] Saeidi | Development of a simulator of damage for evaluation of the vulnerability of buildings in subsidence zones[END_REF]. A house is considered, whose characteristics are shown in figure 2. While it is not possible to make directly a 3D realistic small-scale model of this structure, we have to simplify it. For the first test series, we chose to consider only an elastic behavior and to use an equivalent slab. The followed process to simplify the building and to obtain the small-scale model is presented on figure 2.

Three steps were made for the simplification procedure: first, the equivalent slab is determined such as the bending stiffness and the axial stiffness of this slab are quite equivalent to the 3D structure. Then we choose to reduce the stiffness, in both directions, in order to exacerbate the strains in the structure. Both stiffnesses are approximately halved. From this very simple structure at prototype scale, a small scale model is conceived by applying the scale factors.

As shown in figure 3, two parts are used for the structure model. The first one is a hollow slab while the second one is composed of lead powder in a plastic bag. This allows the model to present a stiffness and a stress transmitted to the ground equivalent to the prototype's ones.

This small scale model is laying on the ground without any foundation system which limits the interaction between the soil and the structure. However, this procedure makes it very simple to reproduce on several tests. The next small-scale building model should integrate a foundation system so as to respect a configuration closer to the reality.

Achievement of a test

The sand is manually placed in the apparatus by layers of 15cm thick. Each layer is compacted by a compacting tool equipped by 15cm long needles in order to obtain a certain level of density, 14th International Conference on Experimental Mechanics repeatable for each different test. This is repeated until the total height of the soil layer is reached, in our case, 30cm. Afterwards, a 130cm wide metallic rule is used to obtain the right level on the whole ground surface.

A snapshot is then taken by both cameras to insure that the ground surface is flat enough with a difference less than 5mm in altitude between the highest and the lowest points. The building model is then placed carefully on the ground surface, its position depends on the modeled case (see part 4 of this paper).

The displacement acquisition by the two cameras is then started with a frequency of 0.5Hz. The motion program of the jack is launched at the same time a snapshot is taken by the cameras. The jack is moved down at a speed of 0.125mm/s for a total of 30mm.

At the end of the test, the displacements of the ground and the structure are computed by the DIC (Digital Image Correlation) program.

Results of the test series

The first test campaign is dedicated to study the influence of the building position relatively to a subsidence trough. After the presentation of the reference result, the subsidence trough is in greenfield condition, the results from the parametric study will be presented.

Greenfield condition

Four identical tests were performed in order to ensure a good level of repeatability of the tests. As shown on figure 4, the greenfield trough is symmetric. The average global characteristics are summarized in table 2. Differences from one test to another are not very important, but cannot be neglected.

Parametric study

Three positions (A, B, C) are used for the building model. They are defined with respect to the main component of the subsidence trough. Position A corresponds to the building being on the extension area and position B puts the building completely in the compression area, position C is where the slope is the greatest. Figure 5 presents these three different positions. Three particular axes are then defined for the exploitation of the results which pass through the center of the building to the center of the subsidence trough.

Four tests are performed for each position of the building, as for the greenfield condition. Results are then presented for two points of view. First the displacements measured at ground surface will be compared in order to identify the effect of the building on the ground behavior. Secondly the strains in the building model will be analyzed.

Ground behaviour

The building produces important differences in the soil displacement at ground surface. The trough, formerly symmetric in greenfield condition, shows clearly some dissymmetry with the presence of the building. However, the building being in B position, centered in the subsidence trough, remains symmetric. Two examples of ground displacement curves are shown on figure 6, corresponding to the axis plotted for the positions A & C of the building.

It appears that the soil movements are reduced due to the soil-structure interaction. Different areas may be distinguished, depending on the relative displacement of the ground to the building model. In the center part of the trough, the ground falls off the building due to a more important displacement, while on the other side of the building, its rotation causes another detachment. For the C position, three different behaviors are identified. For the part laying close to the center of the trough, a loss of support appears, thus the soil behavior is not very disturbed by the soilstructure interaction. For the center part of the structure, the contact between ground and building is kept and so important modifications are visible in the behavior: the slope of the soil is largely reduced and the horizontal displacements are restrained to the average horizontal displacement of the building. For the last part, another loss of support appears where the rigid body rotation of the building engenders uplift.

For the A position, the chosen axis for the behavior of ground and the building reveals a very important loss of support concerning two third of the appearing length. In fact, this is not so important due to the fact that the structure is supported on its sides, not visible here. The impact of the soil-structure interaction relative to the vertical displacement is less important than for position 14th International Conference on Experimental Mechanics C. For the horizontal one, the shape of the displacement curve is very close to the one in greenfield condition. The values of maximal displacements are the same.

Consequences on the building

The optical setting allows measuring the movements and the strains of the building. Those measures are synthesized in table 3. It can be seen that the measured strains are very small comparing to the strain of the soil. Several reasons are identified:

The elastic behavior chosen for the building model;

The fact that the building is just laying on the ground surface;

The stiffness of the building is relatively too important. The soil-structure interaction is thus clearly identifiable: the strains measured on the building are very different from those measured in greenfield condition. The behavior of the soil is notably modified due to the presence of the building. The influence of the soil-structure interaction depends moreover on the position of the building relatively to the subsidence trough. In a general manner, the strains measured on the structure model are smaller than those in greenfield condition. The main differences are shown for the horizontal strain and for the bending radius.

Conclusions and prospects

The study presents a new 3 D physical model allowing the simulation of ground movements caused by the collapse of underground cavities. A simple structure model was designed in order to represent an individual house, was used for two purposes: to observe the evolution of the ground movements subjected to the presence of the building, and the behavior of the building itself, and, to shed some light on the importance of the soil-structure interaction.

As a conclusion for this study, we would like to emphasize the fact that these tests represent the first step of a more global research for the consequences of ground movements on building, by taking into account the soil-structure interactions. It was established than the soil-structure interaction phenomenon must not be neglected and that it depends greatly on the relative position of the building to the subsidence trough.

Still, many technical improvements should be brought to the small-scale physical model. The initial state of the sandy soil must be clarified and several jacks would be set up to increase the variety of subsidence trough geometry that the apparatus is able to achieve.

However, in parallel, the structure model should be improved in order to represent more closely a real building. The main points are the design of the foundation system and the upper part of the structure. A new test series should be then conducted with the final objectives to study the performance of mitigation techniques.

Fig. 1 .

 1 Fig. 1. Granulometry of the Fontainebleau sand

Fig. 2 .

 2 Fig. 2. Simplification procedure of the 3D building to an equivalent small-scale slab.

Fig. 3 .

 3 Fig. 3. The resulting small-scale structure model composed of a hollow slab and small bags of lead powder representing the load.

Fig. 4 . 5 .

 45 Fig. 4. Displacements in Greenfield condition Fig. 5. Building position for the parametric study

Fig. 6 .

 6 Fig. 6. Behaviour of the ground for two different positions of the building (left: pos. C; right: pos. A).

Table 2 .

 2 Geometric characteristics of the surface ground displacement (S max : maximal subsidence, i: distance to point of inflexion, R min : minimal radius of curvature, α: maximal slope, ε h : maximal horizontal strain)

	Characteristics	Average
	Smax	24mm
	i	100mm
	Rmin	1.7m
	slope α	22%
	εh	5%

Table 3 .

 3 Comparison between the strains determined in greenfield condition and using the building model.

	Case	Horiz. strain (%)	Bending radius (m)	Slope (%)
	Greenfield (pos. C)	5	-1.06	9.5
	SSI (pos. C)	0.08	-25	6.4
	Greenfield (pos. A)	10	-1.67	10.4
	SSI (pos. A)	0.04	-9.42	7.1

Acknowledgment

The authors would like to thank the French Ministry for Environment, Sustainable Development and the Sea for the partial founding of this research program.


