Limitations Of MUF For MSRs

Existing light water reactors utilize item accounting for tracking discreet fuel assemblies, while bulk handling facilities calculate material unaccounted for (MUF) as part of safeguards verification. However, some next-generation reactors are moving away from countable fuel rods and proposing new design aspects that could require a MUF calculation for materials accountancy. One of the next-generation reactor classes of interest is the molten salt reactor (MSR), which dissolves fuel in the coolant. The MSR design calls for the actinides to stay in the reactor for the lifetime of the fuel with some designs calling for minimal online salt processing. The MSR design is being proposed for use in various markets including cargo ships and scalable electric markets, so identifying accurate ways of tracking the actinides is imperative to the deployment of the MSR. This study uses the molten salt demonstration reactor (MSDR), a 300 MWe reactor designed by Oak Ridge National Lab to calculate theoretical MUF for the MSR design. The MSDR’s 8-year fuel lifetime is modeled using beta tools in SCALE, which represent a good estimate given the currently available depletion tools, to understand the evolution of the actinides over the fuel lifetime in the reactor and calculate a theoretical MUF calculation for the MSR design. The MSDR uses LEU fuel and produces hundreds of kilograms of plutonium over the eight-year life of the fuel, leading to a naturally increasing standard deviation in the uncertainty for plutonium over the fuel’s lifetime. Two variables are controlled in diversion scenarios: the first is the time the diversion occurs, as the actinide population is different over the course of the fuel life, and the second is the amount of time taken to remove the actinide. Diversion scenarios are investigated at three times: one year after operation, five years after operation, and seven years after operation. The material loss time was modified to evaluate both abrupt and protracted loss scenarios. MUF calculations show that with increased time in the reactor the likelihood of detecting the diversion decreases as the plutonium inventory increases. These results suggest that differing safeguards approaches may be needed for MSRs.