Introduction

The Commission of the European Communities, in a cooperative program with the US Department of Energy, is developping an accounting framework for identifying and quantifying the external costs associated with fuel cycles.

The initial phase consisted of the development of a general methodology (cf. bibliography). This methodology is based on : * an analysis of the different stages of a fuel cycle and an estimation of pollutants emissions, * a modelisation of transport and chemical transformation of pollutants from sources to receptors (impact pathways), * a modelisation of physical effects supported by receptors (human population, crops, materials) depending on their exposure, * a monetary valuation of physical effects.

INERIS has participated to the implementation of this methodology in France for the fossil fuel cycles (coal, oil, natural gas) in the context of the production of electricity.

As part of this programme, it has been tried to evaluate some of the effects on public health of sulphur dioxide (SO2), nitrogen oxides (NOx) and particulate matter emissions by generation plants.

Two directions have been privileged in the approach :

* to present the principles of the evaluation in simple terms, understandable by a non-specialized public;

* to put in evidence the fundamental parameters of the phenomena and the nature of the related uncertainties.

Background

Observation of the harmful effects on human health, plants and building materials of smoke from combustion is almost as old as the use of fossil fuels. Thus as far back as 1273, Edward 1st published an edict prohibiting the use of coal in London because of the danger it posed to health. In fact, this ban, although renewed, failed to be applied. Pollution in London, again, in the fifties of our own century, which on one episode reached such proportions that thousands died in only a few days, contributed greatly to our awareness today of environmental problems.

The first studies on the cost of the effects of atmospheric pollution were carried out at the same time as the theoretical research on social costs conducted by A.C. Pigou at Cambridge University (Wealth andWelfare, 1912, andThe Economics of Welfare, 1920).

* The first study made of the effect of pollution on the cost of maintaining a building complex (Pittsburgh) dates back to 1913. A similar study was conducted in Manchester in 1918.

* An evaluation of the cost of damage to harvests caused by sulphur dioxide was carried out in 1949 in the Columbia River Valley (USA).

* A French study (G. Pannetier, 1957), quoted in a work published by the World Health Organization (WHO), estimated the cost of pollution at 6000 French francs per head of population per year, a quarter of this sum going towards medical expenses.

Today, in Western Europe, atmospheric pollution caused by smoke from combustion plants is very different from what it was at the time when the foregoing studies were being conducted. As regards sulphur dioxide and particles in particular, the concentrations commonly observed in the air are very much lower than the levels they might have reached in the past in certain industrial areas or major conurbations.

Air quality standards, expressed in terms of maximal admissible concentrations of the principal pollutants, have been laid down and are periodically revised in the light of new scientific knowledge. Compliance with these standards is designed to protect the population from the dangers that pollution can mean for public health; and generally speaking, standards are complied with. Thus an institutional approach should in principle lead to an absence of health riskand thus of associated external costs -from the emission of particles or of sulphur dioxide in the present environmental context.

New information, which epidemiological research has been providing for the last few years, does however tend to question a number of hypotheses on which current standards are based. It suggests that, even at the concentrations currently encountered in the ambient air, there is a measurable effect of pollution, notably acid particles, on human health.

Translating these new data into external costs, associated, for example, with fuel combustion, raises two problems. First, that of determining, in the complex phenomenon of air pollution, exactly what are the factors (e.g. chemical compounds) whose presence constitutes a risk and evaluating their harmful effects. The second problem is that of assessing to what extent a combustion plant contributes to exposure of the population to this risk factor.

Harmfulness of particulate pollution

Two types of epidemiological approach may be adopted to reveal the effect of atmospheric pollution on public health: time-series analyses and horizontal analyses. The former deal with a region; the level of pollution is assumed to be uniform in space and variable in time; an attempt is made to see whether there is a statistical relationship between (a) atmospheric concentration values in respect of SO2, particles in suspension, etc. measured for a short period (one or more days); and (b) levels of morbidity or mortality observed over the same period or else with a short time gap. Horizontal studies, on the other hand, monitor an average situation over a long period and compare mortality or morbidity in areas subjected to different levels of pollution. The population as a whole within an area is included ("ecological" study), or selected individuals whose characteristics are known are monitored ("cohort" study).

Data of time-series epidemiological studies

A considerable number of studies of this type have been conducted, especially in the United-States (cf. The Harvard School of Public Health Programme), dealing with large populations (for example, monitoring of the daily mortality rate among 10 million inhabitants over 10 years) in varying climatic contexts with different pollution structures. Such studies reveal notably the existence of a constant statistical relationship between different measurements of particle pollution and the daily mortality rate together with indicators of morbidity such as hospital admissions or consultations for respiratory complaints.

Such observations had already been carried out in the past. What is new, is that it has been possible to observe a relationship between air quality parameters and morbidity indicators to include levels of particle pollution which had previously been regarded as low and which are appreciably inferior to the usual air quality standards in respect of the ambient air.

Similar studies have been carried out in France, notably as part of the ERPURS Project (Evaluation of the Risks of Urban Pollution for Health). The first data were published recently (see Bibliography). On a more modest statistical basis, these data reveal relationships analogous to those observed the other side of the Atlantic.

The measured quantitative relationship between atmospheric pollution and, for example, mortality varies according to the study ; that is to say, according to the site and the method of statistical processing used. If we take ten or so American studies published since 1990, it can be seen that the mortality rate observed in a region on a given day increases by an average of 0.1% each time the particle concentration of the air (PM 10 particles of a size inferior to 10 jim) increases by 1 fig/m 3 . The ratio observed in different studies varies from 0.07% to 0.16%, this range also representing the "95% confidence level" of the ratio calculated by the major study (Philadelphia).

If the base mortality rate of a population is in the order of 1 death per year per 100 inhabitants, the foregoing relationship can be expressed by saying that 1 additional death is observed when 100,000 inhabitants are exposed for 1 year to a particle concentration increase of 1 jig PM^g/m 3 . It can then be said that "acute harmfulness" from the particle pollution revealed by these studies is in the order of 10" 5 deaths per inhabitant, year. ju,g PM 10 /m 3 . To illustrate the potential significance of this concept, particle pollution measurements taken in France give average "black smoke" concentrations of generally between 10 and 40 p,g/m 3 .

On the basis of the same principle, some American studies (Ostro, Krupnick, etc.) monitored a population sample and studied the statistical relationships between variations in time of particle pollution and the appearance of symptoms such as respiratory problems or the inability to work. Thus measurements, in a specific context, have been taken of "respiratory discomfort" acute harmfulness of 0.46 days with respiratory symptom per inhab. yr. jig PM 10 /m 3 and "reduced work potential" acute harmfulness of 0.05 days at lowered activity per inhab. yr. jxg PM 10 /m 3 ).

Data of horizontal epidemiological studies

The studies referred to above attempt to evaluate acute harmfulness, i.e. an immediate effect of atmospheric pollution (instantaneous level). They give no indications as regards chronic harmfulness, i.e. the cumulative effects liable to arise from long-term exposure to pollution. Every effort is being made to evaluate these chronic effects by means of horizontal epidemiological studies.

For a long time, "ecological" studies of metropolitan areas in the USA revealed a relationship between particle pollution levels and mortality rates. Such relationships, whilst they provide food for thought, do not enable direct conclusions to be drawn, as differences of pollution levels from one region to another are also related to general differences between the various geographic, economic, urban and social contexts which may themselves help to explain divergent mortality rates.

Cohort studies, although more costly, provide increased control of factors likely to be significant and have therefore been undertaken. An initial study took place in 6 American cities and monitored 8000 adults over a fifteen-year period.

In addition to pollution, the study examined determining factors such as a age, sex, race, educational level, weight, the consumption of cigarettes and alcohol, professional exposure and so on. It revealed, beside the influence of these factors, a strong statistical relationship between particle pollution, in particular a fine fraction of PM 2 5, (particle size inferior to 2.5 |i,m) and mortality rates. The increased mortality recorded in the worst-polluted cities was due to death from cardiopulmonary diseases. Furthermore, there was an appreciably higher incidence of bronchitis and chronic coughs in these cities.

A wider study is currently in progress, embracing 151 American towns and cities and designed to monitor 500,000 adults over 7 years. The initial results published confirm those of the previous study; they would lead to an evaluation of chronic harmfulness from particle pollution in the order of 8.10" 5 deaths per inhab. yr. fig PM 2 5/m 3 (standard deviation of the evaluation 1.4.10 5 ). The difference between this value and that given for acute harmfulness can be partially explained by the choice of a different pollution indicator : PM 2 5 concentrations represent on average 60% of PM 10 concentrations, thus harmfulness is revealed as 1.6 times greater. In addition, it would mean that the incidence of chronic harmfulness is approximately 5 times higher than that of acute harmfulness.

Discussion

Overall, these evaluations should, needless to say, be regarded with some caution. Observation of a statistical relationship between two variables does not imply a direct causal relationship between them. It is always possible that the relationship can be explained, partially or totally, by a third variable, correlated with each one of both.

