Frédéric Masson -Jean-Philippe Pineau

EXPERT POINT OF VIEW ON THE INVESTIGATION OF LARGE ACCIDENTS

Accidents are taken into account in risk assessment and shall be a basis for knowledge to be considered for improving the safety level at the design and operation stages in various industries and activities (transportation). Unfortunately, in many cases, detailed data are difficult to be accessed. Lessons learnt from French accidents in plants and transportation will be developed. In the detailed investigation of accidents, emphasis should be given on the need to consider product(s), process, causes, circumstances, evolution (chain of events) and consequences. Some examples will be developed on involved materials and products, on accidental phenomenon, on the general design of a plant or transportation system, on the reliability of safety related equipment, on procedures and on operator's influence.

INTRODUCTION

For long, very large accidents occurred in explosive factories. Later, accidents were reported in flour mills (end of 18th century), in mines (coal dust and firedamp explosions), in chemical plants, during the transportation of dangerous goods such as fertilizers, explosives ... or even with very common substances such as fuels or grain.

In all cases, the consequences could be more or less extended : thermal effects from fire, toxic gas and fire fumes releases, toxic spills in soil and water.

Especially, when the extension of damage was large and when people died, various enquiries were undertaken :

• at the company level for a better understanding of the deficiencies of plant design and operation including human factor, management and maintenance • at competent authorities level for investigating the causes and for improving the existing regulation • at the insurance company level for defining the extent of the damages to be paid and for reevaluating the insurance premium for a given plant, industrial field or technical system ... Thus, investigations to be carried out have various goals, but the methodology could be the same. This paper is given as an attempt to summarize the experience gained by INERIS (and formerly CERCHAR) experts when accidents occurring with dangerous substances took place in a variety of plants and systems.

In general, this experience is based on direct investigation and analysis of accidents. In this paper, the logical steps to be used will first be given. Then, lessons learnt will be examplified.

INVESTIGATION OF ACCIDENTS

Logical steps for investigating and analysing an accident are the following 1. characteristics of the plant or system in which the accident occurred, 2. collection of data on damages, testimonies, analysis of samples, 3. definition of the operation when accident occurred, 4. consideration of all possible accident scenarios, 5. plausibility of a scenario.

Such steps will be illustrated in the oral presentation on the accident occurring at Blaye -France -on August 20th, 1997 where a grain dust explosion, initiated in a grain silo, was at the origin of 11 fatalities and of the collapse of silo cells.

Characteristics of the plant or system

Any information about the exact location of the plant (system) involved, other plants, buildings and communication routes in the vicinity, the operation procedures when the accident occurred, the types of products involved (if relevant), the presence and location of operators or other persons, are to be collected. At this stage, preventive and protective measures, existing regulations and codes of practices, emergency response should also be reported. In many instances, emergency response data collected can give valuable information about the development of an accident, the effectiveness of preventive and protective measures, but can also explain some major modifications of the plant system before the investigation by experts.

Data on damages and testimonies, sampling

Observation of damages is to be done as soon as possible, and if possible during the development of the phenomenon.

Any injury and its consequences (immediate, mid-or long-term effects), fatalities and ecological effects, are to be reported clearly including the locations and the exact dates. Photos, video films, witnesses interviews, distribution of fall-outs and missiles, plumes distribution after releases shall be collected to be considered later.

Samples shall be collected for further investigations about toxicity, combustibility, explosivity ... even modified substances or construction materials can be interesting as a proof of the fire development, thermal effects, involved materials ...

A lot of accident investigations proved that the system was neither designed nor operated in the conditions in which it was supposed to be. Any investigation must consider the exact design and operation of the system, the maintenance, the training of operator, the application of operating procedures and the management system, the possible interaction of third parties (passengers in transport systems), the staff actions, the exact nature of products (if relevant), the monitoring system (human or technical) and shall lead to a definition of the state of the system (with existing uncertainties) and should be related to the other data collected.

All possible accident scenarios

At this stage, no consideration is to be paid to the likelihood of occurrence of the sequence of events which constitute the possible accident scenarios.

The possible scenarios may be chosen from a preexisting list based on the possible phenomena related to the use of hazardous substances, or to the materials present in buildings or transportation vehicles. Mechanical effects from pressure vessels or containers under pressure, from operation of plant, should also be considered as potential scenarios : collision, derailment, sudden engulfment by fires or by fire fumes in a building. Only the main characteristics of scenarios are to be taken into consideration. In a further step, the causes and sequences of events related to these scenarios will be reported, checked and evaluated.

Some damages and testimonies can prove the likelihood of a scenario (bursting of a vessel or building for explosion, thermal effects with flame for fire, visible colored smoke for releases). At this stage, the collection of data from past accidents and incidents in the plant (system) or in similar plants and/or with similar products, can induce some ideas about the sequence of events.

For further investigations, an event tree for the considered damage or for a given phenomenon can be established.

In such event trees, comparison with facts and damages, operation conditions, products involved can focus the attention on the most logical and plausible scenarios.

Plausibility of a scenario

In explosion, the characteristics of products involved, the modelling of the phenomenon for defining the pressure levels are to be considered. In other cases, the thermal effects for fires, the gas or liquid releases flow calculations, and the dispersion effects can explain the extent of damages.

When results of tests or modelling are obtained, they shall be compared with data collected. Such an analysis is examplified in the summary report of Blaye accident (see annex 1).

The reports of such investigations shall be made available to a large public and especially stored in databases for further use by systems designers and operators, insurers, competent authorities.

Such public databases are in existence and are described in a Directory issued in 1997 by European Safety, Reliability and Data Association -ESReDA -.

An expert group of this organisation dealing with "Accident Analysis" is currently preparing a guidance document to be issued in 1999 for the improvement of the quality of data and will especially consider the role of the database operator and how to structure the database and to collect, record and validate data.

LESSONS LEARNT FROM ACCIDENTS

INERIS experts were directly involved in the following field investigations :

• an unconfmed vapour cloud explosion (UVCE) in a fuel depot at St Herblain, October 7, 1991 (J.F. [START_REF] Lechaudel | Assessment of an accidental vapour cloud explosion. A case study -Saint-Herblain[END_REF] with one fatality and extensive damages outside the depot premises • another major UVCE in the TOTAL refinery's fluid catalytic cracking unit at La Mède (November 9,1992) (P. Michaelis et al., 1995) with 6 fatalities and 2 billion francs damage • confined explosion in a grain silo, Blaye in 1997, 11 fatalities, 1 injured and extensive damages (F. Masson, 1998) • confined gas explosion in a plant connected to a flare stack (J.F. [START_REF] Lechaudel | Assessment of two accidents involving safety devices" -Loss Prevention and Safety Promotion in the Process Industries -Barcelone[END_REF] • a fire in a fuel depot at St Ouen, June 1991, with 15 fire fighters injured and the evacuation of 1.000 persons • ammoniac accident release (30 tons) in a chemical plant at Mazingarbe when unloading a railtank (December 16, 1994) (J.F. [START_REF] Lechaudel | Assessment of two accidents involving safety devices" -Loss Prevention and Safety Promotion in the Process Industries -Barcelone[END_REF] • fire and explosion following derailment of unleaded fuel railtanks on August 13, 1993 in La Voulte. 1.900 persons evacuated • fire in a NPK ternary fertilizer storage, Nantes, in 1987. 30.000 persons evacuated.

Most of these investigations were carried out at the request of French competent authorities, the rest directly for industrial firms.

Further details on other accidents investigated, either in chemical industries or in other industries are given elsewhere (J.P. Pineau et al., 1993).

From these accidents, many lessons can be drawn, either for the general design or the operation of the system, or on the efficacy of existing safety measures.

These field investigations allowed to draw some general assumptions.

The accidents at St Herblain (fuel depot) and La Mède (refinery) pointed out the influence of the location of ignition sources when flammable gases can be released and the absolute requisite of the definition of dangerous areas for choosing the adapted equipment.

The fire in the St Ouen fuel depot emphasized the role of repair and maintenance works and some difficulties for closing valves during fire fighting, mainly in relationship with the design.

A grain dust explosion as in Blaye, a rare occurrence with so extended damages and fatalities is dealing with dust, a by-product in grain elevators. The extension of fatalities is related to the location of offices and to the design of confined premises in which the explosion can propagate.

The case of Nantes accident (fertilizers) is an illustration of a dangerous release of toxic fumes from a fire and the necessary emphasis to be paid to proper distribution of goods in storages and use of adapted fire fighting methods.

The railtank accident in La Voulte is dealing with a transportation accident for which the geographical location of the occurrence can play a major role : explosion in a sewage system and pollution of the drinking water system.

The emission of toxic gases as in Mazingarbe and the explosion in a plant connected to a flare stack could be considered as an illustration of accidents in which the use of safety measures was not analyzed regarding their functionality and dependability : safety systems for avoiding releases in the first case and the running of utilities in the second case had poor reliability.

Within the cases of these accidents, only technical aspects were dealt with either for the design stage or for the operation stages in which safety measures are to be used. An improvement of the situation can only be obtained if the same level of experience is shared between manufacturers and designers of equipment and the users. This could be based on a risk analysis in which the intended use of the equipment (generally included in a process or a plant) is adequately defined, as well as the potential accidental scenarios with the hazardous zones concerned and the use of safety measures (including their reliability).

But more emphasis should be paid on organisational measures (safety management) and human behaviour.

All these aspects being dealt with, they could be introduced in a database in which a summary report can be the basis for further use.

CONCLUSION

The analysis of very large accident must follow logical steps :

• an understanding of intended use and operation procedures when the accident occurs • identification of all possible scenarios considering data on damage, testimonies, analysis of samples • choosing the plausible one considering all data collected in relationship with testing modelling of events and phenomena.

