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

Abstract

In support of the Radioactive Waste Management (RWM) safety case for a geological disposal facility (GDF) in the UK, there is a regulatory requirement to consider the likelihood and consequences of nuclear criticality. Waste packages are designed to ensure that criticality is not possible during the transport and operational phases of a GDF and for a significant period post-closure. However, over longer post-closure timescales, conditions in the GDF will evolve.For waste packages containing spent fuel, it can be shown that, under certain conditions, package flooding could result in a type of criticality event referred to as 'quasi-steady-state' (QSS). Although unlikely, this defines a 'what-if' scenario for understanding the potential consequences of post-closure criticality. This paper provides an overview of a methodology to understand QSS criticality and its application to a spent fuel waste package.The power of such a hypothetical criticality event is typically estimated to be a few kilowatts: comparable with international studies of similar systems and the decay heat for which waste packages are designed. This work has built confidence in the methodology and supports RWM's demonstration that post-closure criticality is not a significant concern.

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).…”