The biosphere in post-closure radiological safety assessments of solid radioactive waste disposal

The principal aim in geological disposal of solid radioactive wastes is to provide long term isolation of the wastes from the accessible environment, so that most of their radionuclide content can decay to stable products. However, on very long timescales, small quantities of some long lived radionuclides can be transported back to the biosphere, typically by migration in groundwaters. In these circumstances, there is a need to determine whether the releases that are assessed to occur give rise to radiological impacts that are judged to be acceptable. There is, therefore, a need to have tools in place to calculate the distribution and transport of radionuclides in the range of biosphere systems that may exist in the future, and to evaluate the radiological impacts of those radionuclides on hypothetical future individuals and population groups. Furthermore, criteria have to be developed by which the acceptability of these radiological impacts can be judged. In this paper, an account is given of the radiological protection criteria that have been developed for geological disposal of radioactive wastes. This outlines international and national regulations and guidance, with a particular emphasis on material relevant to the UK. The regulations and guidance emphasise the need to undertake assessments applicable to very long timescales, sometimes of the order of a million years, and to ensure that individuals or small population groups do not incur annual radiation exposures or risks in excess of specific limits, targets, or constraints. Such assessments require innovative approaches to characterising the range of potential environments that may occur at any specific site and to modelling radionuclide transport through those environments. Zn order to develop and justify such innovative approaches, it is appropriate to complement assessment studies with a substantial programme of research and site characterisation activities. Therefore, having set out the criteria relevant to geological disposal of radioactive wastes in the UK, an account is given of the approach adopted by UK Nirex Ltd for demonstrating compliance with those criteria, emphasising the role of biosphere modelling in that approach and the close links with both a long term, generic research programme and the site characterisation studies undertaken at Sellafield (Cumbria, UK). Finally, some reflections are provided on issues arising from the work undertaken to date. In particular, attention is given to the role of the biosphere component of an assessment in determining the acceptability of a proposed facility. Specifically, emphasis is placed on the need to complement acceptance criteria with a methodology for demonstrating compliance that is consistent with the principles upon which the acceptance criteria were based.