Startup and burnup strategy for Th-U/U-Pu fuel cycles in an EM2 reactor

General Atomics (GA) is developing the Energy Multiplier Module (EM2) which is a compact gas-cooled fast reactor as one of candidates of the Generation-IV nuclear energy systems. In the EM2 core, low enriched uranium is used as igniting fuel and depleted uranium is used for converting and burning. It indicates that EM2 can maintain critical operation for more than 30 years without refueling. To further study the Th-U fuel cycle performance in the EM2, two kinds of start-up strategies with Th-U (Th + 233U) and semi Th-U (Th + enriched 235U) are evaluated. Neutronics characteristics, such as the effective multiplicity factor (keff) and conversion ratio (CR) are analyzed from neutron usage point of view. The simulated results for the two kinds of fuels are compared with the U-Pu fuel from the design of GA. The analysis gives an insight into the pros and cons of U-Pu and Th-U fuel cycles in terms of the breeding capability and the discharged radio-toxicity. The breeding performance of the second generation EM2 is also presented and compared with that of the first generation EM2. It indicates that the multi-generation EM2 can deepen the burnup and reduce the waste management pressure for each kind of fuel loading strategy.