Annex 1 Grain dust explosion in a silo

August 20 th, 1997

Type of plant

In a grain elevator (port silo along the Gironde river), under operation (unloading grain from lorries and moving grain from a cell to another storage area through a conveyor belt), an explosion occurred.

Facts on accident

The explosion induced the collapse of more than half the 44 silos cells (30 m high and 6 m in diameter, storing wheat, barley and maize), and of the working tower, with blow out of debris 100 m around ; 11 people were killed and 1 injured. Minor damages to neighbouring houses occurred : glass breakages.

Immediate cause

The immediate cause of this explosion is still unknown, but reports from witnesses proved that the explosion initiated at the top of the working tower. Due to the presence of dust in the centralised dust collection system, it is supposed that the explosion initiated there and propagated to some cells, the working tower and other parts of the silo

The investigation proved no evidence of self heating of stored grain, excluding the formation of an explosive atmosphere of pyrolysed gas from grain stored in cells.

Extended damage does not allow to define the nature and location of the ignition source of grain dust air mixture. Friction through mechanical shocks in the ventilator upstream of the bag filters (centialised dust collection system), or self ignition in the dust collection chamber are suspected. The equipment was located in the upper storeys of the working tower.

Underlying causes

The propagation of the explosion in concrete confined structures was at the origin of the collapse of concrete structures. Location of offices in these concrete premises, too close to silo cells, was an aggravating factor and the cause of fatalities of people not directly related to the operation of the silo.

Dust was collected through bag filters in a dust chamber. The non enclosed bags were located at the top of the working tower. Such a device should be avoided.

Recommendation

Revision of the French legislation on grain storage (dated 1983) which will imply a risk assessment.


E Thybaud

UTILISATION DES MESOCOSMES POUR LE CONTROLE DES DANGERS LIES AUX PRODUITS CHIMIQUES

A l'heure actuelle, l'écotoxicité et le comportement des substances chimiques dans l'environnement sont évaluées à l'aide de méthodes de laboratoire conventionnelles et standardisées.

Sans remettre en cause l'intérêt de tels outils dans le cadre d'une démarche de classification des substances chimiques, force est de constater leur faible représentativité écologique. Ceci conduit à utiliser des facteurs d'incertitude plus ou moins importants lors de l'utilisation de ces résultats de laboratoire dans le cadre d'un processus d'évaluation des risques.

Dans le but de diminuer ces facteurs, les écotoxicologues ont été conduits à se rapprocher de leurs collègues écologues et à utiliser des moyens d'essais développés par ceux-ci depuis de nombreuses années, les mésocosmes, pour étudier les processus écotoxicologiques fondamentaux tels que l'influence des éléments minéraux nutritifs sur la productivité piscicole des eaux douces.

Les mésocosmes, du grec meso : milieu, et kosmos : monde, constituent selon la définition de Odum (1984), des écosystèmes artificiels placés dans des conditions environnementales naturelles. Ils présentent donc une complexité intermédiaire entre la simplification que représentent les essais de laboratoire et l'extrême complexité des écosystèmes naturels (figure 1).

Ces dispositifs expérimentaux permettent donc d'étudier de façon concomitante, les effets des contaminants ou de leurs produits de dégradation sur des populations ou des communautés simplifiées et le devenir et le comportement de ces contaminants dans des conditions propres aux biotopes naturels.

Ces installations permettent donc le contrôle de critères variables (nature du biotope, composition des communautés) au sein d'une expérimentation réalisée en milieu complexe.

25ème Congrès de toxicologie aquatique, Québec, 17-23 novembre 1998

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Des dispositifs de taille et de volume très variés ont été développés dans le but d'étudier les propriétés écotoxicologiques de divers contaminants potentiels de l'environnement (tableau 1).

Tableau 1 : Variabilité des dispositifs expérimentaux utilisés (d'après [START_REF] Kosinski | Artificial streams in ecotoxicological research[END_REF] Caractéristiques Nombre de canaux par expérience [START_REF] Heimbach | Methodologies of aquatic field tests : system design for field tests in still waters[END_REF].

En fait cette classification est basée sur le fait que la taille de l'installation va directement conditionner son degré d'autosuffisance, c'est à dire sa capacité à fonctionner sans apports extérieurs autre que les apports naturels (Voschell 1990[START_REF] Bélanger | Literature review and analysis of biological complexity in model stream ecosystems : Influence of size and experimental design[END_REF].

Ce degré d'autosuffisance est conditionné par la nature et la quantité des différents éléments nécessaires à un fonctionnement autonome sur une période de temps donnée.

La taille de l'installation va donc directement influer en terme de durée (court ou long terme) et de nature des effets (aigus ou chroniques) ou des populations et communautés étudiées.

Ainsi les installations de petite taille (1-5 m 3 ) sont adaptées pour des études à court terme (1 mois) prenant en compte des critères d'effets aigus (tableau 2) (SETAC Europe 1991, OCDE 1996). [START_REF] Lalli | Enclosed Experimental Marine Ecosystems : A Review and Recommendations[END_REF]).

La taille des dispositifs doit donc être un compromis entre la taille assurant une autosuffisance optimale et celle permettant une reproductibilité acceptable.

A l'heure actuelle, il peut être considéré que la taille optimale pour les études à long terme est comprise entre 10 et 20 m 3 pour les mares artificielles et entre 10 et 20 m de longueur pour les rivières artificielles.

La forme des dispositifs expérimentaux conditionne l'importance de l'effet de paroi pouvant entraîner par là même un facteur d'hétérogénéité non négligeable entre les diverses installations.

Ainsi les parois ont elles tendance à favoriser le développement d'organismes encroûtants tel que le périphyton. Cette communauté fixée est susceptible d'absorber une partie du polluant diminuant de ce fait la concentration en toxique dans le milieu étudié. Par ailleurs, le métabolisme de cette communauté modifie les caractéristiques de la colonne d'eau et en particulier la concentration en oxygène dissous et celles de divers éléments nutritifs.

Dans ces conditions, il est conseillé que les parois des mésocosmes soient légèrement inclinées de façon à limiter le développement de ces communautés.

COMPOSANTES ABIOTIQUES

Si la composition d'un mésocosme est fonction des objectifs de l'étude, il n'en demeure pas moins qu'un certain nombre de constituants s'avère indispensables. Il s'agit en particulier des composants du biotope que sont l'eau et les sédiments.

En ce qui concerne l'eau, son origine peut être diverse, eau du réseau de distribution, eau souterraine ou eau de surface.

Dans le premier cas, du fait de la présence de produits de traitement et en particulier de chlore, un vieillissement de 48 heures préalablement à l'introduction des organismes vivants doit être envisagé (SETAC Europe 1991).

Lors de l'utilisation d'une eau de surface, divers paramètres devront être pris en considération. Parmi ceux-ci, la teneur en particules en suspension devra faire l'objet d'un suivi et si possible d'un contrôle afin de limiter les phénomènes de sédimentation dus au plus faible débit de l'eau dans les mésocosmes [START_REF] Rodgers | Design and construction of model stream ecosystems[END_REF].

Par ailleurs, l'introduction d'eau de surface peut s'accompagner de celle d'organismes indésirables tel que des oeufs ou de jeunes alevins de poisson. De plus la présence dans ces eaux d'organismes planctoniques (phyto ou zooplancton) peut entraîner un facteur d'hétérogénéité entre les diverses installations. Dans ces conditions, une filtration préalable des eaux (SETAC Europe, 1991) et/ou un mélange de celle-ci avant la répartition dans les différents systèmes peuvent s'avérer indispensables (Touart, 1988 ;[START_REF] Hill | Freshwater Field Tests for Hazard Assessement of Chemicals[END_REF].

L'utilisation de sédiments naturels provenant d'un écosystème non contaminé fait maintenant l'objet d'un consensus (SETAC Europe, 1991 ;OCDE, 1996).

En effet, du fait de leurs caractéristiques physico-chimiques et biologiques, leur utilisation rend les mésocosmes rapidement fonctionnels.

En revanche, une caractérisation physico-chimique et structurale de ces sédiments est nécessaire avant toute utilisation.

Une attention toute particulière doit être apportée à la répartition granulométrique des sédiments, celle-ci ayant une influence non négligeable pour la colonisation par certains invertébrés.

Bien que présentant de nombreux avantages, l'utilisation de sédiments naturels n'est pas sans poser divers problèmes et en particulier ceux relatifs à leur disponibilité en quantité suffisante dans le temps et ceux concernant la présence au sein de ceux-ci d'organismes pouvant entraîner du fait de leur prolifération, un déséquilibre dans la structure de l'écosystème.

Pour tenter de limiter ces phénomènes, l'utilisation d'un mélange de sédiment naturel et de sédiment artificiel pourrait être un bon compromis.

La hauteur de sédiment introduit est fonction des installations envisagées. Dans tous les cas celle-ci devra être supérieure à 5-10 cm (SETAC Europe, 1991 ;OCDE, 1996).

COLONISATION

Au sein des mésocosmes doivent être présentés, outre des populations animales ou végétales nécessaires aux objectifs de l'étude, des organismes représentatifs des écosystèmes aquatiques naturels simulés.

Ainsi au niveau de la flore, devront être présentés des populations représentatives du phytoplancton naturel. Celui-ci sera soit introduit sous forme d'inoculum soit fera l'objet d'une colonisation naturelle.

Les macrophytes quant à eux seront soit introduits avec le sédiment naturel soit plantés.

Dans tous les cas, ces organismes ne devraient pas occuper plus de 25% de la masse d'eau. Si un faucardage de ces végétaux est nécessaire il devra être réalisé dans la mesure du possible avant tout traitement (SETAC Europe, 1991).

