The consequences of hypothetical post-closure criticality

Abstract

The Environment Agency Guidance on Requirements for Authorisation (GRA) of a geological disposal facility (GDF) 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." A neutron chain reaction that is just self-sustaining is also known as critical.Waste packages can be designed to ensure that criticality is impossible during the transport and operational phases of a GDF, and for a significant period post-closure. Over longer times, however, packages may degrade, and groundwater flows could lead to a localized accumulation of fissile material. Hence, even though the initial distribution of materials would need to change substantially, criticality cannot be ruled out completely.This paper describes how an accumulation of fissile material could, hypothetically, lead to a critical configuration; how such a system could evolve; what the local consequences could be; and how the engineered and geological barriers could be affected. The conclusion from studies to date is that, even for large (and very unlikely) fissile accumulations, the consequences of a post-closure criticality event are not a significant concern.

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

“…Another type of transient criticality is a short-lived but energetic transient and can result from a critical system with positive temperature feedback, where k effective increases with a rise in temperature. These are termed Quasi-Steady-State (QSS) and Rapid Transient (RT) criticality events, respectively, and could potentially have very different physical consequences for a GDF (Mason et al, 2012a). A hypothetical post-closure criticality event would not, necessarily, evolve as a QSS or RT event.…”