Assessment of the occupational dose rate during fuel handling and storage of various fuel cycles used in the super-critical water reactor and the impact on deep geological repository performance

The Canadian concept of the Pressure-Tube Super-Critical Water-Reactor (PT-SCWR) uses Pu/Th in a once-through fuel cycle. The implication of using Pu/Th in a PT-SCWR on the safety of the workers in the Deep Geological Repository (DGR) facilities as well as on the public after the closure of the DGR was investigated. A comparison between the Pu/Th and other fuel cycles, namely, U/Th (two U/Th fuels with different enrichments and different UO2/ThO2 ratios), and the regular UO2 was also made. The enrichment of U-235 in U/Th and UO2 was adjusted to give the same exit burnup of the proposed Pu/Th fuel, namely 60 GWd per ton of heavy metals (THM). The dose rate as a function of distance from the surface of a canister holding different types of spent fuel, and the thickness of the shielding required to reduce this dose rate to 10 μSv/h were estimated. The dose rate after 50 years of decay for Pu/Th fuel channel is higher than that of uranium by about 8%. This will increase the dose to the workers in DGR facilities unless more shielding is used. Pu/Th will need 13% more thickness of concrete compared to UO2. Using U/Th fuel will also increase the dose received by workers, compared with uranium-only fuel, but less than using Pu/Th. The neutron dose rate of the Pu/Th fuel is the highest dose rate from all fuels studied. Pu/Th spent fuel will cause the highest dose to the public in case of an accident of the DGR after closure as it will produce the largest amount of I-129 per THM. Although the exact dose received by the critical group is not expected to exceed the dose limit for DGR, it is higher than alternative fuels.