Thus in the case of time-series analyses, we know that both the mortality rate and the level of pollution vary according to weekly and seasonal cycles, the weather and other factors. The value attributed to acute harmfulness from pollution varies according to the method used to adjust it in relation to seasonal variations and other influences; and there does exist a danger of both over-and under-correction. However, when studies conducted in different climatic contexts, using different methods to process cyclic phenomena, continuously reveal a relationship between atmospheric pollution and health, the hypothesis of a single statistical artifact becomes somewhat improbable.

The evaluation of chronic harmfulness raises even more difficult problems.

Pollution is only one of a number of factors that influence mortality ; for example, its effect is outweighed by that of smoking. Thus inappropriate evaluation of the effects of the foregoing individual factors can skew the evaluation of the effect of pollution. Other factors which are not explicitly taken into account in the statistical analysis and which are possibly correlated with the level of pollution could be significant . A sedentary way of life is such a factor ; another such factor is the level of pollution during previous decades, generally much greater than the current level of pollution.

In fact, the chronic effects of pollution on health can only be measured to the extent that all the principal factors are controlled. Current data provided by epidemiological research must therefore be regarded as provisional indications. Nevertheless, their convergence and consistency lend credibility to the hypothesis of the effect of atmospheric pollution on health at commonly encountered concentrations. However that may be, considering the magnitude of the differences of mortality rates between regions, to explain these differences is a challenging research objective.

For the rest, epidemiological studies cannot at the present time identify accurately which components or characteristics of pollution are determinant in respect of effects. Indeed, such studies can only include parameters used for the purpose of routine measurements. Furthermore, variations in these parameters are themselves very often correlated and therefore it is difficult to measure the respective influence of each one. Depending on the study, the indicator most closely correlated with health may therefore be particle concentrations (TSP, PM 1Q , PM 2 5, black smoke, etc.), or again sulphate, ammonium radical or H + concentrations and so on. This is matter not of contradiction but of indeterminacy. It would seem, however that a factor of harmfulness is more particularly related to the particle phase, especially to the fine fraction component of this phase.

The toxicological approach, complementary to epidemiology, cannot at the present time remove this indeterminacy. The determinant parameter of harmfulness has yet to be identified : mass, number, area, chemical nature of particles, and other factors. It has still not been determined by which mechanisms relatively low particle pollution levels might cause acute cardiopulmonary complaints liable to cause the early death of a patient.

It is evident that fine particles, since they penetrate deep into the lungs and settle there, can represent a highly active impact vector ; research is being carried out to determine which processes would be likely to exacerbate drastically the condition of certain already weakened subjects. Some hypotheses point to ultrafine particles (size less than 0.01 um) whose overall mass is low but which are very numerous. Others lay the fault at the door of oxidants (notably hydrogen dioxide Yi^O^)' the concentrations of which in the ambient air correlate with particle concentrations and which may be dissolved in the aqueous phase of these particles. Still other hypotheses involve iron ions which may be fixed to the surface of the particles. It is not impossible that several pollutants are active, or that there exists a synergetic effect. It is to be hoped that certain toxicological studies taking in all these different hypotheses will bear fruit.

4

Health effect of particulate pollution caused by a combustion plant

In order to relate an effect on health and an external cost to the operation of a combustion plant, we have (a) to accept that one or another observed statistical relationship, for example between mass particle concentration and mortality or morbidity, expresses causality, and (b) to calculate the increase of particle concentrations in the ambient air resulting from combustion plant operations.

Following the previous discussion, it is clear that the first term of this approach implies a risk of error which it is difficult to quantify. It would seem to be a question of a pragmatic approach which, on the basis of the principle of precaution, could highlight what is potentially involved.

Origin of particle pollution

Particle pollution of the ambient air, as measured for the purposes of epidemiological studies, consists of three components whose relative influences are commonly of comparable orders of magnitude:

* Dust from the ground, raised by the wind or by traffic, and possibly dust which has settled on the ground and then once again put back in suspension. Dust of natural terrestrial origin is usually of a relatively coarse grain size; it is regarded as relatively less harmful to health because generally it is arrested within the upper respiratory tract and does not penetrate the pulmonary alveoli.

* Fine mineral or carbon-laden dust directly given off in exhaust gases from motors and fumes from stationary combustion plants or by certain industrial processes.

* Secondary particles caused by formation in the atmosphere, from gaseous pollutants (SO2, NOx, NH3, COV, etc.), through chemical conversion, condensation or adsorption, of chemical compounds in the form of aerosols (ammonium nitrates or sulphates, organic particles and so on).

Thus coal combustion, like the combustion of other fuels, generates an atmospheric load of particles, consisting on the one hand of particles expelled directly in smoke in a solid state (primary pollution); and on the other hand, salts (sulphates and nitrates) produced subsequently by the conversion of gaseous pollutants.

Fine particle pollution circulates and is dispersed into the atmosphere over great distances as it penetrates, moreover, closed premises. In order to evaluate the overall effect of an emission, one has to consider the resultant increases in particle concentration in a very large area of influence around the point of discharge; if we take France as an example, we should have to consider an area taking in the entire continent of Europe.

Exposure of a population to the pollution generated by an emission

For a given individual, the risk to health engendered by atmospheric pollutionthat is, the probability of a respiratory disease or of death -is assumed to be proportional to both the harmfulness of the pollutant and the amount of the pollutant absorbed by the individual. This second term is itself proportional to the concentration of the pollutant in the ambient air and the time during which the individual is exposed to this concentration ; exposure of the individual is referred to as the product of these two factors.

In the case of a population, the collective exposure is the sum total of exposure of the individuals making up the population. The overall effect of an emission is thus the product of harmfulness multiplied by collective exposure.

The collective exposure caused by the emission, at a given place and time, of a puff of pollutant depends on the location of the population and also on the diffusion of the pollutant in the atmosphere, its chemical evolution and its deposition on the ground. These processes obey complex physical laws which, in some cases are not fully understood and which models are being used to elucidate. The sophisticated nature of certain models cannot disguise the fact that the real values of the base parameters (rate of deposition, the kinetics of the chemical reactions) are often imperfectly grasped. Given the magnitude of the various uncertainties, we shall refer, in order to illustrate a "standard" situation, to very simplified models.

Using a model of this sort on a large scale, it can be shown (see Appendix) that the collective exposure resulting from pollution within a given geographical area is the product of three factors : the mass of pollutant falling on the area concerned, average population density per unit of ground surface area, the inverse of the deposition velocity of the pollutant on the ground.

The model in question assumes that the "emitters" and "receivers" of pollution are distributed uniformly or at least independently over the area concerned; if this is not the case (emissions in an urban environment), we need to include an additional term: over-exposure to the primary pollution. This additional term depends on the size of the conurbation, population density and average wind speed within the area.

Effects on public health of a combustion plant

On the basis of a hypothesis of harmfulness and of an exposure model, we can calculate, for example, the effect of a coal combustion plant. The effect will depend mainly on the following:

* the properties of the coal (e.g. sulphur content); * the characteristics of the plant (particles removal from the combustion gases, desulphurization, control of the production of nitrogen oxides, etc.); * the emission site (especially in the case of emission within a conurbation).

Tables 1 to 6 illustrate the calculation. The social costs appear considerable in certain scenarios, whether worked out per tonne of pollutant, ton of coal or kWh of electricity. It will be noted, for example, that these are comparable, in order of magnitude, to certain measures designed to prevent the discharge of harmful emissions.

Aspects requiring further research

Earlier emphasis was placed on the uncertainty associated to the value of the harmfulness of atmospheric particulate pollution. The hypothesis considered in the example above refers to American time-series analyses and the relationship PM^g/acute harmfulness. A hypothesis based on the relationship PM 2 5/chronic harmfulness would lead to evaluations of the effects in terms of mortality rates and the corresponding external costs, multiples of a factor 8 of those shown in the table. Conversely, if one assumes that an increase in concentration is only harmful where the concentration exceeds a certain threshold, the estimated effects would be lower ; for example, if effects were restricted to urban areas, they would be divided by a factor of the order of 2.

Other uncertainties are related to the calculation of exposure : * As regards direct particle emissions, the statutory standards provide a reference point (an emission ceiling); but there is incomplete knowledge of actual particle emission according to the coal used and the various types of plant (combustion, dust removal, etc.); particle size distribution in particular has a major influence on the deposition velocity. * Evaluation of the mass of nitrate particles produced by the emission of nitric oxides poses a special problem: there is in effect a balance in the atmosphere between nitrate of ammonium particles and nitric acid and ammonia gas. This balance depends on temperature, humidity and the composition of the atmosphere. In the calculation shown here, it has been assumed that 50% of the NO 3 radicals present in the atmosphere are in the form of particles ; this is a provisional hypothesis which will require further research.

* As regards the additional term for emissions in an urban area, the proposed model has the advantage of very easy operation. However, as far as we are aware it has not been tested or parameterized in the french urban context.

In expressing the results in terms of external costs, the level of unit costs applied in respect of morbidity and mortality is obviously a major factor. The unit costs of a "day of discomfort" or a "day of reduced activity" are those adopted for the ExternE European Programme. The unit cost attributed to a death refers to french studies on road safety. If for example we applied in the case of this unit cost the value proposed by the ExternE study, the external cost of mortality would be multiplied by a factor of 5.