Les végétaux flottants type lentille d'eau ne devront pas occuper plus de 1% de la surface de l'installation au moment du traitement.

En ce qui concerne la communauté zooplanctonique, celle-ci devra renfermer des populations de rotifères et de crustacés (cladocères et copépodes) (SETAC Europe, 1991).

Les macroinvertébrés quant à eux seront représentés dans la mesure du possible par des annélides, des crustacés, des gastéropodes et des larves d'insectes.

Enfin, lorsque le volume des installations le permet des populations de poissons pourront être introduites dans les dispositifs. Celles-ci feront l'objet soit d'étude de mortalité, dans le cas des expérimentations court terme (< 1 mois) soit d'étude de croissance.

Dans tous les cas, la charge biotique ne devra pas excéder 50 g de poisson par m 3 d'eau (SETAC Europe, 1991) et il faudra veiller à ce que la pression de prédation induite par ces organismes ne mette pas en péril l'équilibre des communautés vivantes présentes dans les mésocosmes.

PHASE DE MATURATION

Préalablement au démarrage de l'étude proprement dite, une phase de maturation des différentes installations est nécessaire. Celle-ci a pour objectif l'homogénéisation des paramètres physico-chimiques et des communautés vivantes au sein des divers dispositifs.

Sa durée est fonction de la taille de l'installation (Figure 2), celle-ci s'échelonne de quelques heures à plusieurs mois pour les installations les plus grandes, le temps médian étant d'environ 50 jours. Selon les experts de la SET AC, la densité des communautés de micro et macro-invertébrés ainsi que celle de producteurs primaires se stabilise en 2 à 4 semaines.

APPROCHES EXPERIMENTALES

Deux approches expérimentales reposant sur des analyses statistiques différentes peuvent être envisagées, l'une par régression, l'autre par analyse de variance.

L'approche par régression est appropriée lorsque la relation dose-réponse est de type monotone. Cette méthode n'impose pas l'emploi de réplicats pour chaque traitement. En revanche, elle nécessite qu'une réponse soit enregistrée pour au moins trois doses différentes.

L'utilisation de l'analyse de variance est conseillée pour comparer différents produits entre eux ou lorsque la relation effet-dose n'est pas monotone.

Les résultats obtenus pour chaque traitement sont alors comparés directement avec ceux enregistrés dans les milieux témoins. Un minimum de 3 réplicats par concentration est nécessaire pour une telle analyse.

Dans tous les cas, la répartition des mésocosmes traités et non traités sur la plate forme expérimentale doit se faire de façon aléatoire.

CONDITIONS DE TRAITEMENT

La méthode de contamination utilisée doit dans la mesure du possible être représentative de la voie d'entrée du contaminant dans l'environnement. En effet, le mode d'introduction du polluant dans les mésocosmes peut avoir une grande influence sur le comportement, le devenir de la substance ainsi que sur l'exposition des organismes.

Si l'objectif de l'expérimentation est l'étude de l'influence du ruissellement, le contaminant sera introduit sous forme d'un mélange avec de l'eau et du sol. Le ratio soleau devra préférentiellement être basé sur le ratio susceptible d'être rencontré en milieu naturel.

Ce mélange sera introduit en un ou plusieurs points du mésocosme.

Que ce soit en mésocosme lotique ou lentique, l'introduction du contaminant peut se faire sous forme unique ou répétée.

En mésocosme lotique, l'introduction unique correspond à une exposition des organismes au contaminant pendant une courte période de temps, ce qui sauf dans le cas des composés très toxiques n'entraînera que peu d'impact sur ces organismes. Dans la majorité des études, c'est donc un mode de contamination chronique qui est utilisé.

En mésocosme lentique, l'introduction unique sera utilisée pour l'étude d'un phénomène aigus, tandis que l'introduction répétée le sera dans le cas de l'étude de phénomènes chroniques.

Le choix des concentrations d'étude est fonction de l'objectif de l'étude, impact sur l'ensemble des communautés animales et végétales ou impact sur une communauté particulière.

Dans le premier cas, les concentrations seront déterminées en fonction des concentrations prévisibles dans l'environnement, celles-ci étant déterminées soit à partir de modèle de devenir dans l'environnement [START_REF] Mackay | A quantitative water, air, sediment interaction (QWASI) fugacity model for describing the fate of chemicals in lakes[END_REF]) soit à partir des concentrations de rejets dans l'environnement.

Dans le cas d'une étude de l'impact sur une communauté donnée, les concentrations d'étude seront choisies en fonction du mode d'action de la molécule [START_REF] Graney | Aquatic Mesocosms Studies in Ecological Risk Assessment[END_REF].

DUREE DE L'EXPERIMENTATION

La durée de l'expérimentation est fonction des objectifs de l'étude (tableau 4), et du type de système expérimental utilisé (figure 4).

Dans la mesure du possible, la durée d'expérimentation sera déterminée en fonction des informations disponibles sur la substance étudiée, telles que toxicité et taux de dégradation biotique et abiotique.

Celle-ci est également dépendante du mode et de la fréquence d'application de la substance.

Dans tous les cas, l'expérimentation devra si possible se poursuivre sur une période suffisamment longue pour pouvoir étudier les effets directs et indirects de la substance.

Tableau 4 : Critères d'étude en fonction de la durée (d'après OCDE, 1996 ;SETAC Europe, 1991) Etude court terme (^ 1 mois) Etude long terme (> 6 mois) -Toxicité aiguë -Devenir de la substance -Toxicité chronique -Devenir de la substance

CRITERES DE VALIDITE

L'utilisation de telles études dans un cadre réglementaire nécessite un consensus large autour des critères de validité de celles-ci.

A l'heure actuelle, seule l'OCDE a proposé de tels critères et ce uniquement dans le cas des mésocosmes lentiques.

Selon les experts de l'OCDE, une étude en mésocosme peut être considérée comme valide si :

• Une relation effet-dose est observée,

• Une concentration sans effet par rapport au témoin est utilisée,

• La mesure de la substance au cours du temps est réalisée,

• Les paramètres physico-chimiques de l'eau sont déterminés,

• Et enfin, si la durée de l'étude est suffisante pour étudier la résistance des organismes vivants.

EXEMPLES D'UTILISATION

De très nombreuses études en mésocosmes ont été réalisées jusqu'à présent. Ainsi Bélanger en 1997 a t-il pu en référencer 276.

Celles-ci concernent de très nombreuses substances telles que des HAP, des pesticides, des détergents, ou des métaux (zinc, cuivre, aluminium, cadmium) ou des effluents industriels.

PLACE DES MESOCOSMES DANS LA PROCEDURE D'EVALUATION DES RISQUES

A l'heure actuelle, l'évaluation des risques liés à l'utilisation des substances chimiques est réalisée au moyen d'une comparaison entre la concentration prévisible dans l'environnement (PEC) et la concentration prévisible sans effet (PNEC), le rapport PEC/PNEC permettant d'évaluer le niveau de risque potentiel.

La PNEC est actuellement évaluée à partir de données de toxicité issues d'expérimentations de laboratoire réalisées dans des conditions conventionnelles et standardisées sur un petit nombre d'espèces considérées comme représentatives des écosystèmes aquatiques. Ces résultats sont ensuite pondérés par des facteurs de sécurité fonction de la nature et du nombre de données disponibles (Tableau 5). Le réalisme écologique de ces dispositifs est la question centrale dans le cadre de leur utilisation à des fins d'évaluation des risques. En effet, celui-ci doit être le plus important possible tout en permettant une répétabilité des expérimentations.

Ainsi, si les dispositifs de grande taille présentent un réalisme écologique important, en revanche leur utilisation et la réalisation de réplicats sont particulièrement délicates.

A l'inverse, il est plus aisé de réaliser des études en réplicats sur de petites installations mais leur degré de réalisme écologique est faible [START_REF]An Outdoor Replicated Artificial Stream System : Design, Operating Conditions, and Initial Invertebrate Colonization[END_REF][START_REF] Kosinski | Artificial streams in ecotoxicological research[END_REF].

Les mésocosmes peuvent également être utilisés pour améliorer les modèles de devenir et de comportement des substances chimiques en milieu aquatique [START_REF] Hommen | Application of a plankton simulation model to outdoor-microcosm case studies[END_REF]Ratted al, 1994).

La question de leur utilisation dans le cadre réglementaire reste une question ouverte.

Ainsi si le recours obligatoire à de telles expérimentations lors de la mise sur le marché de produits phytosanitaires a été suspendu en 1992 aux Etats-Unis du fait des nombreux problèmes économique et scientifiques posés par leur utilisation (Shaw et Kennedy, 1996) il n'en demeure pas moins que leur utilisation permet d'introduire une dimension écologique dans l'évaluation du risque par une substance chimique.

Dans tous les cas, ce type d'expérimentation ne devra être utilisé qu'en deuxième intention à la suite de la mise en évidence d'un risque potentiel en milieu aquatique.

