Probabilistic safety assessment: work for UK Nirex Ltd

Probabilistic calculations using the computer code MASCOT-3 played a major role in post-closure safety assessment studies carried out for UK Nirex Ltd during 1988. This code estimates risk using mathematical models of the system in which many parameters are subject to uncertainty, described by probability distribution functions. Comparatively simple models, with no explicit time-dependency, are used. Laplace transform techniques are used to calculate solutions efficiently. Very general types of correlated parameter distributions may be specified in the form of algebraic formulae. Details are given of the sub-models used for release of radionuclides from the vault, transport through the geosphere, and transmission in the biosphere to produce doses to Man. Altogether, some 13 combinations of disposal concept and geological environment were treated using MASCOT-3. Some example results are shown, and the overall conclusions reached are listed. 3 refs., 5 figs