Conclusions

To date, science has only an incomplete knowledge of all the direct and indirect effects that industrial activities have on the environment. Any attempt to evaluate the risks and the associated environmental costs of a given activity is therefore necessarily incomplete.

Certain effects such as the effect on health form the classic subjects of evaluation in respect of external costs. Such evaluations, given the complexity of the systems involved, inevitably remain subject to uncertainty.

The example proposed here shows that the results of calculating the external costs only make sense if the hypotheses on which they are based are fully explained and discussed. It also shows that a major effort is required in order to reduce the margin of uncertainty that accompanies these calculations at the present time.

It remains to say that evaluating external costs presuppose clarification of all current knowledge in respect of the effects of an industrial sector or activity on the environment. It provides a challenging framework for multidisciplinary research. ERPURS, Impact de la pollution atmospherique urbaine sur la sante en Ile-de-France, Observatoire Regional de Sante d'lle-de-France, Paris 1994. (DRAFT may 1995) 14

Bibliography

APPENDIX

Brief outline of methodology : some simple models used to evaluate collective exposure related to the discharge of pollutants into the atmosphere Dispersion of pollutants in the atmosphere, their chemical evolution as a function of meteorological factors and the presence of other components, and their deposition on the ground obey highly complex physical laws. A number of simple models can, however, be used to ascertain the principal parameters in respect of these phenomena and the extent of the effects.

Large-scale model of dispersion in an infinite space

The earth's surface is represented by an infinite plane space. One has to imagine receivers located on the surface of this plane space and sensitive to atmospheric pollution; for example, a population of A uniform density.

Let us take the emission of a "puff" of pollutant, with Q the quantity of the pollutant emitted. The pollutant disappears progressively from the atmosphere with time ; T is the average time during which the molecules of the pollutant remain in the atmosphere.

The assumption is that dispersion of the pollutant is restricted to a lower part of the atmosphere, where there is very considerable turbulence and which is known as the mixed layer. The height h of the mixed layer is constant; at a given time and in a given place, the pollutant concentration is equal at different altitudes in the mixed layer.

On the basis of the foregoing hypotheses, it can be seen that the collective exposure resulting from the puff of pollutant, from the instant of emission to the moment where the pollutant has completely disappeared, can be expressed as follows :

E = A Q T \ 2

Removal of the pollutant by deposition on the ground

Let us consider the case of a stable pollutant, that is to say one that is chemically inert in the atmosphere. It can only disappear by deposition on the ground, either directly in dry conditions or under humid conditions (carried by rain, fog, snow).

The deposition velocity V is defined as the ratio between the mass of pollutant which settle in a given place per unit of surface area and per unit of time and the concentration of the pollutant in the atmosphere directly above the place concerned.

The deposition velocity of a pollutant is an empirical parameter analogous to the sedimentation velocity of particles. We shall consider here an average deposition velocity and assume it to be constant, taking into account both dry and humid deposition.

The pollutant is eliminated from the atmosphere by deposition at a constant V rate per unit of time A, = ~r . The average time the pollutant remains in the atmosphere is: h T = V The foregoing model can be simplified and expressed as follows:

E = A Q \ 3

Exposure to a reactive pollutant

As well as deposition on the ground, the pollutant contained in the puff disappears due to chemical reactions in the atmosphere. This process can often be modelled by a "first order" law : the quantity of pollutant consumed by the reaction per unit of time and unit of volume is proportional to the concentration of the pollutant which remains present in the atmosphere at a given time. The pollutant is eliminated from the atmosphere by chemical reaction at a constant rate per unit of time \L

The quantity of pollutant emitted Q divides into a mass D deposited on the ground and a mass R which disappears due to chemical reaction. Thus we have:

D -r^- Q ; R= r*- Q A, + JLL A + (J,
Collective exposure may be expressed as follows:

1 E = A D y 4

Secondary pollutant

The above formulae also account for exposure to secondary pollution, if we consider Q as the amount of secondary pollutant produced from a mass Qp of a primary compound.

Let Ap be the rate of elimination by deposition of the primary compound, [ip the constant of reaction, Rp the mass of the primary compound that reacts, cr the stoichiometric coefficient (mass of secondary pollutant generated by the reaction of a unit mass of the primary compound). This gives:

(DRAFT may 1995) 16 Up Q = cr RP = a -^ Q P Ap + UP 5
Collective exposure induced in a delimited geographical space

If we retain the foregoing hypothesis, we can specifically consider collective exposure E^of receivers located in a space % representing, for example, one country or a group of countries.

Let A^be the density of receivers in the area ^and D^the total mass of pollutant deposited in this area. We then have: 1 E^ = A<^ D^ ^7 If P^ is the total population of the region and S^ its surface area, we can use the following formula:

The factor q-is the average deposition of pollutant per unit of surface area in the region, the term q-y the integral in the time of a concentration.

Evaluation, in a real situation, of that part of an emission which is deposited in a given area assumes in principle that the circulation of air masses in the atmosphere are known. This means using complex meteorological models such as those developed under the European EMEP Project.

However, if we take a puff of pollutant emitted within a given site, e.g. France, and which circulates above a given area, e.g. Europe, before escaping to others countries or to sea, it will be observed that, if we apply the foregoing hypotheses, the mass of pollutant deposited, and thus collective exposure, only depends on the duration of the path above Europe. EMEP shows, for example, that in the case of emissions of sulphur dioxide, nitrogen oxides and ammonia, the percentages of sulphur and nitrogen falling out over Europe are respectively 64%, 52% and 82%. On this basis, it is possible to reconstruct approximately the distribution of the duration of the path over Europe of french emissions and so evaluate simply the levels of exposure to different pollutants.

Emission in an urban context : additional term for collective exposure to primary pollution

The hypothesis of a homogeneous mixture of pollution over the full height of the mixed layer is not valid near the source of the pollution. Local exposure requires to be studied specifically, especially when receiver density, instead of being uniform, is characterized by clusters and emission is located within a cluster.

Given the limited time the pollutant remains near the point of emission, the phenomena of deposition and chemical conversion of the pollutant are not taken into account. The only vector of elimination of the pollution is the wind.

A classic model for calculating the average concentration of a pollutant in an urban area is the ADTL model, a simplified result of a gaussian representation (Atmospheric Turbulent Diffusion Laboratory, Hanna 1971). This model shows that the term Ex of local exposure depends on the density of the urban population Ax, on the quantity Q of the primary pollutant emitted and average wind speed U: K Ex = A X Q u The form, of this expression is similar to that of the base model, provided the U parameter V is replaced by ^ . The coefficient K is interpreted as the ratio of the length of the urban area to the average height up to which the pollutant is diffused over the urban area. Average height itself depends on the size of the conurbation as well as on meteorological factors (atmospheric stability). Coefficient K also varies, in practice, according to the conditions under which the pollutants are emitted : it is lower in the case of a pollutant of which a large proportion is discharged by tall chimneys (SO 2 ) and higher with a pollutant discharged at ground level (CO).

With certain types of pollution, it would be necessary to take into account other stages of overexposure associated with other containment phenomena : pollution from cars in canyon-streets from which pollution escapes with difficulty, domestic heating appliances in enclosed premises, and so on. (1) The values used for deposition rates and reaction kinetics are those of the TREND model (cf. Atm. Env.,Vol. 23, No. 9,1989).

(2) It is assumed that the secondary product obtained by oxidation of the sulphur dioxide consists of a mixture in equal proportions of SO^NH^ and SO4HNH4. The particulate product from nitrogen oxides is NO3NH4 .

(3) There is a balance in the atmosphere between particulate ammonium nitrate and nitric acid/ammonia gas. The balance depends on temperature, humidity and the composition of the atmosphere. Thus the proportion of NO 3 radicals present in the form of particles varies considerably according to the location and the season. The 50% hypothesis is provisional. (1) Knowledge of the size distribution of the primary particles present in the smoke emitted by various types of combustion plant is incomplete. The deposition velocities and the proportion of emissions from France deposited in Europe shown in the table are therefore hypothetical.

(2) Estimations concerning proportions of secondary particles deposited in Europe were obtained from analysis of EMEP results. Notes:

(1) The hypothesis refers to U.S. studies on the "PMio concentrations/short-term mortality variations" relationship. A hypothesis based on a "PM2.5/inter-regional mortality variations" relationship would give a harmfulness value 8 times higher.

(2) Unit cost of death used in french studies on road safety ; the European ExternE study refers to a value 5 times higher.

(3) Estimations of short-term effects made in an american context (Ostro. Krupnick). ( 4) Unit costs applied in the ExternE study. (1) The case proposed corresponds to a population density close to that of the lie de France.

(2) Hanna's model, developed within an American context, has not to our knowledge been the subject of adjustment studies in respect of French urban areas. It is not applicable to the case of discharge from a tall chimney. The value of the constant used here applies to a large conurbation. Notes:

(1) In the context of situations described in previous tables. The value in respect of general collective exposure from one tonne of sulphur dioxide only differs from that from particles by the factor X / (k + \i) = 0.73, since the deposition velocity of sulphur dioxide is close to that applied previously for particles. Urban overexposure is identical if it is admitted that Hanna's constant has the same value.