G Leroy

S Duplantier

The 1D Iterative Model for Predicting Thermal Radiation from a Jet Fire

Most of the current jet fire models used in the accidental fire risks department are semiempirical. They depend on experimental data and are limited to the experimentally studied fire. Moreover, they are not applicable to very large scales (flame length > 100 m) and tend to overestimate large-scale radiation effects. Therefore, predicting accurately the large-scale jet fires radiation effects is today a challenge in a context where the optimisation of industrial site dimensions is at stake. A1D model based on physical laws has been developed by INERIS for predicting the flame shape and radiation field for large-scale gas or liquid released jet fires. This model is not based on an experimental correlation but takes into account the physical laws like energy balance, momentum balance through an iterative method. Moreover, the model includes the buoyancy forces which gives bent shape to the horizontal or lifted flame and takes into account wind velocity according to the release direction. At a small-scale, good agreement was noticed, first, between the new model predictions and experimental measurements and, second, between the new model predictions and the semi-empirical model results. We expect the agreement to be equally good for a large scale jet fire. Large scale experiments will be carried out in the near future to validate this physical approach. liquid stoichiometric conditions vaporized gas jet exit

KEYWORDS:

Jet fire, thermal radiation, buoyancy forces.

NOMENCLATURE LISTING

A

INTRODUCTION

Jet fires occur as a result of ignition and combustion of flammable release (natural gas, LPG and others) usually from a pipe or tank. They emit high radiative flux creating a risk for humans and plants. A safety engineer needs to determine the flame trajectory, flame length and radiative fluxes to the surrounding plant and personnel. Typical critical radiative fluxes for humans are 3, 5, 8 kW/m 2 which represents respectively threshold for injured, 1% lethality and 5% lethality in French legislation.

In order to predict jet fires effects and especially radiative fluxes generated by flame, several models have been developed. The current software uses semi-empirical models to compute the distances of thermal effects associated with jet fires. This software is based on models which calculate geometry of the frustum based on input data (mass flow rate, orifice diameter, pressure in the pipe or tank, wind speed and others) and flame characteristics are obtained from experimental data [START_REF] Johnson | A Model for Predicting the Thermal Radiation Hazards from Large-Scale Horizontally Released Natural Gas Jet Fires[END_REF][START_REF] Chamberlain | Developments in Design Methods for Predicting Thermal Radiation from Flares[END_REF][START_REF] Cook | A Comprehensive Program for Calculation of Flame Radiation Levels[END_REF]. The semiempirical models are less expensive to implement because they are usually based on simple equations, and these models are easier to formulate, modify and implement in computer programs. Furthermore, they require low computational resources. However, such an approach is highly dependent on experimental data, and therefore it is limited to the types of fires investigated during the experiments. Moreover, this approach is rarely applicable to very large scales (flame length greater than 100 m) because experiments at this scale are scarce. Finally, in the case of horizontal or inclined released jet fires, semi-empirical models do not take into account the buoyancy effect which significantly changes the shape of the flame (bent shape). As an example, the semi-empirical model by Johnson [START_REF] Johnson | A Model for Predicting the Thermal Radiation Hazards from Large-Scale Horizontally Released Natural Gas Jet Fires[END_REF] takes into account buoyant forces but, being based on natural gas experiment; it cannot be used for LPG releases for which it was not validated.

Computational Fluid Dynamics, CFD, is also used to predict jet fire effects. However CFD models are highly CPU time consuming and are not always suitable to produce quick results.

As a compromise, a 1D model based on simplified fluid mechanic equations is presented in this paper for predicting the flame shape and radiation field for largescale gas or liquid released jet fires. In the following section, the mathematical basis used in this computational model is presented. In the next section, predicted values obtained with the jet fire model are compared with semi-empirical model results and with experimental data.

PHYSICAL BASIS OF THE MODEL

Jet fire is one highly directional phenomenon due to high source momentum close to the release point, which means that the 3D fluid mechanic equations can be reduced to a 1D axisymmetric model. The present model uses a phenomenological approach based on global balances for the characteristics of a steady-state jet fire. The model uses the following input data: pipe or tank pressure, orifice diameter, ambient wind speed and temperature. Figure 1 presents step-by-step methodology of the model. In each control volume represented by a frustum located by a downwind curvilinear coordinate, s, physical quantities including mass, momentum, temperature, density, volume are calculated sequentially using the physical laws. Finally, the flame shape is represented by lateral surface of the whole control volume (i.e. the sum of each cell).

Horizontal jet fi re

Wind direction

Release poi

' ^\ i = 1 t -- i=2 - _-- Vol n i = n .-- s i=n + 1 Buoyancy forces U Figure 1.
Step-by-step methodology of the model.

The jet fire methodology is presented and demonstrated for natural gas, propane and butane, although its extension to other fuels or fuels mixtures is straightforward. Following paragraphs describe different sub-models incorporated in this jet fire model.

Source Term Calculation

In order to estimate jet fire effects from a line rupture or a leak, it is required to calculate the release characteristics in terms of mass flow rate, velocity, temperature and liquid mass fraction. Depending on the release phase (gaseous, liquid, or two phase flow) and the nature of the breach (line rupture, leak or others), different models are used [START_REF] Hébrard | Evaluation of Two-Phase Flow Models for Accidental Release and Comparison with Experimental Data[END_REF]. Descriptions of these models are out of the topic of the present study.

The Fundamental Conservation Equations

In a steady flow, the fluid mechanics equations set can be reduced to:

1) Conservation of mass dm ( 1

) ds

The air mass production rate fn a depends on the entrainment rate model, described in the following paragraph. The vaporised mass production rate is calculated with the vaporisation Eq. 4.

2) Conservation of momentum

For the steady state, the momentum balance can be expressed as:

7 b ds
where F b is the buoyant vertical force acting on the control volume, F a is the drag force due to wind which is supposed to act in the direction perpendicular to the jet axis for the computed control volume, shifting the reaction zone of the jet.

The third term in Eq. 2 is related to the air entrainment. Its contribution rises with curvilinear coordinate in relation to air entrainment model as described below.

3) Conservation of energy

The first principle of thermodynamics leads to the energy conservation equation that can be expressed using several quantities. The present model uses the total enthalpy equation that can be expressed for the steady state as follows:

-= Qc-Qv- m f h f -m g h g -m Uq h Uq ( 3 )
The first term on the right is the source term related to fuel nominal heat release and mass of reacting gas:

Qc=Xc<\-Xr)*H c m R , ( 4
)
where % c is the combustion efficiency and % r is the radiative fraction, Q V is the energy consumption due to liquid fuel vaporisation.

Combustion efficiency varies along the jet axis [START_REF] Gómez-Mares | Axial Temperature Distribution in Vertical Jet Fires[END_REF], see Fig. 2. In region I, only a small amount of air is mixed with fuel because of the high velocity of the jet leading to limited development of turbulence; this implies bad conditions for combustion. Moreover, in that zone of the jet, most of the energy released is spent to evaporate liquid phase in case of two-phase or liquid leak. Therefore, burning rate is low in region I. In region II, the gas entering is hot and the gas mixing is facilitated by developed turbulence and lower velocity; the mixture is better prepared for combustion. Consequently, higher temperatures are reached, and the combustion efficiency is close to unity (^c~l). Finally, in region III, although the temperature decreases due to the fresh air entrainment into the jet, buoyancy leads to efficient mixing and high combustion efficiency. Thus, in this model, three regions are identified based on dimensionless variable s/Fl, where s is the curvilinear axial coordinate and Fl is the flame length: The right of Eq. 3 also represents energy consumption for heating non-reacting products, burned gas from the previous control volume, unburnt gas and liquid fuel, respectively. Lateral boundaries of the control volume are considered adiabatic.

4) Perfect gas law

To close the equations sets, an equation of state is required. The perfect gas law was considered:

P =
(5) RT

Combustion Model

The mixture composition is defined by the equivalent ratio: m [START_REF] Hann | Direct Measurement of Entrainment in Reacting/Nonreacting Turbulent Jets[END_REF] where the nominator is the fuel to air mass ratio in the mixture and the denominator is the stoichiometric fuel to air mass ratio corresponding to complete combustion. It can be assumed that no combustion occurs in the lift-off zone, and that the reaction is infinitely fast beyond the lift-off height; the combustion process is then "mixingcontrolled".

