Postoperation Dose Rate Estimates for the Very-Small, Long-Life, Modular Reactor

The very-small, long-life, modular (VSLLIM) nuclear reactor generates 1.0–10 MW_th, for ∼92 and 5.8 full power years (FPY), respectively, without refueling. This factory fabricated, assembled, and sealed reactor is cooled by natural circulation of in-vessel liquid sodium (Na). It offers redundant control and passive operation and decay heat removal features. The VSLLIM reactor, together with other plant components for generating electricity using open Brayton cycle, is deployable on a portable platform. Alternatively, multiple units could be installed at a site and use high thermal efficiency Rankine or supercritical CO₂ cycles for electricity generation. Extensive analyses are performed to estimate postoperation external biological dose rate and the storage time required on-site for this dose rate to decrease to or below the U.S. federal transportation limit (0.2 rem/h or 2 mSv/h) for safe handling, removal, and replacement with a new unit loaded with fresh fuel. Results show that gamma photons emission from the uranium nitride (UN) fuel and the activated HT-9 steel cladding for the UN fuel rods, the core structure, and the reactor primary vessel, is a major contributor to the radiological source term, and the external biological dose rate estimates. After operating at 10 MW_th for 5.8 FPY, using 6 cm thick lead shielding of the reactor guard vessel, decreases the postoperation external biological dose rate to be in compliance with the federal limit, only after∼ 103 days of postoperation on-site storage.