(2) This estimation is based on studies conducted in Dortmund, Birmingham, Stockholm, Sarpsborg and Prague. In each city an analysis was made of the costs of maintaining building complexes located in areas with different levels of pollution. Costs differences have been attributed to SO2 concentrations differencies. That hypothesis is a schematization ; others pollutants (nitrogen oxides, particles), and others factors, could explain a part of the differences.

*

  Concerning large-scale pollution dispersion and the formation of secondary particles, the purpose of the calculations shown is to highlight the principal parameters of the problem ; the values used for these parameters are rough estimates. A more realistic evaluation should be based on a number of studies such as those conducted under the European EMEP Programme (Co-operative Programme for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe).

(

  DRAFT may 1995) 13 Proceedings of the Colloquium on Particulate Air Pollution and Human Mortality and Morbidity, Inhalation Toxicology, Volume 7 Number 1,1995. C. Arden Pope et al., Particulate Air Pollution as a Predictor of Mortality in a Prospective Study of U.S. Adults, Am J Respir Crit Care Med 1995 ; 151 : 669-74.

Tableau 4 :

 4 Additional exposure to primary pollution from the discharge of fine particles in an area of high population density. Additional term of health external costs.

  . Rabl, G. Landrieu, et al., Environmental Impacts and Their Costs : the Nuclear and the Fossil Fuel Cycles, Final report of the project "External Costs of Fuel Cycles : Implementation of the Accounting Framework in France" funded by the European Commission DG XII (february 1995).

	Ministere des Affaires Sociales, de la Sante et de la Ville, Conseil Superieur
	d'Hygiene Publique de France : Particules en suspension dans l'atmosphere ;
	effets sur la sante et proposition pour une revision des valeurs limites, Paris
	1993.

European Commission, Directorate-General XII, Externalities of fuel Cycles, "ExternE" Project, Coal fuel Cycle, prepared by ETSU, U.K., IER, Germany, 1994.

A

Table 1 -

 1 Calculation of the masses of secondary sulphate and nitrate

particles produced by emissions of sulphur dioxide and nitrogen oxides

	Deposition rate
	Oxidation rate
	Stoichiometric coefficient
	Part of the oxidised radical
	present in particle form
	IVirticles mass produced
	JXT ton of primary emission

Table 2 :

 2 Calculation of collective exposure to particle

pollution by emissions of fine primary particles, sulphur dioxide and nitrogen oxides. Exposure at European level, emissions in France (average case).

					primary	sulphur	nitrogen
			formulation	unit	particles	dioxide	oxydes
	Particle mass produced		Q	tonne / tonne	1	0.5	0.7 (prov.)
	per tonne of primary emission
	Proportion of mass deposited in Europe	(1) (2)	a*	-	0.65	0.5	0.5
	Mass deposited in Europe			tonne / tonne |j.g / tonne	0.65 0.65.10 12	0.25 0.25.10 12	0.35 0.35.10 12
	Particle deposition velocity (1)	V	cm/s m/yr	1 0.31.10 6	1.08 0.34 .10 6	1.61 0.51.10 6
	Population density		A*	inhab./km 2 inhab./m 2	100.10 ' 6 100	-id-	-id-
	Collective exposure			inh.ib. vr. |.ig/ni "'
	per tonne of primary emission		per tonne	21(1	74	h7 (prov.)
	Notes:

Table 3 -

 3 Calculation of health effects and corresponding

external costs arising from emissions of fine primary particles, sulphur dioxide and nitrogen oxides. Effect at European level, emissions in France (average case).

					primary	sulphur	nitrogen
			formulation	unit	particles	dioxide	oxides
	Collective exposure to particle pollution			inhab. yr. |Xg/m 3	210	74	67 (prov.)
	per tonne of primary emission		per tonne
	Harmfulness of
	particle pollution: short-term mortality	(1)	N d	No. of deaths per inhab. yr. |Lig/m 3	1 . 10 " 5	-id-	-id-
	Number of deaths attributable to		No. of deaths
	one tonne of primary emission	Id = E^N d	per tonne	2.1.10 " 3	0.74.10 " 3	0.67.10 " 3
	Unit cost/ death External cost of mortality	(2)	u d	francs /death	3 500 000	-id-	-id-
	per tonne of primary emission	C d = IdU d	francs / tonne	7 350	2 590	2 345
	Harmfulness of
	particle pollution: (short-term morbidity)	(3)		No. per inhab.yr. ug/m 3 of
	respiratory symptoms	N g	days of discomfort	0.46	-id-	-id-
	reduced activity		N a	days of red. activity	0.05	-id-	~id-
	Effects attributable to one tonne		per tonne,
	of primary emission			No. of
	respiratory symptoms		I g = E^Ng	days of discomfort	97	34	31
	reduced activity		Ia= E^N a	days of red. activity	10.5	3.7	.3.6 .
	Unit costs	(4)		francs per
	respiratory symptoms reduced activity		U g U a	days of discomfort days of red. activity	45 450	-id--id-	-id--id-
	External cost of morbidity per
	tonne of primary emission
	respiratory symptoms reduced activity		C g = I gUg Ca = IaU a	francs / tonne francs / tonne	4 365 4 725	1530 1665	1395 1620
	HtMllh external cost
	(short-term effect) per loniu 1 of primary emission	C -C L ] + Cjr "I Cj	francs / tonne	K-. 500	* 800	3 400 (prov)

Table 5 -

 5 Calculation of "damage to buildings" cost attributable to the emission of sulphur dioxide

					Additional local effect of
				large scale effect	an emission in a densely
			unit		populated urban area
	Collective exposure		inhab. yr. jxg/m 3	150	1250
	per tonne of SO2	(1)	per tonne
	Harmfulness
	of pollution by sulphur dioxide in terms of additional	francs per inhab. yr. jxg/m 3	10	-id-
	building maintenance costs (2)
	Damage to building
	external cost		h\mcN / tonne	I 500	12 500
	per tonne of SO2 emission

Table 6 -

 6 Elements of external costs related

to coal combustion plant operations. Effects in Europe of an emission in France Short-term effects on public health and damage to buildings

				Old plant
			unit	located in an urban area
	Emissions		kg of
	primary particles	(1)	pollutant emission	0.5
	sulphur dioxide	(2)	per tonne	20
	nitrogen oxides	(3)	of coal burnt	15
	External cost per emission unit
	primary particles	(4)	francs / kg	116
	sulphur dioxide		of pollutant
	public health buildings		emitted	5.8 14
	nitrogen oxides			5.4 (prov.)
	Elements of the external cost
	of the plant
	primary particles
	sulphur dioxide		francs per tonne of
	public health		coal burnt
	buildings
	nitrogen oxides
	Sum of		francs
	(Ali-mal cost dements		per tonne of
			civil burnt
	I'IOLtridlx generating plant: Cost in terms of		centimes
	olivLricitv produced :		par k\\ h

Notes:

(1) Combustion of pulverized coal; use of coal with a 10% ash content; dust removal efficiency 99.5%.

(2) Coal with a 1% sulphur content; desulphurization variant 90% efficient.

(3) Variant. low-NOx burners. (4) Discharge to atmosphere in an urban area is assumed to occur at a height above ground of the same order as average discharge of sulphur dioxide in an urban area.

(5) It takes 0.33 kg of coal to produce 1 kWh.


P Michaelis

J F Lechaudel

Methods applied to investigate the major ÜVCE that occured in the TOTAL refinery's Fluid Catalytic Cracking Unit at La Mede, France

) 5:26 a.m., explosion of the debutanizer head line with fireball. Let us mention also the explosions of a LPG pipeline and a gasoil line. A total of 60 ruptured pipes were observed.

METHODOLOGICAL APPROACH

The methodological approach developed by the investigation team comprises 4 main steps:

Gathering evidence

The search for Information involved namely specific data sources: control room hard copy and electronically stored records: no deviation of process operating parameters were observed on the 50 measurement records analyzed; video tapes from amateurs and newsmedia: 17 fllms collected covering a period starting at 5h22 a.m. (2 minutes after the first explosion) to 12h00; record from a gas detector 3 minutes before the explosion; more than 1750 photographs from audit team, Professional photographers and amateurs were collected and expertised; witnesses Interviews; missiles mapping; seismic waves records collected from 21 stations located between 28 km and 409 km from the epicentre.

Defining potential scenarios

The selection of the potential accident scenarios was based upon four concepts:

The determination of the explosive mass and the location of the explosion epicentre extrapolated from the observed damages in both far-fleld and nearfield, using different ways of modeling: multi-energy approach, TNT equivalency and spherical expanding flame model;

The identification of all potential ignition sources leading to the determination of the one being the origin of the ignition of the vapour cloud: the FCC furnace F301 located at 150m from the epicentre;

The exhaustive numbering of the ruptured equipment: 60 fractured pipes and branch connections, no capacity nor pressure vessel ruptured;

The identification of the ruptured pipes that might generate the main explosion, using 3+5 criteria.

Investigating the possible scenario

Finally, 4 Systems remained selected for a more complex investigation: -a soda tank containing accidentally introduced propane, -the 8" by-pass ofthe absorber stripper cooler, -a l" brauch connection (guillotine break) located on the 8" line between cooler and absorber stripper, -a 3" LPG pipeline.

