The enhancement of a biosphere code for use in the assessment of deep repositories for radioactive waste

A disposal system for radioactive waste is conventionally considered to consist of the engineered barriers of the repository, the geosphere and the surface environment or biosphere. Computer codes have been developed to assist in assessing the impact of radionuclides migrating from the repository through the disposal system. Codes have been developed to represent the repository, the geosphere and the biosphere. The fundamental role of the biosphere codes is, for radionuclide inputs, to estimate the dose, or probability distribution function of dose, to a maximally exposed individual as a function of time. In the United Kingdom, the primary target for long-term radiological impacts from a single disposal facility is that the risk of fatal cancer to an individual in any one year should not exceed one in a million at any time. The recent revival of interest in the UK in the deep disposal of radioactive waste has resulted in the need for the risk to be estimated over timescales up to a million years, since a deep geologic repository might be able to withstand the effects of several future glacial episodes. Biosphere modelling for deep disposal of radioactive waste poses particular problems since the surface environment is expected to evolve as a result of changes in climatic conditions. Consequently the effect of climate induced changes in geomorphology, land use and sea level on the calculated risk have to be considered. This paper outlines the development of a new version of the dynamic biosphere model DECOS, which was developed originally in the context of shallow site assessments. The new version of the code, called DECOS-MG, is capable of simulating the effect of multiple glacial cycles and changes in sea level. (14 refs., 3 figs., 2 tabs.)