EN CONCLUSION

Figure 1 :

 1 Figure 1 : Les différents modèles d'études utilisés en écotoxicologie

Figure 2 :

 2 Figure2: Durée de la phase de maturation en fonction de la taille des installations[START_REF] Bélanger | Literature review and analysis of biological complexity in model stream ecosystems : Influence of size and experimental design[END_REF]

Figure 3 :

 3 Figure3: Evolution du nombre de taxons en fonction du temps de maturation[START_REF]An Outdoor Replicated Artificial Stream System : Design, Operating Conditions, and Initial Invertebrate Colonization[END_REF]

Figure 4 :

 4 Figure 4 : Durée des expérimentations en fonction de la taille des installations[START_REF] Bélanger | Literature review and analysis of biological complexity in model stream ecosystems : Influence of size and experimental design[END_REF]

Tableau 5 :Facteur

 5 Facteurs d'incertitude pour la détermination de la PNEC Nature des données 3 données de toxicité aiguë (CL50) (algue, daphnie, poisson) 1 donnée toxicité chronique (NOEC) (daphnie ou poisson) 2 données de toxicité chronique (NOEC) (algue et/ou daphnie et/ou poisson)3 données de toxicité chronique (NOEC) sur 3 espèces représentatives de trois niveaux trophiques être utilisés dans ces conditions pour diminuer les facteurs d'incertitudes employés, dans la mesure où ils permettent de déterminer les effets de ces substances simultanément sur un grand nombre d'espèces dans des conditions environnementales plus réalistes.

  Le premier aspect à prendre en considération est celui de la taille des dispositifs expérimentaux. Celle-ci est souvent prise en compte comme critère permettant de différencier les mésocosmes des microcosmes. Ainsi il est classiquement admis que le terme mésocosme est réservé aux installations présentant un volume supérieur à 15 m 3 pour les mares artificielles et à 15 m pour les rivières artificielles(SETAC Europe 1992,

		Moyenne ou pourcentage
		4 (1-32)
	Nombre de traitements	3 (1-12)
	Longueur des canaux	7 m (1-100)
	Vitesse du courant	10 cm s" 1 (0-74)
	Habitat	50% courant et calme
		50% habitat unique
	Circulation d'eau	59% système ouvert
		22% recirculation complète
		19% recirculation partielle
	Situation	60% extérieur
		26% intérieur avec lumière artificielle
		14% intérieur sous serre
	Temps de maturation	50 jours(7-1095)
	Exposition	86% continue
		14% ponctuelle
	Organisme	58% macroinvertébrés
		54% périphyton
		23% poissons
	En ce qui concerne les mares artificielles, les volumes varient de 2 à plus de 1 000 m 3
	tandis que pour les rivières artificielles les longueurs varient de moins de 1 mètre à plus
	de 500 m.
	Cette variabilité de conception recouvrant des situations environnementales très diverses
	conduit à une difficulté extrême d'une part d'interprétation et d'autre part d'utilisation
	des données issues de telles expérimentations.
	Dans ces conditions, une standardisation des paramètres structurants majeurs est
	nécessaire.

A l'heure actuelle, deux tentatives ont été conduites, l'une par la SET AC Europe (1992) (Society of Environmental Toxicology and Chemistry) l'autre par l'OCDE (1996) (Organisation de Coopération Economique et de Développement). Dans les deux cas, les travaux ont porté sur des dispositifs d'eaux stagnantes ou mésocosmes lentiques.

  Bien que d'utilisation délicate, les mésocosmes présentent un intérêt incontestable dans le cadre d'une part de l'évaluation des risques et d'autre part de l'acquisition de connaissances fondamentales sur le fonctionnement des écosystèmes aquatiques.Néanmoins, en vue de leur utilisation dans le domaine réglementaire, il est nécessaire de tirer les enseignements du passé afin de pouvoir proposer des installations et des protocoles d'études plus rationnels et standardisés. static field mesocosm tests ». Held at Monks Wood Experimental Station Abbotts Ripton, Huntingdon, UK. 3-4 July 1991.SETAC-Europe (1992). Proceedings of a workshop on aquatic microcosms for ecological assessment of pesticides (Wintergreen, Virginia, USA, Octobre 1991). SET AC Foundation for Environmental Education and the RESOLVE Program of the World Wildlife Fund. Shaw J.L. etKennedy J.H. (1996). The use of aquatic field mesocosm studies in risk assessment.Environ. Toxicol. Chem. 15, 605-607. Touart L.W. (1988). Aquatic Mesocosm Test to Support Pesticide Registrations. Hazard Evaluation Division Technical Guidance Document. U.S. E.P. A., Washington, Rept n°U S-EPA/540/09-88-035, 35 pp. Voschell J.R. Jr. (1990). Introduction and overview of mesocosms. In : Cuffney T.F. (éd.), Experimental Ecosystems : Applications to Ecotoxicology, Technical Information Workshop, North American Benthological Society, Blacksburg : Virginia Polytechnic Institute and State University, 14 pp.

Dans tous les cas, dès lors que ces effets de parois sont prévisibles, ils doivent être pris en compte dans le schéma expérimental.Enfin la réalisation de ces dispositifs expérimentaux fait appel à des matériaux de construction de natures diverses, tel que films de caoutchouc, de polyethylene ou de PVC dans le béton ou de fibres de verre.Quel que soit le matériau choisi, il convient de tester a priori sa nocivité vis-à-vis des organismes vivants ainsi que son influence sur le comportement du polluant testé.En effet, certain matériaux comme le caoutchouc ou certains PVC sont susceptibles de relarguer dans l'eau divers composés plus ou moins toxiques tel que le zinc pour le caoutchouc ou le cadmium et des phtalates pour certains types de PVC.


Research on an Effect of a Short Term Exposure to Low Concentrations of Ozone on V79 Cells Using the Comet Assay

Introduction

During episode of atmospheric pollution in industrialized areas, the ozone concentration can reach up to 0.35 ppm for a few hours. These concentrations are beyond the protection thresholds for human health and vegetation recommended by EU directive 92/72 and based on the guidelines of the World Health Organisation.

Ozone is not by itself a radicalar species, but most of its toxic effects are thought to result from radicalar reactions mediated by .OH or by nucleases activation. So far, most of the genotoxic effects reported have been observed in vitro at concentrations in the range of 1 to 10 ppm during 1 hour [START_REF] Victorin | Review of the genotoxicity of ozone[END_REF], that is higher than the actual individual exposure.

Recent studies performed in vitro or in vivo at lower concentrations, namely between 0.2 and 1 ppm, have shown that the genotoxic effect of ozone is questionable [START_REF] Kozumbo | Products of ozonised arachidonic acid potentiate the formation of DNA single strand breaks in culture human lung cells[END_REF][START_REF] Madden | The use of the single cell gel electrophoresis assay in detecting DNA single strand breaks in lung cells in vitro[END_REF][START_REF] Gabrielson | Comparaison of the toxic effect of hydrogen peroxide and ozone on cultured human bronchial epithelial cells[END_REF]. Therefore, in an attempt to mimic more closely the environmental conditions, we have studied the consequences on cultured cells of an exposure to low concentrations of ozone for a short time. For this purpose, we used the V79 cell line for which extensive data are available regarding the genotoxicity of numerous compounds. Genotoxicity assessment using this cell line has been frequently done with the micronucleus and the comet assays. Due to the oxydative stress induced by ozone we decided, in a first instance, to use the comet assay which is considered as more relevant to assess efficiently this kind of injury.

Methods

The cell line used is V79 cultured in D-MEM medium supplemented with 20 % of foetal calf serum.

Treatments were performed at 37°C during 1 hour on cells at confluency. H 2 O 2 was used as a positive control at concentrations of 75, 150 and 300 uM. Ozone was produced by photolysis of an oxygen flow (Sonimix 6023, LNI).

_ nn -

TEC-LBTV-MBs-99DL0200.DOC Ozone concentrations were measured with an ozone analyseur (O 3 41M Environnement SA). The chambers of exposure were self-contained and regulated for CO 2 , temperature and humidity. For ozone treatment, V79 cells were cultivated on petri dishes set on oscillating plates, in order to facilitate direct contact between ozone and cells.

The concentration of ozone were: 0.1 ppm, which corresponds to usual urban exposures and 0.3 ppm which is representative of an acute environmental exposure. A higher dose (0.9 ppm), irrelevant to reported exposures, has also been included in the study. Ozone was also used in association with HNO 3 (8000 p.M) in order to mimic some aspects of NO 2 exposure.

Cytotoxicity was assessed with MTT or neutral red assay and cell viability was verified before each comet assay by the trypan blue dye exclusion test.

For the comet assay, the cells were embedded in low melting point agarose at 1 % and after cell lysis at pH 10 and DNA denaturation at pH 8, an electrophoresis was performed [START_REF] Fairbairn | The comet assay : a comprehensive review[END_REF]. DNA was stained by propidium iodide before microscopic analysis. Tail moments were measured by image analysis. Image analysis system correspond to a CCD monochromatic camera adapted on a DMLB 100 microscope (Leica) and driven by FLUOVIEW MORPHOSTAR software (IMSTAR). The value of the tail moment was defined by [START_REF] Olive | Heterogeneity in radaition-induced DNA damage and repair in tumor and normal cells measured using the " comet " assay[END_REF] as a product of the percentage of DNA in the tail distribution and the displacement between the head and tail means.

Results and discussion

No toxic effect was detected for H 2 O 2 after a 1 hour exposure at 75, 150 or 300 uM. A linear dose relationship was observed in the comet assay for the positive control for the doses up to 300 uM (Figure 1). These results are in agreement with those previously reported on V79 cells or on human fibroblasts by Olive et ai, 1994 andSingh et al, 1991 respectively. For ozone exposures at 0.1, 0.3 and 0.9 ppm no toxic effect was seen with both MTT and neutral red assays.

Results obtained from the comet assay showed that ozone did not induce any DNA strand breaks for concentrations close to environmental exposures (0.1 ppm). For higher doses (0.3 and 0.9 ppm) no significant effects were shown under our experimental conditions by comparison with unexposed cells (Figure 2). Similar results were observed by alcaline elution on BEAS-2B cells after an exposure to ozone between 0.2 and 1 ppm during 2 to 4 hours [START_REF] Gabrielson | Comparaison of the toxic effect of hydrogen peroxide and ozone on cultured human bronchial epithelial cells[END_REF].

