Understanding the likelihood and consequences of post-closure criticality in a geological disposal facility

R J Winsley,T W Hicks,P N Smith,T D Baldwin,R M Mason

doi:10.1180/minmag.2015.079.6.30

R J Winsley, T W Hicks + Show 3 more

Open Access

https://doi.org/10.1180/minmag.2015.079.6.30

Copy DOI

Abstract

AbstractA geological disposal facility (GDF) will include fissile materials that could, under certain conditions, lead to criticality. Demonstration of criticality safety therefore forms an important part of a GDF's safety case.Containment provided by the waste package will contribute to criticality safety during package transport and the GDF operational phase. The GDF multiple-barrier system will ensure that criticality is prevented for some time after facility closure. However, on longer post-closure timescales, conditions in the GDF will evolve and it is necessary to demonstrate: an understanding of the conditions under which criticality could occur; the likelihood of such conditions occurring; and the consequences of criticality should it occur.Work has addressed disposal of all of the UK's higher-activity wastes in three illustrative geologies. This paper, however, focuses on presenting results to support safe disposal of spent fuel, plutonium and highlyenriched uranium in higher-strength rock.The results support a safety case assertion that post-closure criticality is of low likelihood and, if it was to occur, the consequences would be tolerable.

Highlights

RADIOACTIVE Waste Management (RWM), a wholly owned subsidiary of the Nuclear Decommissioning Authority (NDA), is responsible for implementing geological disposal of the UK’s higher-activity waste inventory
This paper summarizes results from two recent projects that form part of RWM’s post-closure criticality safety research programme
The likelihood of criticality work presented in this paper focuses on the expected post-closure evolution of waste containers and the geological disposal facility (GDF) itself

Summary

Introduction

RADIOACTIVE Waste Management (RWM), a wholly owned subsidiary of the Nuclear Decommissioning Authority (NDA), is responsible for implementing geological disposal of the UK’s higher-activity waste inventory. RWM has produced an initial ‘generic’ Disposal System Safety Case (DSSC) (Nuclear Decommissioning Authority, 2010a) to communicate the safety arguments for geological disposal using a range. A GDF will include disposal of a significant amount of fissile material, which if not managed appropriately, could hypothetically, under very specific conditions, lead to an unplanned neutron chain reaction (‘criticality’). The environment agencies’ Guidance on Requirements for Authorisation (GRA) for a GDF (Environment Agency, 2009) requires a demonstration that “the possibility of a local accumulation of fissile material such as to produce a neutron chain reaction is not a significant concern”.

Results

Discussion

Conclusion