Photoneutron production and characterization in fluoride-salt cooled high-temperature reactors

Abstract

Due to the presence of beryllium-bearing salts, fluoride-salt cooled high-temperature reactors (FHRs) are expected to have higher production of delayed photoneutrons than other common power reactor systems which may impact transient behavior. New methods for computationally quantifying the characteristics of delayed photoneutrons using Monte Carlo particle transport, including absolute fraction, effective fraction, and group decay constants, are outlined. These methods are validated where possible against existing data from CANDU reactors, and then used to make a best-estimate characterization of the delayed photoneutron effect in a prototypical FHR design. It is found that delayed photoneutrons have a worth of approximately 9 pcm in the reference FHR system, and that their importance is slightly higher than that of fission neutrons due mostly to their softer spectrum. Existing emission group structures from literature have been compared to computational data and found to be lacking, particularly in their representation of groups with short half-lives. Therefore, a new 13-group structure is proposed for use in FHR dynamic calculations.