Post-closure safety assessment

This paper describes the approach under development by UK Nirex Ltd to assess the post-closure safety performance of a deep geological repository for radioactive waste. The term ''post-closure'' refers to the period following operation of the repository, when it has been filled with packaged wastes, backfilled and sealed. Assessment deals with the identification and modelling of the processes and pathways by which radionuclides from the repository may eventually return to the human environment. Post-closure performance assessment forms one element of the comprehensive case that must be developed by UK Nirex Ltd before a repository can be licensed and authorized for construction and operation. The paper outlines the demanding safety requirements for the post-closure period and indicates how they have played a significant part in the development of the Nirex disposal concept. Performance assessment makes extensive use of computer modelling. The models used to assess radionuclide migration from the underground repository to the surface environment are outlined. (author)