Probabilistic accident consequence Uncertainty Assessment using COSYMA : uncertainty from the food chain module

This report describes the analysis of the uncertainty in the model predictions resulting from uncertainty in the values to be assigned to the input parameters describing transfer of radionuclides along food chains. The main aim of this part of the study was to identify the input parameters whose uncertainties make large contributions to the overall uncertainty; the parameters identified would then be included in the final analysis of the uncertainty in the whole system. Uncertainty analysis involves specifying probability distributions for the values of each of the parameters involved, sampling sets of values from those distributions and propagating them through the model to derive information on the uncertainty in the model prediction. Those parameters whose uncertainties make major contributions to the overall uncertainty can then be identified using correlation coefficients between the input values and the model outputs. Earlier expert judgment studies have provided distributions on the values of the parameters describing the behaviour in food chains of those nuclides that make major contributions to the doses following reactor accidents. This information has been extended by the project staff to include distributions on other nuclides considered in Cosyma. The study evaluated the uncertainty on individual doses and risks, the extent of countermeasures and the number of health effects in the population. The calculations were undertaken for a number of situations with and without allowing for the effects of countermeasures. Some licensing procedures require estimates of the potential individual doses and risks at points near the reactor site. Potential doses are calculated assuming people are outdoors for the whole of the period of interest, and so make no allowance for countermeasures or shielding by normal occupation of buildings. The study evaluated such potential doses, and the associated risks of health effects. Consequences assuming normal living (i.e. allowing for shielding by buildings but no countermeasures) are considered in the licensing procedures of several countries. Hence, calculations were undertaken for individual and collective doses and risks for normal living. The source terms chosen encompass a wide range of characteristics (e.g. magnitude and composition) of source terms that have been postulated for LWRs. They are taken from analyses of the pressurised water reactor proposed for the Hinkley Point site in the United Kingdom. UK1 is a very large release; it is the risk-dominant source term for early health effects and a major contributor to the overall risk of late health effects from the reactor. CB2 is a smaller, but less unlikely, sequence that also makes a major contribution to the overall risk of late health effects from the reactor. DBA is a design basis accident. The study showed that the uncertainty (expressed as the ratio of the 95th to the 5th percentile of the probability distribution on the expectation value of the consequence) on the extent of the area where food restrictions would be imposed, and its time integral, lies between about 5 and 70 for the CB2 source term, and between about 70 and 500 for the DBA source term. The uncertainty on the extent of the area where restrictions would be imposed on grain production is larger as the area could be very small for some of the parameter values considered. The parameters whose uncertainty makes the largest contribution to the overall uncertainty on the extent of milk and beef restrictions are the interception factors for pasture and for hay/silage, the retention time on hay/silage and the Fm and Ff transfer factors for caesium. The Fm factor for iodine is identified as important for the DBA source term. The most important uncertainty for green vegetable restrictions is the soil contamination, while those for the extent of cereal restrictions are the interception factor and processing loss for cereals and the retention time of caesium on cereals. The study showed that the uncertainty on the number of fatal cancers is between factors of about 3 and 100, with larger uncertainties on the number of effects if no countermeasures are taken. The uncertainties making the most important contributions to the overall uncertainty are those on the values of Fm for ioding and caesium, the interception factor for pasture and the retention time for hay and silage.