Pre-conceptual design activities of the NIST Neutron Source: preliminary thermal-hydraulics assessments

The NIST Neutron Source (NNS) is the proposed reactor for ensuring continued future availability of neutrons for the United States’ scientific needs. The NNS would allow the aging National Bureau of Standards Reactor (NBSR) at the NIST Center for Neutron Research (NCNR) to be decommissioned without affecting the availability of neutrons in the United States. This work describes preliminary thermal-hydraulics assessments of the preconceptual design of the NNS, and the NNS’s behavior during nominal operating conditions and various accident scenarios. Core-level and system-level analyses are conducted via models using the United States Nuclear Regulatory Commission’s (NRC’s) TRAC/RELAP Advanced Computational Engine (TRACE) code. The TRACE code allows for an adequate understanding of the thermal and hydraulic characteristics of the reactor and its components during various analyzed conditions. Safety analyses revealed that the proposed NNS operation maintains adequate margins to ONB, OFI, and CHF. In addition, accident analyses indicated that NNS remains within the safety criterion of blistering temperature throughout all analyzed conditions. Computational fluid dynamics (CFD) analyses revealed the opportunity for various design optimizations, including the design of the inlet to the reactor, the fuel assemblies, and the outlet to the reactor for improved performance and enhanced safety margins. The analyses in this work serve as a first step toward defining a holistic understanding of the thermal hydraulics in the NNS, and it provides a valuable reference for future design iterations.