Performing the validation ofthe assumptious

To emphasize the most probable sequence and to perform the validation of the assumptions herewith associated, 5 different tools were used:

2D model Software for dispersion Simulation: gaussian plume model and dense model. The released vapours are heavier than air: (C3, C4 and light naphtha); the refinery is located on ground with slop; at the time of the accident, Pasquill stability class was FQ 5.

Wind tunnel Simulation with a model of the refinery site on a 1/150 01 scale, using Richardson similarity. Dynamic process modeling of the absorber stripper column for determining the time associated with process operating parameters deviation. Simulation in a similar unit in another TOTAL refinery.

Use of metallurgical and mechanical data to confirm the determination of the primary leak.

INVESTIGATION INTO THE AMOUNT OF FLAMMABLE PRODÜCTS INVOLVED

Inspection of the accident site is one of the most important steps in gathering physical evidence. A detailed examination is made to obtain Information on the apparent origin, the propagation of the explosion and localized damage of the explosion or fire. In the case of explosion damage analyses, the main goal is the evaluation of the amount involved, because hereafter it is a very important criterion to classify possible scenarios. Explosion damage is due to the high and dynamic pressures generated by the sudden release of energy. However, the damage analyses include consideration of the following important items: 1. flammable vapour cloud confinement in the Installation 2. explosion pressures, propagation rates and energy release 3. near-fleld effects 4. blast wave or far-field effects.

All these aspects were kept in mind during the damage investigation. A damage table was set up (see table l). The evaluation of the pressure in the farfield was made by comparison with tabulated blast wave criteria. Related to the near-field, some mechanical caiculations were performed. A map of the fire damage was established too. One may think thafc the damages behind the refinery fence, in particular at Martigues, were enhenced due to the meteorological conditions at the time of the explosion. As a first step, the explosion damage analysis allows to determine the epicentre area of the explosion. This area is defined äs the location where the pressure was the highest. This epicentre area was located between the propene unit and the Gas Plant. It is different from the ignition point and from the origin of the leak, because the explosion pressure depends strongly on the congested parts on the site. (see photograph of figure 3) As a second step, the examination of the projection of missiles allows to evaluate the pressure at the epicentre area: 50 to 200 kPa locally. Then, several methods were used to determine the amount of flammable mixture involved in the explosion: TNT equivalency, multi-energy method and spherical expanding flame model. For example, the multi-energy method allows to make some comparison between the congested volume from the different units and the damages observed. The objective was to obtain the better curve äs possible to reduce the distance between the curve and the damage points (see figure 4). Finally, the different methods led to good agreement about the assessment of the most probable amount of flammable mixture involved in the explosion. The best estimation was about 5 tons scale of sizes.

Table l Some damages observed

SELECTION OF THE FRACTURED PIPES POSSIBLY IMPLICATED WITH THE PRIMARY LEAK

60 fractures of pipework were observed. In order to identify the ones that might generate the main explosion the following methodological approach based on 3 + 5 criteria was defined. A first selection was made, discarding three families of ruptured pipes: • 1. 14 pipes containing products unable to generate an explosion phenomenon such äs non flammable products (DEA, water, soda) or flammable products with pressure vapour less than lOhPa at operating temperature (GO, HV fuel, heavy naphtha). 2. 11 pipes being isolated at the time of the explosion and having a small capacity. 3. 12 pipes containing flammable products with specific physical conditions: vapour phase and small pressure (1050hPa).

For such emission sources we have a mass flow of 2,3 kg/s in vapour phase, an explosive mass of about 2kg and a LEL concentration up to 45m from the source with stability class Fl. For the 23 remaining ruptured pipes, 4 criteria correlated with caiculations and one criterion based upon visual inspection are then applied: 1. vapour mass flow must be superior to 10 kg/s; 2. the LEL of the released gaseous mixture must reach the furnace; 3. 20% of the LEL must be snuffed at the location of the gas detector having given an alarm 3 minutes betöre the explosion happens; 4. the released mass within the explosion limits must be about several tons; -7-5. The physical conditions of the rupture are not a consequence of the main explosion. 18 fractured pipes were consequently elaminated by criteria l to 4. The five remaining ruptured pipes were then submitted to criteria 5. Three Systems then necessitated a more complex analysis:

-8" by-pass ofthe absorber stripper cooler; -1" branch connection on the 8" line from the absorber stripper cooler; -3" butane pipeline T09.

ANALYSIS OF POSSIBLE LEAKS

Four likely scenarios were considered. The first is related to a leak on the LPG line T09. When the accident occured, it was isolated and füll of liquid butane. It presents a bulge-shaped breach (200xl20mm). Owing to the diameter of the line and the pressure inside it, the order of magnitude of the explosive amount of gas would be at the most 80kg, the explosive area extending äs far äs 45m.

Moreover, witnesses saw the line exploding and metallurgical investigation led to plastic deformation due to heating. It was thus possible to discard this hypothesis. The second scenario is related to C24 tank in which is usually sent the used soda coming from the refining of propane, butane and gasoline. When used soda is drained off, hydrocarbons may be accidentally introduced in the tank. The tank has burnt the day the accident occured. Modeling was performed related to leaks ofgas through vents or broader holes, according to the maximum propane release rate that might have been drained to the tank, the maximum explosive mass has been assessed to about 1,2 tons, the maximum extension of the explosive cloud being 100m. The case of leak without kinetic energy has been considered too, by means of a wind-tunnel Simulation äs described below. It was not possible to guess measurable concentrations in the gas plant. All these reasons led to the discarding of the scenario. The two last scenarios are related to the absorber stripper DA101, the first about a l" branch connection and the second about a breach (800x200mm) on the bypass of the EA103 cooler. More complex simulations including wind tunnel and dynamic process modeling were then used. It was thus possible to emphasize that the most probable origin of the initial leak was the by-pass äs shown below.

IGNITION SOURCE LOCATION

In an unit, the possible ignition sources may be: an electrical spark due to the rupture extra-current of a circuit, generated by some manoeuvre, an open flame like a burner of a furnace, a mechanical spark, hot surfaces the temperature of which being greater than auto-ignition temperature of the flammable products involved. In our purpose, the investigation dealt with the electric sources and the F301 furnace. In fact, the mechanical spark was not taken into consideration because there was no work in progress, nor abnormal Situation in an electrical equipment. The possibility of an ignition on the hot surfaces of the disaster area was not held because the auto-ignition temperature of the gases was greater than the operating temperature of the pipes. On the other hand, several fmdings were made under the F301 furnace. The flammable cloud burnt up to the two thirds of the south part of the furnace. Traces of combustion were found on the external box of the third burner at east. More, in the direction of the Gas Plant, several burns, were found on the electrical cabinets and cables indicating the travel of a slow flame front. Several investigations on the electrical equipment were made in order to justify the exclusion of this kind of equipment. Finally, the ignition on the F301 furnace was found to be the only solution that is consistent with evidence and with the observed traces of combustion. (see photograph of figure 5).

VALIDATION OF THE ASSUMPTIONS

Wind tunnel Simulation ofthe dispersion ofthe explosive vapour cloud.

The choice of a physical model Simulation was dictated by the difficulty of reproducing the complexity of flow phenomena on a site such äs La Mede, due to the lack of available accurate modeis. The experiments were conducted in the diffusion wind tunnel of EDF's Lyon Engineering Center located in the premises of the Fluid Mechanics and Acoustics Laboratory ÜRA CNRS 263 of the "Ecole Centrale de Lyon". This wind tunnel is of the feedback type with venting downstream of the test facility. The test section measures 14 meters in length, 3.70 meters in width and the height is variable between 2 and 2.50 meters. The speed inside the test section is continuously adjustable from 0-lOm/s. The model of the La Mede reflnery site on a l/ISO 111 scale, represents the whole of the zone affected by the explosion and incorporates the potential release points to be investigated (see photograph of figure 6). The ground incline is perfectiy reproduced on the model. The physical model study of the dispersion of an explosive gas cloud heavier than air is performed in Richardson similarity the only scale parameter of which is represented by the Richardson number

Ri^gPszJ^0 pa U 2
where g is gravitational acceleration, ^/p a g' is the gravity term, pg and pg., L and U are respectively the speciflc masses of gas and air, the characfceristic length, and speed scales defined in the case of a continuous gas release at the break of volume flowrate Qv via O 2 1 i L = (^sandü = (Qvg') 3 . g

This definition of the Richardson number, used by Köning-LangIo and Schatzmann (1990) is routinely used for continuous releases in a calm atmosphere, characterized by wing speeds under l m/s. A different definition is used for non-zero wind velocities, where the local flow velocity is used to determine the characteristic scales (Britter and McQuaid 1988). Validation tests, presented in particular by Köning-LangIo et al (1990) Hall et al (1982) and Y. Riou (1987), have shown good agreement of the wind tunnel results with those obtained in situ, mainly at Thorney Island (MC Quaid 1984), Porton (Picknett 1978, 1981) for instantaneous gas releases and at Burro (Koopmann et al 1982), Maplin Sands (Puttock) et al 1982) and Thorney Island for continuous gas releases. Instantaneous concentrations are measured by means of a hydrocarbide analyzer (Cambustion Ltd) the passband of which is about 500 Hz for a 150 mm sampling probe lenght. The tracer gases used in the wind tunnel are mainly hydrocarbides whose densities are dose to the considered release conditions. The main test results for the three scenarios under consideration have shown, despite the very conservative hypotheses adopted for this Simulation, that the maximum gas leak from storage tank C24 could not be the cause of the accident, since the concentration levels reached at the ignition point are far lower than the gas inflammability limit (LEL). Regarding the two absorber stripper cooler EA 103 scenarios, the tests showed that scenario break at the by-pass bounds the second one (failure of l" branch connection). The gas concentrations at the ignition point (furnace F 301) only reach the LEL for the first scenario. The Simulation made for this scenario by east wind led to the results dosest to the evidence. Note in particular that the gas doud reaches both the gas detector P80-1 and the ignition source. The dispersion and transport of the gas doud particularly made it possible first, to characterize the progression of the explosive mixture äs far äs the presumed ignition source and, secondly, to evaluate the average time to reach the ignition point by the LEL at 675 ±110 seconds for 98 simulations, compatible with the estimations made elsewhere (see figure 7).