Irreversible one-step reaction of hydrocarbon fuel and oxygen is considered, in which if è < 1 (lean mixture) then: [START_REF] Becker | Entrainment, Momentum Flux and Temperature in Vertical Free Turbulent Diffusion Flames[END_REF] else (rich mixture):

C.H.+-\x + y 4

Air entrainment Model

The air entrainment in reacting turbulent jet is a fundamental parameter that determines the jet fire development since it controls the mixing rate and, consequently, the fuel burning rate. The air entrainment rate can be written as:

dm a = m 0 C d ds, d* (9)
where d" is the jet momentum diameter

(d* = d o {p o /p a f 12 )-
According to [START_REF] Hann | Direct Measurement of Entrainment in Reacting/Nonreacting Turbulent Jets[END_REF], the local entrainment rate coefficient, C e l, depends on buoyancy, axial position and temperature. To describe the effect of buoyancy, Becker and Yamazaki [START_REF] Becker | Entrainment, Momentum Flux and Temperature in Vertical Free Turbulent Diffusion Flames[END_REF] introduced a parameter that can be expressed as:

4 = RP- ( 10 )
where R i = gd*/u 0 2 is the Richarson number. Parameter £ contains the integrated effect of buoyancy along the jet. Han and Mungal [START_REF] Hann | Direct Measurement of Entrainment in Reacting/Nonreacting Turbulent Jets[END_REF] related the buoyancy parameter F with the air entrainment coefficient:

Ce l = ( 11
)
As shown in Fig. 3, according to the Ricou and Spalding approach [START_REF] Ricou | Measurements of Entrainment by Axisymmetrical Turbulent Jets[END_REF], the air entrainment coefficient is constant. Alternatively, according to Han and Mungal [START_REF] Hann | Direct Measurement of Entrainment in Reacting/Nonreacting Turbulent Jets[END_REF], buoyancy increases the air entrainment coefficient along the jet axis.

Thermal Radiation Intensity

According to the solid flame model, the Surface Emissive Power (SEP, kW/m 2 ) can be related to the fraction of heat radiated from the surface of the flame &, fuel mass flow rate m, total heat released AH C by the following equation: [START_REF] Cleaver | Predictions of Free Jet Fires from High Pressure, Sonic Release[END_REF] where the flame surface area A is given by the sum of lateral area of all the control volumes. The fraction of heat radiated from the surface of the flame & is given by [START_REF] Cook | A Comprehensive Program for Calculation of Flame Radiation Levels[END_REF]: In Eq. 13, u 0 is the jet inlet velocity and M w is the molecular weight of the fuel. This model assumes that the flame emits homogeneous surface radiative flux and does not take into account the fact that the radiative emissions depend on temperature and chemical composition of the flame zone which vary along the flame axis. Moreover, the thermal radiation is also dependent on soot concentration.

x, = • [0.21e-°-°°3 23i "+0.
Finally, the radiative flux received by a target outside the jet fire is expressed as:

q=VFxSEPXT, ( 14
)
where VF is the view factor. It depends on location of the flame in space relative to the target position. The view factor between an elementary receiver surface C and an elementary emitter area dA from the control volume surface is given by:

ldA , ( 15
)
Ttr where 6j is the angle between local normal to surface element dA and the line joining elements dA and target, and 8 2 is the angle between normal of the target and the same line. The atmospheric transmisivity % is obtained by the Brzustowski and Sommer's empirical law [START_REF]Guide for Pressure Relieving and Depressing Systems[END_REF].

RESULTS AND DISCUSSION

Comparison Between Predicted and Measured Values

A safety engineer is mainly interested in predicting the worst case scenarios for the accidental phenomena. That is the reason why, in case of released jet fire, the downwind effects of horizontal jet fire are most thoroughly investigated and the critical thresholds of 3, 5, and 8 kW/m 2 are examined.

Large-scale experiments data are scarce in the literature. Moreover, in the jet fire experiments, the radiometers are not normally located downwind, in the flame axis; instead, they are usually located on the flame side, which is out of the topic of the present paper.

Experimental data used in this work are listed in Table 1. It includes data for natural gas, propane and butane jet fires releases. 4 and Table 2 show a comparison of computed incident radiative flux and flame length with experimental results.

At small and medium scale, the level of agreement between the model predictions and experimental measurements is good for far field. All predicted jet fire radiative flux and flame length are within 10% of the measured value, 15% for flame length. However, it can be observed that the model tends to underestimate the results which can be problematic in safety department. It is essential to ensure that the discrepancy between prediction and measured values does not increase at large scale and in case of horizontal LPG released jet fires.

Two parameters could be reviewed in order to make results more conservative: the radiative fraction of the flame and the air entrainment parameter of the jet. A radiative fraction prediction model based on Stefan-Boltzmann law which gives the radiation of a black body in relation to its temperature, coupled with a simplified soot formation model, for example, the model in [START_REF] Delichatsios | Smoke Yields from Turbulent Buoyant Jet Flames[END_REF] would distribute radiative heat on the flame surface. This model will be integrated in the near future, with the contribution of medium-scale experiments, in order to compute more accurately the surface emissive power.

Soot concentration peaks at the fuel-rich side of the flame. in region I, see Fig. 1. At the same time, the peak temperature is located in the region II, as mentioned in the combustion model section. As a result, in any jet fire regions I and II emit the largest radiative flux. However, in the present model, the radiative flux is assumed to be uniformly distributed over the flame surface. More experiments will also be necessary to improve the air entrainment model.

Comparison of INERIS and Semi-empirical Models

Two representative medium scale scenarios of accidental liquid jet fire were computed in this work using two models. The first one is the new model presented in this paper and the second model is the commonly used Phast v6.5 (DNV software) jet fire module of the Cook model [START_REF] Cook | A Comprehensive Program for Calculation of Flame Radiation Levels[END_REF], which is recommended by DNV [START_REF] Jfsh | Jet Fire) Theory Document[END_REF] for releases of combustible liquids. The Cook model assumes the flare flame shape to be of a cone shape. The cone characteristics are based on semi-empirical correlations. Table 3 presents the mass flow rates in propane and butane released jet fires and the Fig. 5 shows the comparison between the results obtained by both models in terms of the incident radiative flux from the jet exit. The new model significantly reduces the incident radiative flux related to horizontal jet fire when comparing with the values obtained using Phast v6.5. The difference is between -25% and -15% for the 8 kW/m 2 threshold and reaches -30% for the 15 kW/m 2 flux. This is due to the fact that the model flame shape takes proper account of the balance between the initial jet momentum and the buoyant forces, given bent shape to the flame, increasing the distance between the flame and the target and so that decreasing the incident radiative flux. On the other hand, the radiative flux related to vertical jet fire and computed by both models are relatively close (no more than + 15% of discrepancy).

CONCLUSIONS

The calculation of jet fires effects is essential when assessing the safety of highpressure processing of flammable materials. A new model has been developed for the jet fires occurring in large-scale industrial accidents.

Unlike more complex modeling approaches as CFD model, this model provides the capability to yield satisfactory predictions of the radiative flux from the jet fire with a little computer run time. Both at small and large scale, the predictions are in good agreement with the measurements for far field. Furthermore, the results computed with this model have been compared with those computed with semi-empirical approach based on the Cook's model [START_REF] Cook | A Comprehensive Program for Calculation of Flame Radiation Levels[END_REF]. This highlights a smaller incident radiative flux from the horizontal jet fire. A discrepancy up to 25% can be noticed. If this model is validated at large-scale, it can then be used to assess safe separating distances in industrial sites.

  surface area (m 2 ) force acting on volume control flame length (m) gravitational acceleration (m/s 2 ) enthalpy (J/kg) heat of combustion (J/kg) momentum (kg.m/s) mass flow rate (kg/s) molecular weight (kg/mol) pressure (Pa) incident radiative flux (kW/m 2 ) gas constant (J.K/mol) Richardson number temperature (K) velocity (m/s)

•

  region I is positioned as s/Fl < 0.4, • region II corresponds to 0.4 < s/Fl < 0.7, • region III represents the flame tip (s/Fl > 0.7).

