Probabilistic simulation of the long-term evolution of radioactive waste disposal sites

A number of approaches to modelling site evolution over periods of thousands to millions of years have been tried within safety assessment programmes for radioactive waste disposal facilities. Of these approaches, probabilistic simulations have been found to be of great value because they can provide and quantify limits on the uncertainty which is inherent in our understanding of the behaviour of the natural system. Development of the Geologic Simulation Model and the Far Field State Model in the United States and the more recent work on the TIME2 and TIME4 models in the United Kingdom illustrate the history of these approaches. The most recent of these, TIME4, is coupled to the time-dependent pra code, VANDAL. Application of such approaches to specific sites is illustrated by the TIME2 and TIME4 models which have been developed to model site evolution of shallow and deep disposal environments. The models are based on stochastic simulations of future climatic cycles derived from long-term records of climatic change in the last million years. The limitations and advantages of these methods highlight some of the inherent problems of long-term safety assessments but also demonstrate the value of time-dependent models for assessing sites in different environments, at different depths and in different host geologies. 23 refs., 8 figs