TEC-LBTV-MBs-99DL0200.DOC Others studies on BEAS-2B cells using the comet assay, have shown that there is a significant increase of effects for 40 or 60 minutes exposure at 0.4 ppm or 1 hour at 0.1 ppm [START_REF] Madden | The use of the single cell gel electrophoresis assay in detecting DNA single strand breaks in lung cells in vitro[END_REF]. These results seem to confirm the hypothesis of differences in cell sensitiveness.

Adjunction of HNO 3 , without modification of the pH of the medium during ozone exposure, did not change the result of cytotoxic evaluation. By comet assay, increasing concentration of HNO 3 up to 8 mM did not induce any genotoxic effects (Figure 3).

Conclusions and perspectives

Comet assay appear to be fairly sensitive in the detection of genotoxic alterations induced on V79 cells by the oxydative stress produced by a 1 hour exposure to H 2 O 2 , Under the same experimental conditions, ozone alone or in association with HNO 3 did not induce any effects. As in the case of oxygen peroxide, ozone is known to induce DNA strand breaks through the radicalar reactions that are mediated by oxygen species. In this study we worked at low levels of ozone concentrations and short time of exposure (1 hour). One can assume that a prolonged exposure to ozone would have induced more DNA damages and in addition would be more relevant in order to mimic an environmental exposure. In the same way we have chosen not to modify the pH of the medium during HNO 3 exposure but these conditions are not representative of the reality of this kind of exposure since in the lung, a pH value may be around 6.8. The V79 is a well known cell line but it is not the most representative of the lung which is build up of a large variety of different cell types. Other cell lines such as BEAS-2B or epithelial cells in primary culture should be more representative. All these questions are currently under investigations.

  22

Figure 1 :Figure 2 :Figure 3 :

 123 Figure 1 : Tail moments measured by the comet assay after a 1 hour exposure to H 2 O 2


Emmanuel Ruffen

André Carrau

Claude Cwiklinski

NUMERICAL SIMULATION OF LOCOMOTIVE FIRES IN THE L YON-TURIN TUNNEL

Among the ventilation studies for the tunnel of the Lyon-Turin project, located between St Jean de Maurienne in France and Susa in Italy, a particular work has been devoted to locomotive fires and smoke propagation. The purpose of this study was to bring information on the outcome ambient conditions for passengers during a locomotive fire and to confirm the ventilation strategy worked out by Alpetunnel who is in charge of the project. Furthermore the study has been focused on the thermo-aeraulic and physicochemical conditions in the tunnel during a rear locomotive fire. For that INERIS has adopted a deterministic point of view based on lessons issued from the past and results of research projects like Eureka Firetun Project. This study is divided into three parts : bibliography and search for data on materials and locomotives fires, calculation of few probable fires with various possible ignition locations and simulation of the combustion products propagation for the two most probable fire scenarios. Results show that the escape conditions in the tunnel during the fires are acceptable for the two studied scenarios.

INTRODUCTION

Generally rail tunnels are not the cause of accidents except when the loss of their structures' integrity is the cause. In fact, the major risk in tunnel is due to the possibility of effects aggravation induced by the enclosed space. Among the possible accidents, fires can induce very problematic evacuation situations and a lot of difficulties for the rescue access and operations.

All over the world, important accidents have occurred, with or without victims and they widely justify the research projects such as EUREKA FIRETUN 499 (1) or MEMORIAL TUNNEL (2).

Concerning rail tunnels, fires are frequently rolling-stocks fires. Many studies have been conducted on coach fires but few results are published on locomotive fires and their consequences (3). It is true that locomotives have often got security systems for fire detection and extinction and then consequences of these fires are generally reduced to incidents causing only little harm. Nevertheless a fully developed locomotive fire can alter the mobility of a First International Conference and International Seminar « Tunnel Fires and Escape from tunnels », Lyon, 5-7 mai 1999.

train and therefore the consequences have to be studied as one of the major scenarios for the safety in rail tunnels.

LOCOMOTIVE FIRE MODELLING

This study is based on characteristics of a TGV's locomotive type. As a first hypothesis we consider that a fire ignites in the rear-locomotive and the train has to stop in the tunnel. After the train has stopped smokes are convected towards the passenger coach by the ventilation system. Note this direction of propagation is the same than the one induced by the piston effect in the case of a single way tube. In this study we assume that the fire effects have begun when the piston effects have stopped. At this time only the rescue ventilation is working.

Figure 1 The materials can be distributed in different ways :

-localised in a defined volume : « box » which can be separated from other empty « boxes », -homogeneously distributed along a geometrical space (e.g. cables path), -applied on walls (e.g. paint film).

The first study step has been devoted to collect the combustible raw material characteristics. Three main materials types can be listed as following :

-Oil (the major part fills up the power transformer) 1 : Main combustion characteristics of two typical flammable materials (5,6,7).

The fire modelling needs an homogeneous approach in regards of the various and complex chemical and physical processes. In the case of a locomotive fire in a tunnel we have to evaluate, on the basis of an ignition scenario, the temporal and spatial propagation of the fire into the locomotive. Furthermore we have to define the combustion products which leave the locomotive during the fire. This work has been made with the zone code MAGIC1 by Sechaud & Metz, on the basis of the data collected by INERIS for the flammable materials and their locations and distributions into the locomotive. This code takes into account the following time variable parameters :

-walls and gases temperatures into the locomotive, -height of the hot zone 2 , -thermal fluxes received by the flammable material, -self-ignition of a flammable material if its self-ignition temperature is reached, -wall or roof fusion/consumption, and then -propagation of the fire in the locomotive, -multi-fire locations... The studied scenarios have been choiced, in one hand, because the knowledge of motor block ignition in the past, and in an other hand, the potential gravity of an ignition of the oil's transformer.

These scenarios have been studied for two ventilation levels in the tunnel, 3m/s or 6m/s of bulk velocity.

Results give the temporal variation of the fire locomotive source terms which include the wall temperature of the locomotive and the chemical and physical properties of the combustion products by the way of the following parameters : locomotive walls temperature, smoke mass flow, unburned mass flow, and concentrations of CO 2 , CO, CH, HCN, O 2 in the smoke flow.

FIRE SOURCE TERMS AND PROPAGATION FIELD GEOMETRY

The results of the fire propagation scenarios into the locomotive have been used in our unsteady smoke propagation in the tunnel by the way of boundary conditions given on four geometrical zones of the locomotive. Then source terms are the following :

-a fresh air admission in the locomotive from the tunnel, -a smoke release from the locomotive in the tunnel, -the apparition of a new opening due to the combustion or fusion of a material through which air may enter or smoke be released. Source terms are set on these four surfaces during the simulation by giving, • in case of a fresh air entrance, the mass flow • and, in case of a smoke release, the emission velocity, the smoke temperature and the CO 2 concentration. The wall temperature of the locomotive is also given by the zone code at each time step in order to take into account to the heat induced by the locomotive in the tunnel.

Four scenarios have been kept for the smoke propagation simulation : motor block -3m/s and 6m/s ; transformer -3m/s and 6m/s. As only a small difference has been found for the fire locomotive power between the 3m/s ventilation rate and the 6m/s one, the study cases can be resumed in two major cases : a motor block fire which produces around 0.35 MW of convected power and an oil pool fire which can produce up to 7 MW of convected power in the tunnel. Note that the fire power is divided into two parts : the radiative part which is absorbed by the locomotive itself and participates to the elevation of its walls and structures temperature, and the convected part which is released in the tunnel through the openings. The ratio of radiative power to the total power is about fifty percent on an average but can significantly vary with each combustible material.

The propagation smoke geometry is the following :

NUMERICAL RESOLUTION

Simulations were done assuming that the train has stopped before the fire growth and at this time piston effects had vanished. So the unsteady flow field is related to the unsteady boundary conditions of the fire sources terms. The flow is calculated with Phoenics TM 3 by resolving the mean turbulent Navier-Stokes equations and the perfect gas law. The well-known general form of the transport equations is :

S, ( 1
)
where 0 is a transported massic variable.

In this study the following variables are solved : the mass, the momentum in a three-D space geometry, the energy, the smoke concentration, the turbulent kinetic energy and its dissipation.

For convective momentum and heat transfer to the walls, the friction has been modeled with wall functions and fitted (by the way of the mean height roughness) in order to respect the given friction coefficient level. Considering the Reynolds Analogy hypothesis this procedure ensures also quite correct heat fluxes. In our simulation only the forced convective heat transfer has been modeled, the free convective heat transfer appearing to be small in our study cases.

For radiative heat transfer a simple model has been applied only for the walls/gas heat transfer assuming that the smokes were very thick. This choice was justified by the fact that source materials filling the locomotive can produce dense smokes (transformer oil). In this case the wall radiative transfers are assumed to be present in a small region near the wall. Numerically, the radiative transfer was applied only on the first fluid cell near the walls with the following equation :

; d -Tt all ) ( 2
)
where 1 is the smoke emissivity and a the Stefan-Boltzman constant.

RESULTS

First scenario : Motor block fire (0.35 MW)

Source terms indicate that ratios between CO 2 and the other combustion products are quite constant during this fire. As the CO 2 concentration is calculated in the whole geometry it was easy to estimate the possible effects of combustible products on passengers during the evacuation. In such a situation the most relevant parameters are, at least, temperature, toxicity, and visibility. This last parameter is very important due to the rapid increase of the emergency difficulties when visibility decreases.

3 Phoenics is a computer code for solve three dimensionnal Navier-Stokes equations. This code is developped by CHAM. To evaluate this parameter we have used the methodology elaborated during the EUREKA FIRETUN 499 project (8) : the visibility is calculated on the basis of an empirical formula for the Optical Density (O.D.). This formula gives us a relation between the Optical Density, the CO 2 volumic concentration and the temperature at the considered point In our study case the empirical constant k is given by the heptane FIRETUN experiments. Furthermore the visibility is related to the optical density by curve in figure 5. To establish our visibility calculation we have considered thesuggested mean relationship for the various light sources.

