Microbial metabolism has the potential to control the biogeochemistry of redox-active radionuclides in a range of geodisposal scenarios. In this study, sediments from a high pH lime workings site were incubated under carefully controlled anaerobic conditions, at a range of alkali pH values with added electron donors and electron acceptors, to explore the limits and rates of bioreduction in a sediment system analogous to intermediate-level nuclear waste. There was a clear succession in the utilization of electron acceptors (in the order nitrate > Fe(III)-citrate > Fe(III) oxyhydroxide > sulfate), in accordance with calculated free energy yields and Eh values over the pH range 10–12. The rate and extent of bioreduction decreased at higher pH, with an upper limit for the processes studied at pH 12. The biochemical limits for such processes are discussed, alongside the potential impact of such forms of microbial metabolism on the solubility of a range of redox active radionuclides that will feature heavily in the safety case for the geological disposal of intermediate-level nuclear waste.

The implementation of a geological disposal facility requires the demonstration of confidence that such a facility would be safe during both the operational period and in the long-term after the closure of such a facility. The generic environmental safety case described in this paper is the vehicle used to demonstrate an understanding of environmental safety. It will be used to prepare a site-specific environmental safety case in due course. The approach taken will be consistent with a staged development and approval process, as advocated by the environmental regulators.

The safe implementation of geological disposal must be underpinned by sound science. This paper describes the approaches taken by the Nuclear Decommissioning Authority Radioactive Waste Management Directorate, the implementing body for geological disposal in the UK, to build an evidence base of scientific data and understanding which is robust to scrutiny and so provides confidence in the safety of geological disposal.

In safety assessments for nuclear waste disposal, the biosphere is a completely open system, whereas the bedrock can be treated with comparably simple boundary conditions. The bedrock has a vital role in providing and maintaining favourable conditions for the waste, but the public interest tends to focus on the biosphere. More importantly, the bedrock groundwater does arrive from the biosphere. Also, the regulations usually set the safety criteria in terms of doses occurring in the biosphere. Thus, it is reasonable to address the biosphere as a part of the disposal system, performance of which should be described based on conceptual models that cover phenomena and processes controlling radionuclide release and transport. From these, the actual assessment models and data are expected to be derived based on high-level research and expertise obtained through empirical study. In this paper, the complexity expected of a biosphere assessment within a mature safety case is discussed through the example of the Finnish regulations and recent safety cases for nuclear waste disposal. Both elaborate and complex presentations and stylised approaches have their benefits, and a balance needs to be sought.