SENSITIVITY ANALYSES OF THE THERMAL-HYDRAULICS SAFETY MARGINS IN THE PROPOSED NIST NEUTRON SOURCE DESIGN

抄録

The NIST Neutron Source, or NNS, is a proposed new research reactor at the NIST Center for Neutron Research in the United States of America. The NNS will serve as a state-of-the-art source for neutron scattering and irradiation experiments for the domestic and international community, replacing the currently operational but aging National Bureau of Standards Reactor (NBSR) onsite. The reactor will utilize U-10Mo LEU plate fuel assemblies and be moderated and cooled with light water in an upflow forced convection setup. A custom thermal-hydraulics 1D model is developed to perform preliminary assessments of the core’s flow behavior and evaluate the core’s safety margins. This paper is concerned with the critical heat flux ratio (CHFR) and the onset of flow instability ratio (OFIR) as safety margins. Under nominal conditions, it is found that both limits remain above the minimum threshold of 2, which is recommended by the US nuclear regulatory commission’s NUREG-1537 publication. To evaluate the uncertainty in the safety margins, input sensitivity analyses are performed with stochastic and deterministic approaches. The custom model and sensitivity analysis methodologies are detailed in the text, where uncertainties are provided for both safety margins in terms of expected power, mass flow rate, and inlet coolant temperature variations. The results demonstrate how the sensitivity of the safety margins to the inputs varies based on the sensitivity analysis method, where a deterministic approach shows more linearized trends compared to the stochastic trends. Discussions regarding how the results affect the design are included in the text.