Figure 2 .

 2 Figure 2. IR picture of the jet fire (INERIS experiments).

Figure 3 .

 3 Figure 3. Local air entrainment.

  .(0.21e-000323 "°+0.1l) M w > 60

Figure 4 .

 4 Figure 4. Comparison between measured and predicted results.

Figure 5 .

 5 Figure 5. Comparison of incident radiative flux between INERIS and Phast prediction.

Table 1 .

 1 Experimental overview.

	Reference	Johnson [1]	Cook and al. [3]		INERIS [10]
	Test	Test 1083, TypeC	3,2 GW	Test 1	Test 2	Test 3	Test 4
	Fuel	Natural Gas	Natural Gas	Propane	Butane
	Release Phase	Gaseous	Gaseous	Liquid	Liquid
	Mass
	flow Rate	8,4	65.1	1,5	4,1	1,5	3
	(kg/s)
	Release direction	Horizontal	Vertical	Vertical	Vertical	Vertical	Vertical
	Figure

Table 2 .

 2 Comparison between measured and predicted flame length.

		Flame length (m)
		Measured	Predicted
	Test 1083 [1]	33	30
	3,2 GW [12]	60	55
	Test 1 [10]	16	15
	Test 2 [10]	24	25
	Test 3 [10]	16	15
	Test 4 [10]	23	20

Table 3 .

 3 Scenario assumptions.

				Scenario	Mass flow rate [kg/s]
					1		15
					2		50
			PROPANE 15 kg/s				PROPANE 50 kg/s
	14 ^.12	\	2	Horizontal	^-Shell Model || -+ INERIS I	14 : 12 -
	10						10 -
	1 8-						8 -
	|e-						6 -
	ce 1 4 "	Vertical-20%-16%			4 -2 -
	£ o -				*•
			50	100	150	200		50	100	150	200
				BUTANE 15 kg/s				BUTANE 50 kg/s
	14						14 -
	ï!o-I 8 " I 6 -i 4 -						12 -10 -8 -6 -4 -
	? 2 -						2 -
	0		50 Distance from the jet exit (m) 100 150	200	0	50 Distancefrom the jet exit ( m ) D 100 150 200

Further work remains to be performed to validate this model, and to assess the ability of the model to predict a wider range of fires types, especially for horizontal and inclined jet fires. Large scale experiments with the horizontal and inclined releases will be carried out in the near future to validate this new model.


Wassila Benaissa
email: wassila.benaissa@ineris.fr

Douglas Carson Ineris

Comparison of the Results from Six Calorimeters in the Determination of the Thermokinetics of a Model Reaction

Keywords: calorimeter, thermal runway, kinetics

This paper deals with the comparison of experimental results from several types of commercially available calorimeters: a screening calorimeter (DSC), a Calvet calorimeter (C80), a reaction calorimeter (RC1), and various pseudo-adiabatic calorimeters (VSP 2, ARSST, and Phi-Tec 1). One exothermic reaction was selected as a case study: the esterification of acetic anhydride by methanol, a system which has been well studied in the literature. This reaction is put to the test in the six calorimeters under similar operating conditions. The objective of this series of tests is to obtain the thermo-kinetics of the reaction with each apparatus and compare the results. The goal is to determine which calorimetric method can be used (or not be used) for what purpose with respect to the reactive chemistry hazard. This approach gives also the opportunity to illustrate and summarize the potential of the experimental tools used in a complete risk assessment of a particular chemical process. The result can also be used in the education field as a simple and complete model of a case study.

Introduction

The catastrophic accident of Bhopal (India) in 1984 and the toxic cloud spread on Seveso (Italy) in 1976 are often used to illustrate the potential damages, destructions and number of victims caused by a chemical process accident. From the 80 's, numerous publications and studies have focused on the risk of thermal runaway in the chemical industry in order to prevent major explosion, fire and toxic dispersion (Gygax, 1988 ;[START_REF] Stoessel | Thermal Safety of Chemical Processes[END_REF]. Despite this effort and the number of risk assessment methodologies available, the reactive incidents are still a serious problem in the U.S and all over the world. For instance, more recently, in December 2007, a major accident due to a runaway reaction occurred in T2 Laboratories, Inc., a chemical manufacturer located in Jacksonville, Florida. This accident killed four T2 employees and injured 32. In the conclusions of its investigation report, the CSB confirms that the reactive incidents -such as T2 explosion -continue to occur. In this investigation, the CSB also explain that the most important thing to managing reactive chemistry hazard is to have a thorough and complete understanding of the chemistry of the process under normal operating conditions but also in the case of all potential failures of the system (CSB, 2009). Many data bases, theoretical methodologies and experimental devices are available. Moreover, it is always possible to find a process safety laboratory well equipped in calorimetric devices to test all kind of process configurations. So why is there a lack of knowledge of the thermal behavior of the chemical reactions that are carried out in the industry? One of the reasons could be that experimental studies stay obscure for the non specialists, and can represent a non negligible cost and investigation time. The aim of this paper is to illustrate and compare how the kinetic parameters of a chemical reaction can be extracted from different calorimetric techniques commercially available. The reaction studied has been deliberately chosen as a well known chemical system: the esterification of acetic anhydride by methanol, a system which has been studied in the literature. This reaction is carried out in six different calorimeters in similar operating conditions: screening calorimeter, DSC, isothermal calorimeter, C80, reaction calorimeter, RC1, pseudo-adiabatic calorimeters, VSP, ARSST and Phi-Tec 1. The objective of this series of tests is to obtain the thermo-kinetics of the reaction with each apparatus and compare the results. The goal is to determine which calorimetric method can be used (or not be used) for what purpose with respect to the reactive chemistry hazard. This approach gives also the opportunity to illustrate and summarize the potential of the experimental tools used in a complete risk assessment of a particular chemical process. The result can also be used in the education field as a simple and complete model of a case study.

Experimental methods

Chemicals

High purity methanol and acetic anhydride (>99%) were purchased from Merck.

Calorimeters

Since thermal risks of a chemical process is directly associated with the absorption or release of heat, calorimetry is a suitable tool to investigate the thermal safety of such processes. Since its inception by Lavoisier in the 18 th century, a vast choice of different calorimeters, using a variety of designs and measurement principles has been developed. A fairly complete description of the design and use of these calorimeters is given in the recent publication by Stoessel [START_REF] Stoessel | Thermal Safety of Chemical Processes[END_REF]. One can find different ways to classify calorimetric methods: the size of the sample, the design (for example with or without reference cell), the thermal sensitivity and the way of using the heat balance. In this study, six representative calorimeters have been chosen:

• Differential Scanning Calorimeter (DSC) -The DSC 131 commercialized by Setaram measures a heat flux difference between a few milligrams of sample and a reference, placed in a same temperature controlled oven and following the same scan rate [2]. It is used to detect exothermic and endothermic behaviors: phase transitions, heat capacity, chemical reactions/decompositions, etc. The test gives the thermal power and energy released by the sample [START_REF] Hôhne | Differential scanning calorimetry[END_REF].

• Calvet calorimeter -The C80 commercialized by Setaram is also a differential calorimeter that may be operated isothermally or in the scanning mode as a DSC [2]. It allows to work with more important sample volumes (several grams) and shows a higher sensitivity (around 0,1 W.kg ) essentially due to the measurement principales, based on a thermocouple pile totally surrounding the cells (sample and reference). As in the DSC, the test gives the thermal power and energy released by the sample [START_REF] Calvet | Recent Progress in microcalorimetry[END_REF].