Dynamic process modeling of the absorber stripper

If one of the breaks is the origin of the accident, it must be demonstrated that, during the estimated release duration about 10 minutes, 1. the break can release a sufficient mass of product 2. the break is not inducing variations of the operating parameters, äs oberved on the hard copy stored records.

To verify both criteria a realistic dynamic process modeling of the absorber stripper column has been performed by the IFP (Institut Francais du Petrole) in order to determine the time associated with process operating parameters

Figure 1 .

 1 Figure 1. Chronology of Events Q epicentre 0 location ofvictims

Figure 3 .

 3 Figure 3. Gas Plant area after explosion and consecutive fires.

Figure 4 .

 4 Figure 4. Multi-energy approach: correlation with observed damages corresponding to an hemispherical cloud with a 29m radius.

Figure 5 .

 5 Figure 5. Ignition source: FCC main furnace.

Figure 6 .

 6 Figure 6. Wind tunnel Simulation: 1/150^ scale model of the zone affected by the explosion.

Figure 7 .

 7 Figure 7. Wind tunnel Simulation: dispersion and transport of the vapour cloud (8" by-pass case) äs a function oftime,


Antoinette Accorsi

Didier Gaston

Jean-Philippe Pineau

Philippe Villeneuve De Janti

Philippe Villeneuve

Fiabilité des données et sûreté de fonctionnement fournies par des capteurs atmosphériques en cas de situation accidentelle sur un site pétrochimique

ouverte pluridisciplinaire HAL, est destinée au dépôt

For each type of adverse conditions, the evaluation includes three different aspects :

-a theoretical study to make Provision of the possible effects on the sensor, such äs sensitivity, malfunction, -a definition of thresholds for effects on site, -a definition ofcriteria for performing off-site teste.

The final step will be issuing a specification document for the examined sensors.

-Introduction.

Lors Les effets ä quantifier sont:

-le flux thermique et les fumees provenant d'un incendie, -l'onde de pression consecutive ä une explosion, -la projection d'eclats consecutive ä une explosion, -l'emission de vapeurs toxiques ä la suite de fuite, eclatement ou perte de confinement.

-Types de capteurs utilises et essais realises.

5 -1 -Types de capteurs.

-3. -Autres effets en cours d'examen.

Elles sont de trois types :

-les effets des ondes de pression dus, soit ä la detonation d'un explosif (onde de choc), soit ä la deflagration d'un melange methane/air (onde de deflagration),

-les effets de fumees d'incendie ; en fait, ce sont des flux de particules solides qui sont etudies ä un niveau d'empoussierement de 2 g/m 3 provenant d'un feu de gasoil de 30 minutes dans la galerie d'essais d'incendie.

-l'effet d'empoisonnement du au benzene sur les detecteurs explosimetriques.

-Conclusions.

Certains w -s

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ACCORSI, D.GASTON, J.P. PINEAU, P. VILLENEUVE DE JANTI INERIS, Parc Technologique Alata, B.P. 2, Verneuil en Halatte Abstract

  Fiabilite des donnees et sürete de fonctionnement fournies par des capteurs atmospheriques en cas de Situation accidentelle sur un site petrochimique A.

	During accidental scenarios such äs
	explosions, emissions and fires, data
	from atmospheric sensors must remain
	reliable and not influenced by the
	rapidly changing environmental
	conditions. Relevant conditions have
	been defined refernng to the Situation
	of a refinery in which processing,
	storage and transfers offlammable
	and toxic materials are located. The
	effects could be accidental (thermal
	fluxes, pressure waves, fire fumes, ...)
	or chronical (pollutants, rains, high
	temperatures, ...)
	Two types o f sensors in this industrial
	area : a catalytic explosimeter and a
	semi-conductor hydrogen sulphide
	sensor were chosen for further
	examination.

Methode de travail utilisee.

	La technologie des capteurs est en Des complements d'etude ont ete ä 55 °C), enfin les plus « lourds » du	pertinentes pour la gestion d'un C'est l'illustration de cette methode qui
	pleine evolution. Des nouvelles necessaires pour appliquer ces type gasoil sont peu inflammables	« accident majeur ». Ce travail a ete est presentee dans la suite de cet
	techniques telles que les semi-resultats aux capteurs. Ces (point eclair superieur ä 55 °C).	realise avec des partenaires industrieis articie.
	conducteurs, les depöts en couches complements sont determines apres	et l'appui des Directions Regionales de
	minces, la spectrometrie infrarouge, ... l'inventaire detaille des capteurs mis Parmi ces produits, certains sont	l'lndustrie, de la Recherche et de
	permettent de diminuer le coüt et la en place sur le site et l'evaluation du egalement toxiques : hydrogene	l'Environnement dans le but 3 -Presentation d'un site
	taille des capteurs tout en augmentant comportement previsionnel de ces sulfure, benzene, etc...	d'appliquer concretement sur des sites petrochimique.
	les performances. Ces capteurs, capteurs lorsqu'ils sont soumis aux	industrieis les specifications emises ä
	comme les capteurs plus anciens effets de ces diverses agressions. Les situations accidentelles prises en	l'origine de la definition de ce projet.
	(detecteurs electrochimiques, Cette evaluation est realisee compte lors de l'etude de securite	Une raffinerie fabrique des produits
	explosimetres ä elements catalytiques principalement en s'appuyant sur la sont:	petroliers commercialisables ä partir
	chauffes, ...) sont sensibles ä l'environnement dans lequel ils sont places. Or, la normalisation actuelle sur les capteurs de gaz ne permet connaissance des capteurs issue de notre experience (4), peu de travaux bibliographiques ayant aborde ces -fuite sur une canalisation, -eclatement d'un reacteur, problemes. -perte de confinement d'un	du fonctionnement des installations industrielles, des rejets de gaz et vapeurs inflammables et de petrole brut. Pour ce faire, eile comprend des activites de transformation, de stockage et de 2 -La premiere etape de la demarche transfert. Elle est divisee en differentes
	absolument pas de prevoir leur stockage ( « ruine d'un	toxiques peuvent se produire impliquait de prendre en consideration unites de fabrication.
	comportement dans des conditions reservoir, rupture d'une	accidentellement. Lorsque ces rejets un site « pilote ». II s'agit d'un site
	d'environnement anormales canalisation, ..).	sont massifs, il se forme des nuages petrochimique sur lequel sont situes Le stockage des produits s'effectue
	consecutives ä une Situation	de volumes importants. Dans certains differents etablissements presentant dans differents types de reservoirs :
	accidentelle (temperature elevee,	cas, ces nuages peuvent s'enflammer des possibilites de risques spheres pour les gaz liquefies, bacs ä
	fortes pressions, gaz interferents,...) ou simplement suite ä des agressions	technologiques majeurs^. Pour toit flottant pour les produits volatils, et les effets de l'explosion affectent la population. Des plans d'intervention en simplifier notre approche nous bacs ä toit fixe pour les liquides peu
	quotidiennes particulieres (pluies	Situation d'urgence sont alors illustrerons uniquement le cas de la volatils, certains etant calorifuges,
	violentes, gaz interferents, ...).	raffinerie qui est le « coeur » de ce voire rechauffes pour les produits appliques soit par les responsables de l'usine, soit par les autorites complexe petrochimique. lourds et visqueux.
	C'est exactement au moment oü la fiabilite de l'information est la plus	competentes. La methode de travail utilisee est Une raffinerie moyenne traite plus de
	vitale pour la gestion des risques que celle-ci est susceptible d'etre erronee. Comme l'analyse d'accidents recents l'a montre, cette information « douteuse » rend le management de Enfin, la realisation de tests de la Situation d'urgence moins efficace robustesse des capteurs hors site, ou peut entramer l'aggravation de la pour differents types d'effets, fournit Situation accidentelle (2). des informations importantes sur leur comportement effectif dans des Dans le cadre du projet EUREKA 904 situations severes et bien definies.	schematisee sur la figure 1. II faut 7 000 000 t/an de petrole brut et Le retour d'experience sur l'analyse d'accidents industrieis, oü des dispose d'aires de stockage de preciser que seuls les effets des agressions sur les capteurs sont pris 1 000 000 m 3 de produit brut et une melanges inflammables se sont formes en compte. capacite similaire pour les produits dans l'atmosphere, montre qu'entre le finis et semi-finis. Sur le site pilote, debut de la fuite et l'inflammation un Les documents de base pour environ 750 capteurs sont installes. delai de plusieurs dizaines de minutes l'application de notre methode sont les existe souvent et permet donc de etudes de securite et les dossiers de prendre des dispositions pour tenter de specification des reseaux de detection. 4 -Scenarios accidentels et leurs diminuer les consequences. effets sur les capteurs.
	« MEMbrain : plate-forme d'integration Lors de ces essais, les conditions d'aide ä la decision dans la gestion operatoires sont plus dures que celles des crises majeures », l'lnstitut imposees dans les normes traitant des National de l'Environnement Industriel conditions environnementales mais et des Risques (INERIS) est elles restent ä des niveaux responsable d'un sous-prog ramme « realistes » Simulant des situations dont l'objectif est de realiser un cahier accidentelles que l'on peut rencontrer des charges definissant les capteurs dans l'industrie. atmospheriques et leurs systemes d'acquisition associes permettant d'obtenir des donnees fiables et	Les premiers documents nous Le suivi de l'evolution de rejets permettent d'evaluer les agressions inflammables et toxiques necessite de dues aux scenarios accidentels Tous les produits utilises ou fabriques disposer en permanence sur un site susceptibles d'affecter la securite des sont des produits inflammables : concerne de capteurs atmospheriques personnes ou de degrader les biens certains comme les gaz liquefies et les qui doivent constamment fournir des ou l'environnement, et de donner une essences sont tres facilement mesures fiables. Ces capteurs cartographie des effets selon chaque inflammables ( point eclair inferieur ä atmospheriques (toximetres, la temperature ambiante), d'autres tel explosimetres...) sont donc des que le kerosene sont simplement elements essentiels de la gestion des inflammables (point eclair superieur ä risques. la temperature ambiante mais inferieur
		page: 3