For all parameters results show acceptable conditions of evacuation for both ventilation rates. The following results are given at 1.6 m heigth in the evacuation zone, along the train and after the train on 800 m long. The maximum values are logically reached for the 3m/s ventilation case:

-maximum temperature elevation : • maximum concentration elevations : -minimum visibility :

Figure 2 :

 2 Figure 2 : Possible openings for smoke convection (pre-existing and due to melting).

Figure 3 :

 3 Figure 3 : Convected power source terms (see figure $Jbr the openings definition).

Figure 4 :

 4 Figure 4 : Simulated smoke propagation field.

Figure 5 :

 5 Figure 5 : Relationship between visibility and optical density of smoke from fires (9).

Figure 6 :

 6 Figure 6 : Results for the Motor block 2 scenario.

  schematically shows how the locomotive is filled with flammable materials. The driver's cabin is separated by a fire resisting wall and thus the materials included in it do not participate to the fire.

	Fire resisting wall
					-paint
					-SH cables
	Cabin Block : -polycarbonate -polyurethane epoxy	Motor Block : -polyurethane -polyester -epoxy -wood	Auxiliary Block : -polyurethane -polycarbonate -polyester -epoxy -wood	Transformer : -phenolic resin -polyurethane -oil	Common Block: -polyurethane -polyester -polycarbonate -epoxy
		Figure 1 : Distribution of flammable materials.

MAGIC is a zone model developed by Electricité de France.

In a zone code two layers are considered : the hot and the cold layer.

Second scenario : Transformer fire (7 MW)

In this case, ratio between CO 2 and other combustion products are also constant except during the first ten minutes because of the rapid combustion of synthetic materials producing HCN. The maximum temperature elevation (reached in the various simulations) does not exceed 44 °C. But, during the first ten minutes toxicity and opacity peaks occur due to the high combustion rate and the PE-Roof consumption.

-maximum concentration elevations : -visibility : < 8000 ppm CO? < 250 ppm CO,< 8ppm HCN 2m < visibility < 10m After this toxicity peak concentrations and visibility are the following :

-maximum concentration elevations : -visibility : < 5500 ppm CO 2 , < 170 ppm CO, < 1 ppm HCN 3 m < visibility < 10 m Note that for all parameters reached values are about twice less in the 6m/s ventilation case than in the 3m/s ventilation case, the flow geometry being unchanged. The flow geometry along the train is a stratified hot layer flow for the two ventilation levels. No back-layering has been found, the bulk velocity being larger than the critical velocity. This is confirmed by the analytical study of the critical velocity. Visibility and CO concentration distributions in the DD section are given in the figure 8. Along the train (figure 9) the concentration and temperature vary slowly due to the calm distratification of the hot layer. After the front locomotive the hot layer brakes itself because of the nozzle disturbance.

CONCLUSIONS

For the two studied scenarios the evacuation outcome conditions are reasonably acceptable especially for the 6m/s ventilation rate.

The maximum values listed above are reached behind the train when the no^e of the front locomotive breaks the stratified hot layer. Along the train the air quality slowly varies because the train acts as a protection.

The total collapse and disappearance of the PE-roof (3rd opening) and later of the Aluminumroof (4th opening) allow smokes to propagate in a hot layer above the train and decreases the smoke releases along the locomotive through the first and second openings.

Temperature is not the relevant parameter in the studied cases but visibility seems to be very important for the evacuation duration.

The toxicity could also be an important parameter because of concentration peaks which can occur during high burning rate moments of synthetic materials.

In long tunnel, piston effects can be present during up to ten or fifteen minutes. During this time period, this effect could destratify a hot layer and could impeach a safe evacuation. But in the same manner if the rescue ventilation strategy is adequate, piston effects can help to dilute the fire power and/or a toxicity peak.

BIBLIOGRAPHY

[1] HAACKA


P Beguin
email: beguin@cnam.fr

M Kazmierczak
email: marc.kazmierczak@wanadoo.fr

R Cottura

J Leininger

P Vicot

Design of an alarm and risk management in chemistry Some lessons Searned from interdisciplinary research 1

Keywords: Chemical runaway, technique transfer, innovation, design and risk, latent errors

Chemical runaways" consdtute major sources of accidents in chemical processes. In this communication, we underline the lessons we learned from the implementation of the System on an industrial site. Firstly, introduction of the system may be defined äs a know-how and technique transfer. Secondly, the interest of the approach of introducing the technical system is not limited to use of the system and its characteristics, but should be extended to all the action conditions. On this basis, we argue that the industrialization of an artefact like the one we have designed should be done with each installation.

INTRODUCTION

In this communication, we would like to contribute some elements from inderdisciplinary research whose purpose was to develop an aid system designed to prevent "chemical runaways" taking place along with their consequences at human level (death), environmental level (SEVESO type companies) and industrial level (destruction of installations). This project follows an inquiry we made into a chemical runaway in a nuclear plant in 1994. The accident was fatal for one operator. In this case, two hours before the explosion, the operators realised that something was wrong. The five operators tried to get out of the unit a few seconds before the explosion. This type of result (which was largely confirmed by other analyses) shows that process operators often think that the time they have before an explosion is more than it really is. Our aim was the design of a system that would warn operators of the arrival of a chemical runaway and would predict the time remaining until an imminent explosion.

AIM AND OBJECTIVES OF THE RESEARCH

Design is a complex process and, in our case, it appeared necessary to dissociate two phases: (i) a first one, which comes under innovation, meaning the design of the principle, (u) and a second phase which comes under industrialisation of the principle, meaning the implementation of the innovation in a given Situation. The two stages have their own research objectives even though, in this communication, our objective is to deal with the industrialization phase.

An innovative system: The Chemical Runaway Detector/Forecaster (DPE).

The innovation consists of specifying an artefact of which a large number of examples will be used in multiple situations in life or at work. Therefore the challenge consists of dealing with the general case. In our case, INERIS engineers developed a method for predicting the critical moment of chemical runaway. The method was implemented in a Version called "DPE" -Runaway Detector/Forecaster -and tested in an experimental Situation ("large scale" experiment) in order to assess its accuracy. Marss & Coll, 1989) analyzed the typology of around 200 accidents which took place in chemical reactors operating non-stop. The DPE is a system which detects around 60% of the accidents described in the Marss & Coll database. Sudden reactions (reactive loading en-or) or explosive reactions which take place in a few minutes (except for specific adaptation) are excluded from its scope. Naturally, the reliability of the information given increases if we have studies of the chemical kinetics of the expected reactions However, the kinetics of chemical runaways might not have been studied either due to the lack of a preliminary study or because of a minor change in the quality of a basic product. The most interesting aspect of the system is that, despite the lack of specific studies, it can assess the Time to Maximum Rate (TMR) with a moderate error level.

-From technical innovation to technology transfer

We have just seen that the innovation deals with the general. Industrialization is a much different phase which deals with the singularity of a given Situation. It could be described äs the phase of "discovery of a Company ".

A considerable amount of research has shown the existence of contradictory organization and/or technical logics (De Terssac, G. 1992;[START_REF] Leplat | Overpressure protection ofbatch chemical reactors[END_REF]. These logics can be converted into forms of latent error [START_REF] Reason | Human error[END_REF]), meaning the presence ofelements which can remain dormant for a long time and whose dangerous character most often only appears at the time of an accident (Reason, J., op.cit.). In this context, the control of technical Systems should not only be considered äs a negative factor but äs a factor of reliability [START_REF] Amalberti | La conduite de systemes ä risques[END_REF]. Concretely, we wanted to know how and under what conditions the DPE would fit into the socio-technical system, including the cognitive functioning of operators and the use of Systems placed at their disposal.

At the same time, examining the implementation of the artefact in existing structures is not fully satisfactory. The DPE is a technical system whose purpose is to make the socio-technical system reliable. Therefore, we also wanted to know if evolutionary dynamics existed following the introduction of the DPE and, if so, their conditions and limits.

And yet the industrialization of an innovation is a stage that is not very conceptualized. In ergonomics, the technology transfer has practically only been studied in the specific context of the modernization of an industrially developing country [START_REF] Wisner | Anthropotechnologie. vers un monde industriel pluricentrique[END_REF]. But for intra-national transfers, most of the work (done in France) concerned the socio-economic field. The main lesson learned from this research is that industrialization should be done systematically since the innovation cannot be defined simply in terms of technical bases. On the contrary, it is the environment of the installation which will determine if the technical system has an innovative character or not.

CHARACTER1STICS OF THE PILOT SITE AND METHODOLOGY

The DPE was introduced in a small catalyst production unit which produced synthetic optical lenses. The process of the pilot site (where industrialization took place) consisted of synthesizing a few dozen kilos of product from glass reactors. The operators were located beside the reactors which were installed in the "control room". The synthesis of the catalyst was exothermic: the temperature of the product could rise by itself. From the outset, the catalyst also started to self-destruct in an exothermic way by releasing secondary gaseous and explosive products (this is the "chemical runaway"). To prevent any chemical runaway, the catalyst is cooled during the synthesis by circulating glycolated water in coils placed in the reactors. In addition, its thermal and chemical homogeneity is ensured by agitators. Symmetrically, an excessive cooling can lead to cystallization of the product, preceded by an unstable "super-undercooling" phase. Between the solid state of erystallization and the gaseous state of chemical runaway, the product is liquid.