• Reaction calorimeter -The RC1 commercialized by Metier Toledo is a 2 liters laboratory batch reactor designed to carry out the procedure as close as possible to plant operating conditions in a safe environment for the worker [5]. All actions (feed, mixing, safety procedure...) are controlled by dedicated software so that the operator can stay at a safety distance. The heat released by a reaction is then calculated by measuring the difference between the reaction mixture temperature and the jacket temperature. Three temperature programmed modes are available: isothermal (reaction mixture temperature is maintained constant by controlling the jacket temperature), isoperibolic (jacket temperature is maintained constant) and adiabatic (the jacket temperature is programmed to be equal to the reaction mixture temperature). This test gives access to the themokinetic behavior of the reaction mixture [START_REF] Riesen | Reaction calorimetry in chemical process development[END_REF].

Pseudo-adiabatic calorimeters -the objective of pseudo-adiabatic calorimeters is to reproduce as close as possible adiabatic conditions by limiting the heat exchanged with the environment. They are bench scale calorimeters, which can be used for simulating the way in which exothermic runaway reactions will be likely to occur in large-scale equipment. Indeed, in industrial practice, in the case of a chemical reactor failure (agitator or cooling system unexpected shutdown for instance), the reaction medium in instantly placed in adiabatic conditions. Depending on the technology, adiabatic conditions can be reproduced either by insulating the sample, or by heating the environment of the sample. In practice, part of the heat released by the reaction serves to heat the test cell. Thus, a correction of the thermal inertia is required. The correction is made by taking into account a correction factor, called 0-factor and expressed as:

m r -Cp r
Where m ce ii is the mass of the test cell, C p , ce ii is the heat capacity of the test cell, m r is the masse of the reacting sample and C p , r is the heat capacity of the reacting sample. Three apparatus were used in this study:

> The ARSST commercialized by Fauske uses 10 mL glass test cells. This cell is first wrapped by a cylindrical heater belt following the shape of the cell and then insulated with layers of foil and felt insulation. The test cell is then introduced in a 400 mL pressure vessel. The reaction mixture and a magnetic stirrer are introduced in the cell as well as a thermocouple. The pressure inside the vessel is measured by a pressure transducer [7].

> The VSP 2 commercialized by Fauske uses 110 mL stainless steel test cells with very thin wall which limits the thermal inertia. The test cell is placed in a first heater used to heat the sample. The assembly is then placed in a second heater, named guard heater, used to regulate the temperature around the cell. Thus the temperature around the cell is regulated to be equal to that of the reaction mixture temperature inside the cell continuously. The test cell is placed in a 4 L pressure vessel. The later and the cell have a pressure transducer. Two main configurations are possible: open and closed cell tests. When the cell is open, the pressure is the same inside the cell and in the pressure vessel. In this configuration, the gaz and vapor produced by the reacting mixture can escape from the cell to go in the pressure vessel. When the cell is closed, the pressure of the containment vessel is programmed to follow the pressure inside the test cell to avoid the explosion of the cell. In this configuration, the gaz and vapor produced by the reaction can potentially be dissolved in the reacting mixture [7].

> The Phi-Tec 1 commercialized by HEL uses 10 mL stainless steel test cells surrounded by metal guard heaters. The guard heater can track the temperature.

The test cell walls are thick so as to resist a pressure up to 200 bar. Thus no pressure compensation is needed [8].

The three apparatus give access in the temperature and pressure rise in pseudo-adiabatic conditions 3. Test results

Operating conditions

The esterification of acetic anhydride by methanol was carried out with operating conditions as close as possible for the six calorimeters. A mole ratio of 1:2 with an excess of methanol was chosen. Considering the critical temperature of methanol, the maximum temperature of the tests never exceeded 240°C. Main operating conditions used for each calorimeters are summarizes in Table 1.

Differential Scanning calorimetry (DSC)

Few grams of the reactive mixture is first prepared in a 50 mL glass flask and vigorously agitated with a magnetic stirrer during 2 minutes. 5.33 milligrams of the preparation are then taken and introduced in the crucible. The latter and the reference crucible are placed into the oven and heated at 5°C/min from room temperature to 210X. The test result gives the heat flow as a function of sample temperature (cf. figure 1). The baseline is used to integrate the heat flow and calculate the energy released by the reaction.

Calvet calorimeter (C80)

Few grams of the reactive mixture is first prepared in a 50 mL glass flask and vigorously agitated with a magnetic stirrer during 2 minutes. 813.9 milligrams of the preparation are then taken and introduced into the test cell. The latter and the reference cell are placed into the oven and heated at rC/min from room temperature to 210°C. The test results give the heat flow as a function of sample temperature (cf. figure 2). The baseline is used to integrate the heat flow and calculate the energy released by the reaction.

Reaction calorimeter (RC1)

The operating procedure is more elaborated in the RC1 which is carried out at a larger scale. First of all, 648 g of acetic anhydride is introduced into the 2 liter reactor. The agitator is then started at 100 rpm. The rotation speed is chosen in order to have the temperature sensor immersed in the acetic anhydride. The product is then heated at 60°C and the reaction calorimeter is switch to isothermal mode in order to stabilize the reaction temperature at this value during the entire experiment. 128.09 g of methanol is pumped into the reactor over a period of 2 hours and 45 minutes. For safety reasons, 20% less methanol was added compared to the other experiments. When methanol is first added, the jacket temperature slightly increases in order to maintain the reaction temperature constant, which means that an endothermic effect occurs. Subsequently, the jacket temperature decreases significantly, which means that an exothermic reaction occurs. During the feeding, a light reflux appears on the reactor walls. The reaction is considered complete when the jacket temperature is exactly equal to that of the reaction temperature. During the experiment, the monitoring screen displays the jacket temperature, the reaction mixture temperature and mass of methanol introduced in the reactor. After the experiment, the software calculates the heat and mass balance to give the heat flow and the thermal conversion (cf. figure 3). The baseline is used to integrate the heat flow and calculate the energy released by the reaction.

Advanced Accelerated Rate Calorimeter (ARSST)

Few grams of the reactive mixture is first prepared in a 50 mL glass flask and vigorously agitated with a magnetic stirrer during 2 minutes. 8.1 grams of the preparation are then taken and introduced in the test cell. After having wrapped and introduced the test cell in the vessel, the latter is pressurized at 20barg to avoid the evaporation of the reaction mixture at high temperature. The heater belt is then started to heat the sample at around TC/min. During the experiment, the temperature of the mixture and the pressure of the vessel are recorded. When the reaction occurs, a significant temperature and pressure rise are visible. When the reaction is over, the temperature follows again the temperature rate imposed by the heater belt. The test result gives the temperature and pressure rates (cf. figure 4).

Vent sizing package (VSP)

The test is performed in closed test cell. 80 mL of the reactive mixture is first prepared, vigorously agitated and directly pumped in the test cell already placed in the pressure vessel. The reaction mixture is first heated to 28°C at around 2°C/min. The VSP is then switched to adiabatic mode to track the exotherm. The test results give the temperature and pressure rates inside the test cell (cf. figure 5).

Phi-TecI

This test is performed in the closed cell test mode. 7.3 g of reaction mixture is introduced in the 10 mL test cell. The sample is then heated at around 0.2 T/min to 250X using the heater wrapped around the test cell. Any exotherm is detected and proceed to completion adiabatically with the guard heaterd tracking the sample temperature. The test results give the temperature and pressure rates inside the test cell (cf. figure 6).

a.

Figured: Phi-Tec-temperature and pressure rates vs. time 4. Kinetic calculation theory

4.i Kinetic equations

The objective is now to extract the kinetics from all results obtain from the six calorimeters. Depending on the calorimeter type, the information readily available is different: on one hand differential calorimeters (DSC, C80) and reaction calorimeter (RC1) give mainly the heat flow; on the other hand pseudo-calorimetric calorimeters give mainly temperature rate. The paragraph bellow recalls the basic theory and proposes for each case, simple equations to be used for the estimation of the kinetics parameters.