type d'agressions. Le tableau 1 donne une evaluation sommaire des agressions potentielles que l'on peut rencontrer sur des sites industrieis (3). page: 2

Le tableau 2 donne les dysfonctionnements que l'on peut observer sur les capteurs et que nous avons pris en compte.

En fonction des seuils d'effets pouvant induire des perturbations sur les capteurs, il est possible de donner un avis sur le niveau de fiabilite de l'information fournie par les capteurs en Situation accidentelle et de proposer eventuellement des modifications ou complements au reseau de detection.

page: 4


G Mavrothalassitis

P Pons

USER'S POINT OF VIEW IN USING SOFTWARE IN RISK ANALYSIS

The first need of the risk analyst is the availability of pertinent software to assess, in a realistic way, the maximum effects of possible accidents such as pressure pulses, thermal fluxes, accidental missiles formation and projection and toxic releases of gases, vapours, mists, soots or dusts.

To be useful to risk management, this approach has to be quite simple but also as realistic as possible. An underestimation as well as a crude overestimation of the effects may prove dangerous.

A lot of softwares aim at this purpose. The analyst has to make sure he is using a suitable tool. This involves :

-The knowledge of how to use it (it is usually time consuming and depends on the aptness of the theory and the user's manual) ; -The analysis of the tool according to its internal consistency ; -Comparisons of the results with calculations that might otherwise be assessed ; -Comparisons of the calculations with existing data when possible (experiments and/or investigations of past accidents).

This paper aims at describing the approach of INERIS which began five years ago, and still goes on, working for French Ministry of Environment, about testing the ability of software to assess consequences of given scenarios.

Our point of view is that of the user who intends to use a reliable tool. * The source term definition, * The number of chemicals in the data base.

2.2-Admissibility

2.2.1-MAIN ASPECTS OF ADMISSIBILITY

Admissibility is made of three indissociable parts :

The Theory Guide, which has to be considered under the following points: * Its pertinency related to the version of the software that is evaluated, * The accurate description of the hypothesis and ways of modelling, * The definition of the limitations and of the areas of competence of the models, * The quotation of the sources and references. The user's manual, which has to be read bearing in mind the following points: * Precise descriptions of all the parameters, * Relevancy and pedagogical features of the examples, * Exhaustiveness of the explanations of the available functions, * Completeness of the informations to enable the user to run the case he intends to assess. * CHARM 6.1 (August 91) * PHAST 3.0 (November 91) * TRACE 2.5.4 (August 92) * CAMEO 3.0 (April 93) * PHAST 4.2 (a first update : June 95) * PHAST 5 (a second update : June 95) * WHAZAN 2.1 (June 95) [6] G.Mavrothalassitis, P.Pons, "Positionnement de Logiciels pour 1'evaluation des risques industriels", May 1994.

Introduction

A number of works exist, that aim at assessing the reliability of software, by means of comparisons with existing data (especially results from experiments). For instance, S.R. Hanna (Earth Tech, USA) has been comparing well known experimental data (Burro, Coyote, Desert Tortoise, Goldfish...) for many years with a great number of softwares [START_REF] Hanna | Hazard response modeling uncertainty (a quantitative method), volume II. Evaluation of commonly-used hazardous gas dispersion models[END_REF]. The European Authorities have taken such an initiative more recently, by means of European Projects (see [START_REF] Dr | Commission of the European Communities[END_REF]), dealing with the evaluation of effects from major industrial hazards, by constituting a large data base.

But it is not easy to realise a parametrized evaluation by means of experimental data. There are two reasons for that state of affair :

1) There is a need of data that were obtained under exactly the same protocol and ambient conditions. Such a requirement is very difficult to fulfil, because of the numerous various sources of data available, and the variation of ambient conditions from a day to another ;

2) There is a need for more data. But there are expensive to get (especially far field data from dispersion experiments).

That is why such useful experimental comparisons had to be completed by another approach, which is part of the Evaluation Model Protocol recently elaborated by the Model Evaluation Group on Heavy Gas Dispersion of the European Community [START_REF]European Communities, Directorate General XII, Model Evaluation Group on Heavy Gas Dispersion (MEG -HGD)[END_REF]. This approach is named « Scientific Assessment », and is part of the approach which INERIS began five years ago with French Ministry of Environment.

The context of the need of such an evaluation approach at INERIS is described in the first part of this paper.

The second one deals with the methods used to assess a software in risk analysis.

The third one is an illustration of two points : l)It shows the way INERIS works to evaluate software ;

2) It points out the evolution that can exist between the two versions of the same software.

1-Context of risk analysis

This context has been described in a previous paper [START_REF]Use of pertinent softwares in risk analysis[END_REF]. For a long time, INERIS has been interested in prevention and protection against the effects of fires and explosions in industrial facilities in various sectors, as well as in accidents investigations.

When carrying out such an accident analysis, as well as in assessing hazards related to a process, it is essential to use different types of computation tools to calculate effects, in order to be quite simple but also as realistic as possible.

To perform such calculations of effects, INERIS developed and uses its own tools to calculate for instance explosion effects, projections of debris, or radiative effects of pool fires.

On the other hand, INERIS had to consider commercially available softwares.

In this case, one of the first cares of the analyst is to make sure he is using a suitable validated tool. That is why INERIS, with the French Ministry of Environment, initiated a methodology of assessment of softwares in risk analysis.

First of all, the point of view is that of a user who wants to be sure he is using a suitable tool. Thus, there is a need to consider the following One or a few versions of the following softwares : CAMEO, PAMPA, CHARM, TRACE, PHAST, WHAZAN have been considered since 1991 [START_REF]Evaluation de l'aptitude du logiciel (*) a traiter des consequences de mise a l'atmosphere de produits inflammables et/ou toxiques[END_REF].

The methodology of assessment is described in the following part.

2-Methodology of software assessment

This methodology can be illustrated by the following scheme (see figure 1 The positioning allows to define the precise scope of risk analysis in which a software works. At the beginning of 1994, about 140 softwares were recorded in various areas : toxic releases in water and in air (the most numerous), gas explosions...(see [6]).

The admissibility allows us to choose quickly (within a month) a few softwares in a large market that may be assessed more completely.

Finally, the evaluation itself is performed within a few months, and concerns the whole package (software, user's manual and theory guide).

The following points are studied : l)User friendliness of the package ;

2) Quality of the data base ;

3) Internal consistency of the software ; 4) comparisons with calculations that can be assessed independently.

There are different ways to perform those comparisons. The risk analyst may either perform a scientific assessment (i.e.; a parametrized study) or compare the results with a few given scenarios of industrial accidents from which consequences are known (see [5 ; 7]) or compare the results with experimental data.

2.1-Positioning

The purpose for positioning softwares in risk analysis is that the user has a general view on a large market in a restricted area (see [6]). It also allows the user to know how many softwares are able to deal with the particular uses he or she intends.

That is why some information was sought at a given time on given versions of given softwares.

There are several criteria that allows the positioning of a tool.

2.1.1-BASIC CRITERIA

Before buying a software, the user must answer a few questions about the origin of the software and its first purpose. An other question related to the origin pertains to the relevancy of answers the user may obtain. The age as illustrated in figure 2 hereafter, the cost of the package and the hardware required to use the software (PC, Mac, Workstation,...,RAM, ROM,...) are interesting points too.