The industrialization of the DPE took place in two phases. An initial phase called "exploratory study" enabled us to characterize the socio-technical system of the pilot site. It lasted four months. During this stage, we used the Ergonomie Work Analysis [START_REF] Wisner | Situated cognition and action: implications for ergonomic work analysis and anthropotechnology[END_REF]De Keyser, V. 1991 ) which Frenchspeaking ergonomists have developed over the last 30 years on the basis of the initial work of [START_REF] Ombredane | L'anaivse du Travail[END_REF]. For reasons of objectivity, this involves the exhaustive recording of the behaviour of operators and the Status of technical Systems during execution of the action; on the basis of these observations, the operators are interviewed in order to obtain pertinent explanations, to validate the interpretations obtained from the observation and to generalize the results.

The purpose of the second phase, called "experimentell", was to characterize the evolutionary dynamics, It lasted eight months. When it was introduced on the pilot site, the DPE had a digital temperature display, a TMR display for each of the reactors and visual and sound alarms which were activated for a given value of the TMR. Two types of methodology were used : Ergonomie Work Analysis and task Simulation techniques. Concretely, the intention was tq manipulate certain determinants of the action on the site and to analyze the activity of the operätörs in ttie" Situation created äs such. In this case, Simulation of the task was considered äs a cognitive finality method [START_REF] Simon | The Sciences of the Artificial[END_REF]) but also äs an approach which could create situations of exchange among players with heterogenec'us knowledge: designers, Supervisors, operators and ergonomists (Beguin., P., & Weill Passina, A. 1997^.

CONTROLLING THE PROCESS WITH THE DPE

The "experimental phase" of introduction of the DPE highiighted paradoxical results. First of all, the operators consulted the artefact more and more often which, a priori, was positive. But an analysis showed that it was the temperature information provided by the DPE which was significant for the operators and not the TMR which actually constituted the interest of the system. The more the operators consulted the interface, the less they consulted the temperature indications which were provided previously (Cf. table l). At the same time, fluctuations appeared in the TMR. This alerted the operators, indicating the existence of a danger they were obviously not aware of. But since they had no possibility of interpretation, this alarm made them perplex. This was considered äs worrying by the operators but did not appear in the process control. How can this result be interpreted? An analysis of the socio-technical system provided some answers. Table l: Comparative development, during the experimental phase, of the period during which information was read on the DPE and the thermometers available previously (in % of the total control time). The DPE temperature was consulted more and more to the detriment of analogue indicators which are less accurate.

Control strategies and daily risks

The operators develop control strategies on the basis of the lower temperature threshold (super undercooling) and not on the basis of the upper threshold which corresponds to the TMR. This control strategy, very dose to crystallization has several origins: -First of all, the system should be kept at dose äs possible to the lower temperature threshold since experience shows that the colder the product during synthesis, the more the catalyst obtained will be of good quality from the physico-chemical viewpoint: the catalyst Starts to self-destruct from the outset. -Secondly, the characteristics of the technical system, in particular the system for cooling the product during synthesis, tend to spontaneously lead to super undercooling. In such a context; the operators are more inclined to let the temperature of the product rise than to take action to cool it.

Yet, process control very dose to the low temperature threshold is a difficult exercise during which the skills of the operators develop. In this way, real crystallization theories were highiighted by an analysis of the activity. These are theories for the action, constructed during the work, whose purpose is to serve the action . The paradoxical result of the Status which the operators give to the DPE information can thus be better understood:

-First of all, it is the temperature information that is used since it is part of the cognitive constructions and the uses already constructed by the operators. At the same time, the TMR appears external since what is important is not the TMR but what the TMR indicates. Yet, äs we have just seen, in Situation the operators develop knowledge which is far removed from the major risk constituted by chemical runaway. Therefore, it could be thought that they do not have all the cognitive instruments necessary to judge this information.

-Symmetrically, it is certain that control very dose to super undercooling is less risky than a chemical runaway. But nevertheless it is considered by operators and their Supervisors äs a serious incident which should be avoided since, in one way or another, the product must be left to warm with all the risks linked to the instability of the polymer provoked by warming of the product. It could even be thought that, in this Situation, chemical runaway could appear following super undercooling that was badly controlled.

Technical transfer and know-how transfer

These two aspects could be summarized by considering the introduction of the innovation äs a technical and know-how transfer. The transfer was from the engineers to operators: the appearance of developments of the TMR required explanations concerning their significance. This was due to particular physico-chemical phenomena (micro chemical runaway) which appear in the reactor but do not require the implementation of emergency procedures. As such, training sessions were organized. The transfer also went from the operators (who develop singular knowledge which comes under "expertise" of crystallization) to the engineers: control very dose to crystallization constitutes a good compromise. But it is not free of danger and there is a need for aid. In our Situation, this vision led us to complement the principles of display of the temperature values provided to the operators. Initially, the temperature information was only given in a digital form. We kept this type of display since its accuracy corresponds to the requirements of the operators. But we added an analogue display with memory storage of the temperature curve. This facilitates the interpretation of the TMR (identification of "micro chemical runaways") and helps with control "very dose tu super undercooling".

PACING UP TO AND MANAGING THE UNKNOWN

Process control very dose to super undercooling constitutes a class of Situation that is well known to operators. But, äs we underlined previously, a runaway could appear following a badly controlled super undercooling. Operators could then find themselves in another class of Situation, the chemical runaway, which they know little about. Knowledge of the runaway in itself does not constitute the only element of doubt. The actions to be taken to deal with it are also relatively uncertain, both äs regards the operators and the supervisory staff (security manager, foreman, methods manager, etc.) on the site. Here again, this is due to the lack of experience available.

One of these actions consists of "destroying the product", meaning that the reaction is inhibited by a chemical process and is evacuated using specific techniques and means Nevertheless, destruction of the product and the technical and organization conditions under which it is done, are problematic and uncertain since they are not really tried and tested. It should be noted that in the present Situation, which has lasted for the 20 years that the site has been open, three "destructions" have taken place. In two cases, this followed technical problems (damage to equipment). In the third case, the problem was due to the production quality.

The DPE: a Simulation tool

Since it provides information about the thermal kinetics of the product in the case of a chemical runaway, the DPE constitutes an opportunity to do a real test on the temporal conditions of the action consecutive to a chemical runaway. In this sense, we can use it like a Simulation tool in order to explore the temporal conditions of a chemical runaway.

In the approach we used, Simulation was done on the production in order to maintain all the physical and material characteristics. The main objeetive difference was the fact that the product contained in the reactors consisted of a chemically inert liquid. But the thermal kinetics of a chemical runaway were provided by the DPE. Computerized, recorded data of a chemical runaway in a laboratory were sent to the DPE which caiculated the TMR on this basis. All the players liable to be present at the time of a chemical runaway were associated. The control operators, naturally, but also the foremen and the technical managers, who are usually not on the site, were also present.

An examination of the technical and organizationai conditions

The most interesting part of this Simulation approach concerned the production and implementation of the scenarios. In our study, several scenarios were produced but in all cases the approach used was the same: (i) initially a meeting was planned at which the scenarios were defmed collectively, (ii) the scenarios were then implemented in Situation by the operators and analyzed by the ergonomists and (iii) the approach ended with a debriefing. Each of these stages had a specific interest.

The interest of the meetings was that they led to the procedures being examined. This examination revealed certain ambiguities. For example, while the quality,procedures indicated certain temperature values, the security procedures indicated other values everi though the action to be done was the same. Yet the procedure which could be defined äs an operative procedure or method, designed to respond to a clearly defined Situation, constitutes an element whose existence is all the more important when the Situation is rare and when action had to be taken in conditions of great time constraints with considerable consequences för production. All the conditions were present here.

The second stage, that of implementation of the action, had two interests. First of all, it was an opportunity for the operators to test, in action, an event that was too rare to be part of the local experiment. Secondly, it enabled the socio-technical system to be requestioned on the basis of execution of the action in an class of Situation that was not well known. Let's take two examples. For one of the scenarios, it was considered, a priori, that three operators were necessary to perform the action. The Simulation showed that, on the contrary, six persons were required. This figure corresponds to the number of persons who should be allocated to the site and be permanently present, which was not the case. This last point deserves to be theorized. The testing of the action, under chemical runaway conditions that are not well known, would reveal latent errors that were present on the site but not identified for a class of Situation which was made rare by the activity of the operators.

Finally, the debriefing also had two interests. First of all, it was an opportunity to transfer the knowledge of the engineers (present on the site) to the operators. Since it is a source of surprise, testing of the runaway conditions in action generates beneficial debates and exchanges. Secondly, the treatment of latent errors was facilitated by the presence of the supervisory staff on the site. It should be noted that each Simulation ended with action decisions which concerned either the procedures, the technical conditions or, finally, the organizational conditions.

DISCUSSION

The industrialization of an innovation constitutes a stage is that little conceptualized. First of all, it could be considered äs an adjustment of the artefact on the basis of a confrontation with reality. Nevertheless, our results show that this should be renewed for each installation of the DPE. One of the Strategie aspects of industrialization concerns the evolutionary dynamics implemented through the impetus of introduction of the artefact. Yet the main result of this research actually concerns the need to manage the said dynamics at two levels.

The first level concerns adaptation of the artefact to the singular. The innovation is based on theoretical knowledge, built on the basis of hypothetical and deductive approaches which come under the expertise of engineers and can be very generalized. Yet, in our Situation, the operators could be described äs the experts of a singular process, that in which they worked for several years: they have very situated knowledge, marked by the socio-technical system of a given Company. Introduction of the DPE brings face to face very generalizable scientific concepts -which may have little to do with the site -and the control strategies used which make the operators experts in their work Situation. But this is an expertise which concerns the most frequent conditions of control. Industrialization consists of putting this heterogeneous knowledge in tune. This process of exchange generates creation which is liable to be translated at the level of the artefact. In our Situation, it is the said dynamics which led to an in-depth change in the criteria and principles of the display of values provided to operators by the artefact. But we consider that, in other situations, other adjustments could be desirable.

