Post-operation radiological source term and dose rate estimates for the Scalable LIquid Metal-cooled small Modular Reactor

The Scalable LIquid Metal cooled small Modular (SLIMM-1.2) reactor has recently been developed at the University of New Mexico's Institute for Space and Nuclear Power Studies, for generating 10-100 MWth continuously without refueling for ∼66-6.3 full-power years, respectively. The SLIMM reactor is cooled by natural circulation of in-vessel liquid sodium during nominal operation and after shutdown. It is assembled, fueled, and sealed in the factory, and transported by rail, a heavy-duty truck, or a barge to the site and installed below ground on seismic isolation bearings. Following end of life (EOL) shutdown, the SLIMM reactor remains temporarily on site until the external dose rate decreases to an acceptable level for safe handling. The conducted analyses calculate the post-operation radiological source term, the photon and neutron emission rates, and the biological dose rates in the reactor and outside the guard vessel at EOL and as a function of time after shutdown. When operating at 100 MWth before shutdown, the calculated biological dose rate outside the SLIMM reactor guard vessel ∼65 days after EOL shutdown is slightly lower than the US federal transportation limit of 0.2 rem/h. It decreases to 0.069 and 0.036 rem/h, 6 months and 1 year after EOL shutdown, respectively. These rates ensure safe handling, removal and transportation of the post-operation SLIMM-1.2 reactor back to the factory or a processing facility, and subsequent replacement by another unit loaded with fresh fuel. Results also show that for the same thermal power of 100 MWth and operation life of 6.3 full power years, the SLIMM reactor generates ∼179% more 239Pu and ∼72-100% less minor actinides than a comparable PWR. The former decreases the fissile depletion during reactor operation, while the latter decrease the toxicity of used fuel at EOL.