The esterification of acetic anhydride by methanol can be written:

A + B -> C + D [Eq. 1]
where A is methanol, B acetic anhydride, C acetic acid and D is methyl acetate. But in reality, the reaction mechanism is more complex and several reactions may be involved. Balland and al. [START_REF] Balland | Kinetic parameter of solvent-free reactions: application to esterification of acetic anhydride by methanol[END_REF] proposed the following reaction path:

A + B -> C + D [Eq. 2] A + C -> H 2 O + D [Eq. 3] B + H 2 0 -> 2C [Eq. 4]
The heat production corresponds to the heat release rate by the reaction. Therefore, it is proportional to the reaction enthalpy and to the reaction rate [START_REF] Stoessel | Thermal Safety of Chemical Processes[END_REF]:

q r = (r)V(-AH r ) [Eq. 5]
Where ris the reaction rate, Kis the volume reactor and AHris the reaction enthalpy.

With two reactants, the reaction can be expressed as:

r = k-CX-C? [Eq.6]
Where k is the reaction constant, C A is the methanol concentration, C B is the acetic anhydride concentration and n+m is the order of the reaction. The reaction constant is a function of the activation energy (E^ and the temperature (T) according to Arrhenius:

k = k 0 • e -£ a/( RT ) [Eq. 7]
The combination of equations 5, 6 and 7 gives:

q r = k 0 • e -£ «/( RT ) -C^-Cf-V-(-AH r ) [Eq. 8] q r = k 0 -e -£ «/( RT ) • n^ • ng 1 • (-AH r ) " V 1 ' 71 ' 711 [Eq. 9]
As methanol is in excess:

n A 0 = 2n Bo [Eq. 10]
And the equation of the heat release rate becomes, with ^as the thermal conversion:

q r = k 0 -e -£ «/( RT ) • ng+ m • (2 -x) n • (1 -x) m (-AH r ) • V 1 ' 71 -711 [Eq. 11]
It can also be written as a logarithmic expression:

ln ( (2 -^-,r) = /n ( fc ° • < m • ( -^) " V 1 -") ~ Y T Pq. 12]
Thus, when the heat flow and the reaction enthalpy are known, kinetics parameters can be estimated from equation 12. This is directly the case for DSC, C80 and RC1 but not for the pseudo-adiabatic calorimeters. For those, another expression can be used as demonstrated bellow.

The reaction enthalpy is calculated from:

= C p -AT ad [Eq. 13] Where C p is the specific heat of the reaction mixture and AT a dis the adiabatic temperature rise.

For pseudo-adiabatic calorimeter, the calculation has to take into account the phi-factor (<£):

AT,,* = (T, -T o ) • 4> [Eq. 14]

Where 7>is the final exotherm temperature and To is the exotherm onset temperature. From equation 6, reaction rate can also be written:

r = 2 n • k • C£+ m • (2 -x) n • (1 -x) m [Eq. 15]
If we assume that the fractional change in temperature is a direct measure of conversion, we can write the following equations:

x=^r [Bq.16] dC B _ dx _ 1 dT w 1R1 HT-C ^ Tt~C^ TpF 0 Tt [Eq.18] C B • -^ = 2» • k • Cr m • ( 2 -I^l) • (I^T [Eq. 19]
oo Tf-T 0 dt u ° \ Tf-T 0 j \Tf-T 0 ) Ŵe obtain two expressions of k:

In (fc) = In (2 n • k 0 • C^7 11 ' 1 ) -^ [Eq. 21]
The kinetic parameters can be estimated in this case from the temperature rate.

Experimental results

From DSC and C80 experiments, the integration of the heat flow gives the reaction enthalpy. The kinetic is then calculated from equation 12 by plotting In ( _ Jf r _ j versus . The slope gives the activation energy. Different values of n and m are tested. The best combination corresponds to the best linear line obtained. Figure 7 shows an example for the calculations made from the DSC test results. For the RC1 experiment, 20% less of methanol was added. The kinetic cannot be directly extracted with the same method.

For the ARSST, VSP and Phi-Tec experiments, the kinetic was calculated using equations 20 and 21 by plotting

In dT/dt 2T f -T Q -T >-T -) • versus
. The slope gives the activation

T f -T Q J \T f -T Q J
energy. Different values of n and in are tested. The best combination corresponds to the best linear line obtained. The calculation results are summarized in Table 2.

-2

Ln[q

Discussion and conclusion

It is possible to determine the enthalpy of reaction with the six calorimeters. With the exception of the results of the Phi-Tec I, the enthalpy of reaction determined by the different calorimetric techniques is -60 kj.mol 4 ± 4%. This value is about 9% less than that reported by Widell [START_REF] Widell | Autocatalytic behavior in esterification between anhydrides and alcohols[END_REF]. Comparing the results between the DSC and the C80 experiments, the onset temperature are both around 40°C and the form of the curves are similar. However, the peak temperature is lower for the C80 experiment because of the lower heating rate, TC/min as opposed to 5°C/min for the DSC. The enthalpy determined by the DSC experiment is about 5% less than the enthalpy determined by the C80 experiment. In principle, C80 is more precise because of the sample size and the more precise flux meter. For the pseudo-adabatic calorimeters, the main difference between the VSP and the Phi-Tec I experiments is in the sample size (74 g versus 7 g) and the CD factor. In terms of the results it can be noted that the maximum temperature attained in the Phi-Tec I experiment is substantially lower than the maximum temperature attained in the VSP (110°C versus 170X). In order to obtain the reaction enthalpy, it is necessary to correct the adiabatic temperature rise with the CD factor. It should be noted that the CD factor for the Phi-Tec I apparatus is much greater than one. In the two tests the filling factor was approximately the same. However the maximum pressure is much greater for the VSP experiment (18 barg for VSP versus 2.5 barg for Phi-Tec). The ARSST experiment is somewhat different from the two pseudo-adiabatic cited above in that the sample is heated during the entire experiment (lX/min). The overall adiabatic temperature rise is very close to that found in the VSP experiment. However, the pressure rise is difficult to compare to the closed cell experiments because the vapors fill the containment vessel at ambient temperature such that the vapors can condense on the walls. It is found that the pressure rise in the containment vessel is substantial and is closer to the overall vapor disengagement found in the VSP experiment. The RC1 reaction calorimeter operates on a different principle and the only value that can be compared with the other is the enthalpy of reaction.

Figure 1 :

 1 Figure 1 : DSC -heat flow vs. sample temperature

Figure 2 :

 2 Figure 2: C80 -heat flow vs. sample temperature

  Figure 4:

Figure 5 :

 5 Figure 5: VSP -temperature and pressure rates vs. time

Figure 7 :

 7 Figure 7: Kinetic modelling from DSC test results ,9

9 -

 9 AH r (kj.mor 1 of acetic anhydride)

Table 1 . Operating conditions

 1

		Acetic anhydride :	Mass	range
		Méthanol
	DSC	1 : 2 mol	5.33 mg	20-250X
	C80	1 : 2 mol	813.9 mg	20-250X
	RC1	1 : 0.6 mol	776.9 g	60°C
	ARSST	1 : 2 mol	8.1g	20-250X
	VSP2	1 : 2 mol	74.4 g	20-200°C
	Phi-Tec 1	1 : 2 mol	7.3 g	20-200°C

Calorimeter | Molar fraction | Test Sample | Temperature

Table 2 . Title of Table

 2

	Calorimeter	E a (kj.mol 1 )
	DSC	83.7
	C80	96.7
	RC1
	ARSST
	Phi-Tec I
	VSP II


Wassila Benaissa
email: wassila.benaissa@ineris.fr

Douglas Carson Ineris

Oxidation properties of "Solar Salt"

Keywords:

Solar Salt is a name sometimes given to a molten salt mixture made up of about 60% of sodium nitrate (NaNCh) and 40% of potassium nitrate (KNO3). This composition is near the eutectic point and is thermally stable until 600°C. It is popular in Industrial Solar Energy Projects and is used for storing energy in the form of heat to smooth out the peaks in electricity production. However for some technologies, combustible substances, like a thermal fluid for example, may come into contact with the molten salt. The aim of the paper is then to study the oxidizing properties of the solar salt in order to estimate the energy released by a potential reaction between the salt and combustibles and estimate safety issues. In that purpose, four types of experiments were carried out: the standard UN O.I test for solid oxidizers, the standard UN 0.2 test for liquid oxidizers, differential scanning calorimetry (DSC) and isothermal calorimetry (C80). The experimental program demonstrates the oxidizing properties of the solar salt and shows that the reactivity of solar salt with other combustibles has to be taken into account in a global risk analysis of a Solar Energy Central.

