Technologies and Approaches for Safeguarding a Liquid Fluoride Thorium Reactor

Efforts are underway to design a liquid fluoride thorium reactor (LFTR) for commercial production of heat, electricity, and radioisotopes. The LFTR is largely based on the Molten Salt Reactor Experiment at Oak Ridge National Laboratory in the 1960s. It is a thermal neutron reactor in which the fissile material is predominantly 233UF4 dissolved in molten LiF-BeF2. 233U is formed via neutron capture by the 232Th fuel to form 233Pa. Reactivity is maintained by recharging the fuel salt with 233U at the same rate which it is consumed, and by resupplying the system with thorium tetrafluoride as needed. Proper design can minimize the production of transuranic nuclides and prevent them from entering the waste stream, leaving a waste stream that consists only of fission products that decay relatively quickly (except notably 99Tc and 129I). The thorium fuel is three times more abundant than uranium, offering promise of sustainable carbon-free power. Inevitable and intentional contamination of the 233U by 232U with its associated high- energy gamma radiation reduces the attractiveness of the fuel for diversion. Nevertheless, a reliable and practical safeguards system should be implemented. The fluid nature of the fuel presents an accounting challenge typically encountered in reprocessing plants, with the added challenge of high temperature and inert atmosphere. It will be necessary to monitor the concentration of 233U in the molten salt and gas streams and integrate over the entire fluid volume to calculate the fissile inventory. This approach is similar to that used at the Rokkasho Reprocessing Plant in Japan, which is being actively safeguarded by the IAEA and handles much larger quantities of fissile material than a single LFTR would use. In the case of the LFTR, rather than taking samples and destructively analyzing them to provide for nuclear material accountancy, it is recommended to utilize sensors that can be monitored remotely in real time and which are compatible with the molten salts. These technologies are based on electrochemistry, optical spectroscopy, and mass spectrometry. They provide tools for the operator to calculate material balances and control the process as well as tools for the inspector to verify operator declarations.