The second level more generally concerns risk management We know that some industrial accidents are due to system defects in their technical and organization dimensions. These are the defects which [START_REF] Reason | Human error[END_REF]) suggested calling "Latent errors". Nevertheless, they can only be expressed in classes of Situation that are very specific when the activity of the operators is found to be lacking, which is very rare. The main question then does not so much concern the imputation of responsibility (which is the question at the origin of the "latent error") äs the ways and means of identifying the "latent risks". Certainly, the DPE is a system which can warn operators of an imminent chemical runaway. But, during the study, it also appeared to be a system which, in the case of a certain type of accident (chemical runaway), provides the opportunity to requestion the socio-technical system äs regards unknown classes of Situation. In this sense, it could be used äs a means ofrevealing latent risks in a given Situation.

help of ANVAR and INERIS. We would like to thank Mrs. G. Beaumont


International Conference on Latest Achievements in the

EMISSION DE GRISOU A LA SURFACE

Comme le montre l'expérience de différents bassins miniers, le gaz qui se dégage et s'accumule dans le réservoir constitué par une ancienne mine souterraine a tendance à migrer vers la surface. Ce gaz représente une source de risque non négligeable, car il peut avoir à la fois un caractère inflammable et asphyxiant.

Dans de différents pays, un certain nombre d'incidents ou même d'accidents liés à la remontée de gaz à la surface ont déjà été observés [START_REF] References | The effect of mine closure on surface gas emission[END_REF][START_REF] Kral | Methane from closed-down mines in the soil[END_REF].

Des situations de risque classiques ayant été à l'origine des événements ont été les suivantes :

l'émanation ponctuelle et concentrée de grisou au travers d'une communication directe entre les vieux travaux et la surface (orifice minier, faille, fissure...) ; l'accumulation de gaz dans des espaces confinés ou semi-confinés (caves ou sous-sols, voire habitations ou bâtiments eux-mêmes, mais aussi réseaux enterrés, etc.) ; la mise en communication d'un piège à grisou avec l'atmosphère (par exemple réalisation de forages ou de travaux de génie civil à proximité de vieux travaux ...) ; le confinement d'une surface émettrice de gaz (par exemple par la construction d'un bâtiment).

OBJECTIFS ET PRINCIPES DE LA METHODE

Pour les émanations concentrées, ou encore les accumulations de gaz dans un milieu confiné, les simples mesures des teneurs en gaz dans l'atmosphère peuvent suffire pour détecter et évaluer les risques.

La solution est bien plus compliquée dans le cas d'une émission diffuse par les terrains, car une simple présence de teneurs mesurables en gaz dans le sol n'est pas toujours équivalente à l'existence d'un flux réellement significatif. En plus, certains gaz peuvent être naturellement présents dans le sol, notamment le dioxyde de carbone et le méthane.

Ainsi pour pouvoir détecter, quantifier et qualifier les rejets des gaz de cette nature, ainsi qu'évaluer l'efficacité des moyens de prévention mis en oeuvre, il est indispensable de disposer de méthodes fiables et éprouvées de mesure du flux de gaz émanant d'une surface de terrain. Dans ce but, des travaux de mise au point et de validation de telles méthodes ont été entrepris à l'INERIS depuis 1992 [START_REF] Pokryszka | Development and validation of a method for measuring biogas emissions using a dynamic chamber[END_REF]. Ces méthodes destinées initialement pour les décharges et les sols pollués, ont trouvé rapidement une application minière.

La dernière technique de mesure développée est basée sur un principe de la chambre à accumulation. Il s'agit d'une mesure de type locale et directe, non tributaire d'hypothèses relatives à l'émission des gaz dans l'atmosphère et à leur dispersion.

Figure 1. Principe de la méthode.

La méthode consiste à couvrir, au moyen d'une chambre, une certaine surface de manière à ne pas trop perturber le milieu (voir figure 1). Les gaz dégagés par la surface recouverte s'accumulent alors dans la chambre. Il est ainsi possible de suivre l'enrichissement de l'atmosphère en méthane. Pour ce faire, un système permet de prélever le mélange qui est envoyé vers un analyseur et ensuite réinjecté dans la chambre, créant ainsi une recirculation des gaz. Le suivi de la vitesse d'enrichissement en méthane du mélange recirculé permet de déduire le flux local de méthane au point considéré. Les dimensions de la chambre et les paramètres de fonctionnement de la méthode ont été optimisés lors de la conception sur un banc d'essais (figure 2) où l'on a pu mettre en oeuvre des flux de gaz connus. On a pu également à cette occasion déterminer l'influence sur le résultat du dispositif de mesure lui-même, de la nature du sol, du vent et bien d'autres facteurs, afin d'en tenir compte dans les calculs. Après avoir effectué la phase de validation au laboratoire, il a été procédé à la mise au point de la méthode dans les conditions réelles in situ. Celle-ci a confirmé la pertinence des principes de fonctionnement de la méthode définis en laboratoire.

Soil

EXEMPLES D'APPLICATION

La méthode a déjà été mise en oeuvre de manière opérationnelle pour le diagnostic et l'expertise sur de nombreux sites (décharges, anciennes mines...). Ici on présente deux cas d'application caractéristiques.

Emission de biogaz d'une décharge d'ordures ménagères

Les ordures ménagères sont une source importante d'émission de biogaz, dont un composant majeur est le méthane. Ces émissions peuvent représenter dans certains cas un risque assez important, notamment d'explosion. Dans le cadre d'une étude pour l'ADEME et le Ministère de l'Environnement, l'INERIS a réalisé une campagne de mesures qui a permis de quantifier le flux d'une décharge de 8 hectares [START_REF] Savanne | Comparison of different methods for measuring landfill methane emissions[END_REF]. Une cartographie de l'émission a également été établie, moyennant 400 mesures réparties sur la surface étudiée (figure 3). Surface : 8 hectares Flux moyen : 95 Ncm

Emission de grisou d'une ancienne mine de charbon

Les mesures effectuées à titre préventif sur un terrain situé au-dessus d'une ancienne mine de charbon ont mis en évidence des teneurs en méthane dans le sol dépassant 10 % (figure 4). Le terrain étant accessible au public, l'INERIS a été appelé pour évaluer le risque.

En utilisant la méthode de chambre, des flux de gaz non négligeables ont été mesurés, pouvant créer des accumulations dangereuses. La source directe de cette émission était très probablement une ancienne descenderie située à une faible profondeur de la surface et débouchant vers des vieux travaux miniers.

Suite à ces mesures, un traitement (remblayage) de la galerie a été décidé. Elle est destinée à détecter, quantifier et qualifier les rejets des gaz, mais peut être également appliquée pour vérifier l'efficacité des moyens de prévention utilisés pour réduire des émissions (barrières d'étanchéité, captage, etc...).

La méthode a déjà été mise en oeuvre de manière opérationnelle pour le diagnostic et l'expertise sur de nombreux sites (décharges, anciennes mines...). Bien que conçue initialement pour mesurer le flux de méthane, elle est tout à fait utilisable ou transposable à tout contexte similaire de flux gazeux émis par le sol, moyennant éventuellement quelques adaptations de détail.

Le savoir-faire lié à la méthode est protégé par un brevet européen d'inventions n° 96-05996 .

Field

  l'Environnement Industriel et des Risques (INERIS) BP2, 60550 Verneuil en Halatte, France

Figure 2 .

 2 Figure 2. Schéma du banc d'essai utilisé pour valider métrologiquement la méthode.

Figure 3 .

 3 Figure 3. Cartographie du flux de méthane d'une décharge d'ordures ménagères.

Figure 4 .

 4 Figure 4. Mesures de l'émission de méthane à la surface d'une ancienne mine de charbon.

  Les travaux réalisés à l'INERIS ont permis d'aboutir à une méthodologie fiable et adaptée pour la quantification d'un phénomène aussi complexe qu'est l'émission de flux gazeux faibles sur une surface de terrain.

  Le montage utilisé pour réaliser les mesures est relativement simple à manoeuvrer. Le temps total nécessaire pour une mesure ponctuelle est de l'ordre de 5 à 10 minutes, ce qui permet d'en réaliser un nombre important pendant une journée (de 40 à plus de 60 points selon les difficultés liées au site). Les densités de flux que l'on peut déterminer sont fonction des capacités d'analyseur utilisé pour mesurer la concentration en gaz donné dans la chambre. Pour le méthane mesuré avec une résolution de 1 ppmv, il est possible de mesurer des flux allant de moins de 0,1 à 4 000 Ncm 3 /minute/m 2 .S'ilest nécessaire de déterminer le flux global d'une surface donnée, les mesures ponctuelles sont effectuées selon une stratégie d'échantillonnage spatial adaptée, au cas par cas, aux conditions locales d'émission ainsi qu'à la précision d'estimation souhaitée. Des méthodes spécifiques d'interpolation et d'extrapolation des mesures ponctuelles sont ensuite appliquées (géostatistiques par exemple), afin d'estimer le flux global et de fournir d'autres informations nécessaires, comme la cartographie, la variabilité spatiale du flux.... Les modalités précises du savoir-faire lié à la mise en oeuvre de cette méthode sont protégées par un brevet européen d'invention (n° 96-05996 déposée le 14 mai 1996 sous l'intitulé "Mesure de flux surfacique de gaz").

		Teneur en gaz
	Pompe	Analyseur	temps
	Chambre
	Sol
	Gaz

/min/m