Introduction

Regarding the potentially dramatic effects of climate change and the estimated diminution of available fossil fuels, the development of new renewable energies have became a priority in research and development in order to anticipate the energy supply solutions of the future. Solar power, allowing conversion of sunlight into electricity, can be seen as one of the most promising ways [1]. In particular, during the last decades, concentrating solar power (CSP) has shown very interesting potentialities, through different sustainable power generation applications at industry scale [START_REF]Concentrating Solar Power -Why Renewable Energy is Hot[END_REF]. Four different technology approaches are currently used for concentrating solar power: sterling dish, linear reflector, parabolic trough and power tower. In each case, the heat source provided by the sunlight is captured then concentrated by mirrors. One of the largest advantages of CSP against other sources of renewable energy such as photovoltaic or wind is the capability to provide a more flexible repartition of the power: to do so, the solar energy can be stored in thermal reservoirs and released during periods of peak power demand, cloudy weather or even at night [START_REF] Skumanich | CSP: Developments in heat transfer and storage materials[END_REF]. Storage technologies can be either "direct" or "indirect". Indirect means that the storage medium is not heated directly by the concentrators. Indirect systems use a heat transfer fluid instead, typically synthetic oil, which passes through a heat exchanger with the storage medium to heat it indirectly. Typically the transfer fluid is synthetic oil and the storage medium is molten salts, generally nitrates. A practical example of this technology named Andasol 1 and implemented in Spain in 2008, use cool tanks (about 290°C) and hot tanks (about 390°C) of molten salts, with about 29,000 tones in each tank. In this configuration the storage capacity is about 1100 MWh that means about 7.5 h of full-load production of electricity and a very interesting benefit in terms of power supply duration [START_REF] Medrano | State of the art on hightemperature thermal energy storage for power generation, Part 2 -Case Studies[END_REF]. In the field of safety, one can ask the risks of storing such a quantity of chemical product on a same spot. Indeed, combustible substances, like a thermal fluid for example, may come into contact with the molten salt. The aim of the paper is to study the oxidizing properties of a solar salt in order to estimate the energy released by a potential reaction between the salt and combustibles and estimate safety issues. The molten salt chosen as a case study is a mixture made up of about 60% of sodium nitrate (NaNOs) and 40% of potassium nitrate (KNO3). Four types of experiments were carried out: the standard UN O.I test for solid oxidizers, the standard UN O.2 test for liquid oxidizers, differential scanning calorimetry (DSC) and isothermal calorimetry (C80).

Literature overview for the oxidizing properties

Pure nitrates

As the solar salt is a mixture of potassium and sodium nitrate, the reactivity of these two pure products has to be analyzed to understand the behavior of the salt. Potassium nitrate is an ionic salt, odorless and melting at 333°C. It is stable at ambient temperature and pressure and can safely be stored during a long period if protected from moisture. When heated, potassium nitrate produces oxygen (cf. Eq.l). This reaction is endothermic and reversible.

KNO3 <-> KN0 2 + V2 O 2 [Eq. 1]
Ullmann's Encyclopedia [START_REF]Ullmann's Encyclopedia of Industrial Chemistry[END_REF] gives a decomposition temperature equal to 530°C but smaller values around 400°C have also been published [START_REF]Hazards in the Chemical Laboratory[END_REF][START_REF] Kramer | Differential scanning calorimetry of sodium and potassium nitrates and nitrites[END_REF]. According to the Wiley Guide to Chemical Incompatibilities [START_REF] Pohanish | Wiley Guide to Chemical Incompatibilities[END_REF], potassium nitrate is noncombustible, but enhances the combustibility or oxidation rate of many materials and chemical reactions can cause fire and explosion. Sodium nitrate is an ionic salt, odorless and melting at 308°C. It is stable at ambient temperature and pressure and can be safely stored during a long period if protected from moisture. When heated, potassium nitrate produces oxygen (cf. Eq.l). This reaction is endothermic and reversible.

NaNO 3 <-> NaNO 2 + l A O 2 [Eq. 2]
Ullmann's Encyclopedia [START_REF]Ullmann's Encyclopedia of Industrial Chemistry[END_REF] gives a decomposition temperature around 500°C but smaller values around 380°C have also been published [START_REF]Hazards in the Chemical Laboratory[END_REF][START_REF] Kramer | Differential scanning calorimetry of sodium and potassium nitrates and nitrites[END_REF]. According to Wiley' Guide to Chemical Incompatibilities [START_REF] Pohanish | Wiley Guide to Chemical Incompatibilities[END_REF], sodium nitrate is non combustible, but enhances the combustibility or oxidation rate of many materials and chemical reactions can cause fire and explosion.

Classification

Potassium nitrate and sodium nitrate are classified 5.1 (oxidizing substances) by the "Recommendations on the Transport of Dangerous Goods" [START_REF]Recommendations on the Transport of dangerous Goods -Model Regulations[END_REF] and class 1 oxidizers by the U.S.

Hazardous Material Code, NFPA 400, Chapter 15, Oxidizer Solids and Liquids [START_REF]Hazardous Material Code, NFPA 400[END_REF]. The last classifies oxidizer materials according to their ability to cause spontaneous combustion and how much they can increase the burning rate. Of four classes, class 1 is the least dangerous and refers to substances which "slightly increase the burning rate of combustible materials" but "do not cause spontaneous ignition when they come in contact with them". The sodium nitrate and potassium nitrate mixture is also clearly identified in the classification 5.1 and UN number 1499.

Solar salt

Solar salt is composed of two powerful oxidizers, both stable until 400°C. The mixture of these two salts without impurity gives a eutectic which melts at 219°C. From this point, the solar salt becomes a transparent liquid which can be used as a thermal fluid. Some industrial applications shows that the operating temperature of a solar central can reach 400°C for the hot storage, for a quantity more than 20,000 tones. Regarding safety, one can ask two questions: is the solar salt thermally stable and what are its oxidizing properties?

Experiments and results

Experimental approach

Two calorimetric devices and two tests from the Transport of Dangerous Goods Manual [11] have been used: differential scanning calorimetry (DSC), Calvet calorimeter, test O.I for oxidizing solids and test O.2 for oxidizing liquids.

The calorimetric experiments are used to study the thermal stability of the solar salt and the reaction with combustibles. One important constraint was met for the UN standards: the tests and criteria are significantly different for solid and liquid oxidizers. Depending on the temperature, the solar salt state can be one or another. Some modification was then made to manage the two states.

Chemicals

High purity (99.99%) sodium nitrate (NaNCh) and potassium nitrate (KNO3) were purchased separately from MERCK. At ambient temperature, these two products are generally white crystal powders. The solar salt is then prepared by mixing these two powders. The solar salt was ground to less than 100 |im and tested as such.

A combustible oil containing biphenyl has been chosen to represent the heat transfer fluid. Pure potassium bromate has been used as reference for the 0.1 test.

DSC experiments

The DSC is a micro-calorimetric technique which measures a heat flux difference between a few milligrams sample and a reference, placed in a same temperature control oven and following the same scan rate (cf. picture 1). It is used to detect all kind of exothermic and endothermic behavior: phase transitions, heat capacity, chemical reactions/decompositions, etc. The test gives access in thermal power and energy released by the sample [START_REF] Hôhne | Differential Scanning Calorimetry and Introduction for Practitioners[END_REF].

