Probabilistic assessment, for Italian sites, of the consequences of severe accidents and of the actions taken for protection of the population

The study performed by ENEL with the CRAC 2 code aimed at obtaining, with a probabilistic treatment of the meteorological data, a detailed evaluation of the effects of the actions taken for protection of the population during a severe accident at two reference sites. The source term taken into consideration during the severe accident refers to two different situations: the first situation represents the worst possible scenario with a large dry containment, the second situation represents an envelope source term that is in line with the present European trends. Of the CRAC 2 code the stratified sampling option was used, with the meteorologic matrix of all the one-year data relating to the two reference sites. The paper also examines the probability distribution that better approximates the individual dose (whole body and thyroid) and the related cumulative probabilistic distribution function. A parametric analysis takes into account the fact that the source term has negligible associated energy and release height, whereas the release duration may vary. With regard to the emergency reference value given for the equivalent whole body dose, a discussion is presented on the implications of comparing with said values the equivalent whole body dose or the effective dose computed in case of a severe accident. A sensitivity analysis was performed of the benefits afforded by evacuation at different speeds, sheltering in the different exposure paths, and iodine prophylaxis started at different times during the accident. The results of the analyses give useful indications for preparation of the emergency plans, and confirm that in the short time the combination of sheltering and iodine prophylaxis, adequately balanced with a graded evacuation of the population from the areas close to the plant, gives the best protection of the population in the case of severe accidents