On the Neutron Kinetics during a Promptcritical Accident in a Heavy Liquid Metal Fast Reactor and the Importance of Low-Energy Neutrons

Accidental scenarios that involve degradation of a fast reactor core and/or relocation of its fuel material call for particular attention with respect to promptcritical reactivity events. The dynamics of a power transient during such an event is governed by the Prompt Neutron Generation Time (PNGT), a parameter that is sensitive to the moderating power of the system. Since Heavy Liquid Metals (HLMs) have a boiling point that is higher than the melting point of stainless steel, the first degradation mechanism to occur in a Heavy Liquid Metal Fast Reactor (HLMFR) is likely to be the loss of structural material. This sequence holds the potential to create conditions for considerable spectrum softening and may thus have important implications for the value of the PNGT. In the framework of this study, a (postulated) complete absence of structural material in and around the reactor core of an HLMFR is demonstrated to lead to an increase in the PNGT by an order of magnitude. The sensitivity of the fission energy release during a promptcritical event to the value of the PNGT is further investigated by employing the severe accident code SIMMER-III. To correctly model a degraded reactor core characterized by a considerably softer neutron spectrum when compared to the spectrum of its intact configuration, a new neutron data set is generated. This is done by introducing new energy groups to the already existing 11-energy-group structure and collapsing multigroup cross-section data by employing a weighting spectrum representative of a degraded core configuration of an HLMFR. Subsequent simulations demonstrate that an increase in the PNGT by a factor of ∼4 yields an increase in the fission energy release during a Core Disruptive Accident (CDA) by ∼50 %. It is therefore established that low-energy neutrons may play an important role during a promptcritical reactivity transient in an HLMFR.