Picture 1. DSC calorimeter

Crucibles Oven

Each few milligrams sample is introduced at ambient temperature in a stainless steel crucible. The crucible is then sealed and put in an oven with an empty crucible as reference. The two crucibles are heated at 5 K/min from ambient temperature up to 700°C. The heat flux and the energy released are then measured (see figures 1 to 7). Table 1 summarizes the results.

Temperature (°C)

500 TOO On figure n°l, the endothermic peak is characteristic of the melting point of the solar salt and it is thermally stable up to 650°C (cf. figure 1).

The cellulose decomposes at 300°C with an enthalpy of-588 J/g (cf. figure 2). When solar salt is mixed to cellulose, we observe a significant difference: the onset temperature is much less and the enthalpy is almost ten times larger. It seems that the solar salt enhances the decomposition of the cellulose (cf. figure 3).

We can reach a similar conclusion for the thermal fluid (cf. figure 4). The last is very stable until 545°C. When mixed with solar salt, the initial temperature decrease around 500°C and the enthalpy is also almost ten times larger (cf. figure 5 and6). The figure 7 confirms that with another inert product, no thermal activity is detected. These results illustrate the influence of solar salt acting like a typical oxidizer i.e. it increases the reactivity of a combustible substance when both are thoroughly mixed.

C80 experiments

The Calvet calorimeter is also a differential calorimeter that may be operated isothermally or in the scanning mode as a DSC. It allows working with larger sample volumes (few grams) and shows a higher sensitivity (around 0,1 W.kg" 1 ) essentially due to the measurement principal, based on a array of thermocouples totally surrounding the cells (sample and reference) (cf. picture 2). As in the DSC, the test gives thermal power and energy released by the sample and allows detecting exothermic activity [START_REF] Calvet | Microcalorimetrie: applications physic-chimiques et biologiques[END_REF].

Picture 2. C80 calorimeter

Oven

Reference cell

Two tests were carried out. Firstly, the cellulose is put in a closed cell and tested in DS mode at 0.5 K/min from ambient temperature to 300°C (cf. figure 8). Secondly a cell constructed of two compartments separated by a membrane is used: 300 mg of cellulose is introduced in the first compartment and 300 mg of solar salt is introduced in the second. During a first step, the two products are heated separately until 300°C. When the heat flux is stabilized, the membrane is broken and the two products mixed (cf. figure 9). When the mixing occurs, a highly exothermic reaction can be observed that is not detected when the cellulose is pure.

O.l test

The UN manual defines the 0.1 test as a method to measure the potential for a solid substance to increase the burning rate or burning intensity of a combustible substance when the two are thoroughly mixed (cf. picture 3). For the 0.2 test, a liquid is also considered as an oxidizer if the mixture made with the combustible substance can spontaneously ignite. Picture 3. UN O.I disposal 0.1 tests have been conducted on the solar salt mixed with dry fibrous cellulose in mixing ratio of 1:1 and 4:1, by mass of sample to cellulose. The burning characteristics of the mixture are compared with the standard 3:7 mixture (around 100 s), by mass of potassium bromate to cellulose. If the burning time is equal to or less than this standard mixture, the burning times should be compared with those from the packing group I or II reference standards, 2:3 (around 54 s) and 3:2 (around 4 s) ratios, by mass of potassium bromate to cellulose respectively. The results are summarized in table 1. The solar salt as tested is assigned to the class 5 (oxidizing substances and organic compounds), division 5.1 (oxidizing substances) and packaging group II (substances presenting medium danger), borderline packaging group I (substances presenting high danger). According to this test and criteria of the United Nation Manual, the solar salt has to be considered as a strong oxidizer.

0.2 test

According to the 0.2 tests procedure, the liquid to be tested is mixed to 1:1 ratio, by mass, with fibrous cellulose, poured into a pressure vessel with an exposed electric heating coil. The mixture heated (by action of the heating coil) in a pressure vessel and the rate of pressure rise is compared with those of reference substances. Since solar salt is a powder mixture at ambient temperature, the procedure had to be slightly adjusted:

• the pressure vessel was placed empty in an oven at 250°C overnight,

• a mixture of 2.5 g of solar salt with 2.5 g of cellulose was then prepared in a glass container.

• the pressure vessel was filled with the mixture to cover the ignition coil,

• the pressure was vessel was covered with aluminum foil and placed in the oven for 30 more minutes, • the pressure vessel was taken out of the oven and closed according to conventional procedure,

• the heating coil was energized.

To complete the approach, two other mixtures were tested in the exact same conditions: a mixture of solar salt and an inert powder, a mixture of solar salt and an inert liquid. Since the operating temperature is much higher than the conventional one, it is important to verify that the cellulose thermal behavior doesn't influence the results. Table 2 summarized the pressure rise obtained. Perchloric acid solution (55%) has been chosen as a typical reference for a solution classified 5.1 (oxidizing substances) and packaging group I: the mean pressure rise time for a 1:1 mixture with cellulose is 59 ms. Since the tests have not been carried out in the exact same condition as the one described in the UN manual, it is not possible to directly extrapolate the classification of the molten solar salt. However, the result shows a significant reaction when the solar salt and the combustible are mixed showing a potential influence of the salt on the combustion.

Conclusion

Despite the good knowledge of the hazard characteristics of the pure components of the solar salt, the oxidizing characteristics of the mixture were not clearly identified. The experimental program has demonstrated the oxidizing properties of the solar salt and shows that there is a potential important reactivity of solar salt with other combustibles. Since it is one of the technical options used in the solar centrals using concentrated solar power, this property has to be taken into account in a global risk analysis of a Solar Energy Central.

2 Figure 3 :Figure 4 Figure 5 :Figure 6 :Figure 7 :

 234567 Figure 1: DSC n°1 -100% solar salt

Figure 9 :

 9 Figure 8: C80 n°1 -100 % cellulose

  The characteristics of the cellulose used to perform the tests follow the recommendation of the manual of tests and criteria published by the United Nation for Transport of Dangerous Goods[11]. It is dried fibrous cellulose, with a fiber length between 50 and 250 |im and a mean diameter of 25 |im. It is dried in a layer no more than 25 mm thick at 105°C for 4 hours and kept in a desiccator until cool and needed for use. The water content is less than 0.5% by dry mass.

Table 1 . DSC test on Solar Salt, cellulose and thermal fluid

 1

		Component A	Component B	Tonset	Enthalpy	Enthalpy
						(°C)	(J/g)	(J/g
								combustible)
	DSCn°l	solar salt	100 %			stable
	DSC n°2			cellulose	100 %	300	-588	-588
	DSC n°3	solar salt	50%	cellulose	50%	240	-3054	-6108
	DSC n°4			thermal fluid	100 %	610	-545	-545
	DSC n°5	solar salt	85%	thermal fluid	15%	550	-707	-4799
	DSC n°6	solar salt	45%	thermal fluid	55%	540	-2454	-4490
	DSC n°7	potassium chloride	60%	thermal fluid	40%	stable
		6 -

Table 2 . UN O.I test on Solar Salt

 2

		Classification	Mixture	Molar
		limits		ratio
	Solar Salt		24 g solar salt / 6 g cellulose 15 g solar salt/ 15 g cellulose	4:1 1:1
		Packing Group I limit	18 g potassium bromate / 12 g cellulose	3:2
	Reference	Packing Group	12 g potassium bromate /
	mixtures	II limit	18 g cellulose
		Packing Group	9 g potassium bromate /
		III limit	21 g cellulose

Table 3 . UN O.2 test on Solar Salt Component A

 3

		Component B	Test n°l	Test n°2
	Perchloric acid (55%)	cellulose	59 ms (typical time taken from the UN manual. Solution 5.1 packaging group I)
	Solar salt	cellulose	23 ms	30 ms
	Solar salt	Inert powder	No significant pressure rise	No significant pressure rise
	Inert mineral oil	cellulose	No significant pressure rise	No significant pressure rise