System-Level Thermal-Hydraulics Analysis of the Proposed NIST Neutron Source Design

The National Institute of Standards and Technology (NIST) is considering replacing its existing reactor with a cutting-edge test reactor. This new reactor, the NIST Neutron Source (NNS), is currently in the conceptualization phase. The current design of the NNS consists of 20 MW compact reactor that uses light-water cooling, features a pool-type configuration, and incorporates a heavy water reflector tank. The core consists of a 3 × 3 rectangular lattice of assemblies fueled with low-enriched U-10Mo curved plates with aluminum cladding, where each assembly contains 21 fuel plates. This paper provides system-level thermal-hydraulics analysis results for the proposed NNS under various normal operation and accident conditions. The system-level model of the NNS is developed using thermal-hydraulics code TRACE V5.0 Patch 5. The model contains the core components, coolant piping, the reactor pool, and the primary coolant pumps to provide the thermal and hydraulic behavior of the coolant in the system and throughout the core. The goal of this analysis is to establish safety margins for the safe operation of the NNS and assess the evolution of pre-determined accident conditions. Analyses include hottest channel temperature distributions and thermal safety margins such as critical heat flux ratio and the onset of flow instability ratio, both during steady-state and transient conditions, such as loss of power accident, loss of heat sink accident, loss of flow accident, loss of coolant accident, and reactivity insertion accidents. The natural circulation capability of the NNS is also assessed in this paper.