2.1.2-SUBJECTIVE CRITERIA

They are difficult to define precisely. But some questions seem to be interesting to consider systematically. For instance, the user can wonder whether the aftersales services are competitive or not. In particular, does a hot line exist ? Then, if admissible, the software is fully evaluated.

2.2.2-EXAMPLE RELA TED TO ADMISSIBILITY

The hereafter example is source term test n°18. The aim of this test is to make sure the results related to mass flow rate, discharge velocity and temperature are close when considering a saturated tank or a padded one, the pressure in the gaseous part of which is equal to the saturated pressure.

Results

#**

Results are nearly the same

2.3-Evaluation

This step is in fact a full development of the work performed during the admissibility phases. All the following points are thoroughly considered (A to D), and a dialogue is held with the seller of the software on the aspects on which the package seems to be inconsistent.

The final report of the evaluation takes into account the positioning and admissibility parts, under the following scheme :

A-USER FRIENDLINESS OF THE SOFTWARE

Use of the software

The steps of installation must be clearly defined in the manuals, and it is necessary to have an efficient help from the retailer.

The user must be interested in the conditions of use : see the basic criteria for positioning a software. This point includes also a question related to the ease to install the software in the computer.

The software itself must be clearly built, and the results have to be clearly explained and referenced. Also convenient is the possibility for the user to store in memory the cases studied. We could illustrate these points in a previous paper [START_REF]Use of pertinent softwares in risk analysis[END_REF].

Quality of tools to assess phenomenon evolution

Another point of concern in user-friendliness of the examined version of the software is related to the quality of tools available to assess phenomenon evolution.

Error messages, guard-rails and limitations of the models

Another point with regard to the user-friendliness of the version examined is related to the error messages and the presence of guard-rails testing the physical consistency of the data introduced to calculate a case, in relation with the description of the limitations of the models.

B -QUALITY OF DATA BASE

One function of the softwares used is to give the physical properties of species involved in the study, in order to allow calculations.

For the analyst, the main features of the data base are the number of species involved and their nature, the ease with which to create a mix of species of the base, to add new species or to modify a property, the capacity of the data base for constituting a library of physical properties, and, finally, the completeness of the base concerning toxicity.

C -INTERNAL CONSISTENCY OF SOFTWARE

The purpose of this part of the examination is to make sure the tool the analyst uses is consistent. Problems of consistency may arise about several topics.

Consistency in the software architecture

These pertains to the results that have to be the same -or nearly the same -whatever the ways of dealing with the case are choosen.

These problems of consistency are mainly relevant when, for instance, flammable properties and modelling have been added on a software built initially to deal with dispersion.

Boundary problems

The main points of consistency arise at the boundary of domains where continuity must be fulfilled, according to the question «when physical conditions are very close, are the results close ?».

The problems may arise according to several boundaries and the results pertain mainly to calculation of source term and dispersion.

D -COMPARISONS OF RESULTS WITH CALCULATIONS THAT MIGHT BE OTHERWISE ASSESSED

For each software, comparisons are systematically made about a lot of calculations. Some examples are given related to source term, dispersion and flammable properties.

Source term

-Calculation of flow rate in a monophasic gaseous flow through an orifice as a function of pressure and temperature inside the reservoir.

-Pressure drop for the same flow through a pipe, as a function of the length of this pipe.

-Calculation of a liquid flow rate through an orifice.

-Calculation of a flow through a pipe.

-Discharge velocity related to phase. Flashing fraction and temperature of release.

-Formation of aerosols.

-Diphasic flow.

-Rain out and mass balance between flash, rain out and aerosols.

-Emptying the reservoir as a function of time.

-Pool formation, extension and evaporation.

Dispersion

Flammable properties

One has to note that the importance given to flammable properties depends on the main features of the software. The investigated softwares are very poor on flammable phenomena for their main purpose is source term and dispersion.

When the software includes flammable effects, comparisons are made considering :

-a given pool fire, various sizes and products involved (hydrocarbons, alcohol,...)

-BLEVEs of propane, butane, ethylene oxide...

-UVCE related to a given flammable mass of product, the concentration of which is between LIE and LSE.

3-An

Example of a lack of physical consistency

This example highlights some features of the evaluation. Generally speaking, PHAST is one of the best among the evaluated softwares.

Let's assume a monophasic gaseous leak of chlorine from a vessel through a pipe (length = 0m, lm, 10m, 100m), all other parameter fixed, let's vary the initial pressure in the vessel (Pres). The observed variable is the mass flow rate (m in kg/s).

We notified that the results are within the right order of magnitude, but we also observed a discontinuity around Pres = 1 barg (see figure3 below).

Such a discontinuity seems to be related to the connection between two different models, and is not always scarce in a software.

But experimentations do not produce continuous results either.

In that case, the order of magnitude for the mass flow rate as a function of Pres is correct. It is the same for its evolution. Therefore, the discontinuity that was pointed out is only a slight problem compared with the expected accuracy of results related to the existing models.

Mass flow rate (kg/s)

2 3
Initial pressure in the vessel (barg) -Lpipe=0m Lpipe=1m -Lpipe=10m -Lpipe=100m -Advantage: The two manuals fully describe the whole models used in the software; -Limitation 1: The user's manual would be clearer if it was integrating schemes and explanations about generic terms like : «early ignition», «late ignition», «flash fire», ... Indeed, the user is not always a specialist in risk analysis, and doesn't necessarily know all the specific english terms.

-Limitation 2: The theory guide would be more complete if it was describing the model used more precisely.

In the new version (4.2) of PHAST, the first limitation is no longer present.

A few schemes were added that help the user to understand the software better.

2) Related to the data base -Advantage 1: It's a full and useful base that allows the user to calculate the consequences of the release of a product chosen amongst 59 toxic chemicals; -Advantage 2: The first data base can be completed with another base that contains 900 chemicals; -Limitation 1: Some data are missing related to the assessment of the effects of a human exposure to a toxic gas; -Limitation 2: It's not possible to define a mixed substance made up with a few products.

In the new version, the second limitation does not remain. In the new version, all the advantages are still convenient, and limitation 2 is no longer present.

Conclusion

The methodology mainly dealing with positioning, admissibility and evaluation was applied to six softwares.

For such a work, the following restraints are to be considered:

1) The evaluation of a package is time consuming, Further developments will deal with other commercially available softwares.

*

  Easiness to use the software, * Internal consistency of the tool, * Consistency of the results as compared with calculations which might be otherwise assessed, * Comparisons with available data (experiments, investigations of accidents).

  figure 1 below) :

Fig. 1 :

 1 Fig.1: This method for evaluating softwares concerns the whole package, that is the software itself + the user's manual + the theory guide.

Fig. 2 :

 2 Fig. 2 : Scheme illustrating different possible classifications in families of softwares.

2. 1 . 3 -

 13 SPECIFIC CRITERIA ABOUT SOFTWARES ON GAS DISPERSION The points to be looked at are : * The models used (Gaussian, box, Lagrangian), The software itself, which is considered under 150 tests performed by INERIS. These tests aim at assessing the physical consistency of the software. They are about source term, dispersion and flammable properties, and aim at comparing results from software with theory. Their contents and philosophy are described and illustrated in the next. Generally speaking, the software is considered as admissible when : a) the answers related to the manuals are generally positive ; b) the tests don't point out a lot of inconsistencies.

1) In a phenomenon itself 2 ) 3 ) 4 ) 5 )

 2345 When the conditions given are close to those of a change of state When the different conditions vary continuously When conditions are close to the boundary of different modelling domains Lastly, when the conditions are close to an internal boundary of modelling which is unknown to the user.

-

  Transition from dense gas to passive dispersion.

  example : from PHAST 3.0 to PHAST 4.2 PHAST is a software from DNV Technica. Two non-Windows versions were evaluated (3.0 in 1991 and 4.2 in 1995 : see [5]). A Windows version exists, the number of which is 5.0. Two illustrations of the work performed are given here. The first one is about a test that pointed out a slight lack of consistency. The second one gives an idea of the improvements performed on software between the two versions 3.0 and 4.2.

Fig. 3 :

 3 Fig. 3 : Gas release through a pipe

3 )

 3 Related to the source term -Advantage 1: The models related to a catastrophic rupture (instantaneous) are correct... -Advantage 2 : The models related to a liquid pool development and evaporation are correct; -Limitation 1: The models related to a release through a pipe perform results (speed of the flow, mass flow rate, liquid fraction) that might be inconsistent... -Limitation 2: The option «Vent from vapour space» is not operational.

2 )

 2 It is difficult to give synthetic results because of the complexity of the points to deal with (user friendliness, quality of data base, internal consistency and comparison with calculations are parts of the assesssment of the software), 3) Only a given version of a given software is evaluated. The conclusions that are drawn about that version are generally speaking no longer relevant concerning a further version, 4) The assessment is strongly depending upon the knowledge of the physical phenomena involved.But the advantages of such a methodology for software assessment are mainly three:1) Eliminate softwares the quality of which is not convenient, 2) Increase the quality of existing softwares, 3) Enable the software developer to take account of the risk analyst's needs.

Acknowledgement

This work was founded by the French Ministry of Environment.