Impact on Safety of the Conversion of the NIST Research Reactor

Abstract

The NIST research reactor is heavy-water moderated and cooled and fueled with high-enriched uranium fuel. It operates at 20 MW and provides thermal and cold neutrons for researchers. A program is underway to convert the reactor to low-enriched uranium (LEU) fuel using a U-Mo alloy. An LEU core has been designed that minimizes changes to the fuel elements and maintains the current optimum fuel cycle length, but incurs a penalty to researchers because the additional 238U in the core reduces the neutron flux that goes into the beam tubes. In the current study, the safety of the converted core is analyzed for normal operation and under postulated reactivity-initiated and loss-of-flow accidents. Neutronic parameters as a function of burnup are obtained from the three-dimensional Monte Carlo code MCNP and transient analysis is done with the system thermal-hydraulic code RELAP5. The parameters that are calculated to assure safety include shutdown margin, reactivity feedback coefficients, critical heat flux ratio, onset of flow instability ratio, and clad temperature. The results show that the conversion will not lead to significant changes in the safety analysis and there is adequate margin to fuel failure during accidents.