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

Abstract

A 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.

Consequence modelling of hypothetical post-closure criticality events for spent fuel disposal

“…Overview: hypothetical post-closure criticality from spent-fuel disposal An illustrative disposal concept for PWR spent fuel in a higher-strength rock is described by Winsley et al (2015). The waste package contains four fuel elements from a PWR in a square array within a nominally dry cylindrical copper and iron waste container.…”

“…Recent research led by RWM has considered the likelihood of criticality extensively for a wide range of waste packages and illustrative disposal concepts and demonstrates clearly that any criticality is either not possible (e.g. due to insufficient fissile material), or has a low likelihood of occurrence (Hicks and Baldwin, 2014;Winsley et al, 2015).…”

“…The focus for this paper is understanding the consequences of a hypothetical criticality event resulting from the disposal of spent fuel from a pressurized water reactor (PWR), including how detailed simulation of critical systems supports the understanding. A broader overview of the RWM-led research programme, discussing both the likelihood and consequences of post-closure criticality has been described by Winsley et al (2015).…”

The IGD-TP Geodisposal 2014: Introduction to the Conference Proceedings

“…Here, papers cover: bentonite buffer erosion in fracture systems (Reid et al, 2015); carbon-14 behaviour in the engineered geological disposal facility system (Doulgeris et al, 2015); modelling post-closure criticality events associated with spent-fuel disposal (Mason et al, 2015); the European Network of Testing Facilities for the 'Quality Checking of Radioactive Waste Packages' (ENTRAP) programme and its potential interaction with the IGD-TP (Tietze-Jaensch et al, 2015); manufacturing of copper canisters for geological disposal (Hernandez-Selva et al, 2015); dissolution of MAGNOX blend nuclear waste glass (Cassingham et al, 2015); the impact of thermal cycling on bentonite permeability (Zihms and Harrington, 2015); and on the likelihood and consequences of post-closure criticality in a geological disposal facility (Winsley et al, 2015).…”