4. Conclusions

In order to utilize thorium fuel in block-type HTRs with the features of inherent passive safety, high burnup, and hard neutron spectrum, two key factors of thorium content and spatial separation levels are chosen to be investigated. The thorium content represents the thorium/ uranium fuel composition and the spatial separation level represents the spatial distribution of thorium/uranium fuels. For every thorium content,the spatial distribution of thorium and uranium fuels are concluded into four spatial separation levels, that is, no separation level (Th/U MOX), TRISO level (SBT), channel level (SBU), and block level (WASB) for the thoriumfueled block-type HTRs.

The nuclear performance of the reactor core, that is, initial effective multiplication factor, average conversion ratio, and initial inventory of U-235, under four spatial separation levels are obtained in the one-batch fixed-pattern refueling mode by the two-step calculation scheme developed based on the DRAGON. For every thorium content, the initial inventory of U-235 decreases with the increase of the spatial separation level from Th/U MOX to WASB, because spatial self-shielding effect is strengthened by the lumped thorium and uranium. However, the SBU level is nearly the same as the WASB level. If the multiple fuel batches and realistic refueling patterns are considered, the difference of the SBU from the WASB could be large. On the other hand, the initial inventory of U-235 decreases with the increase of the thorium content for the SBT, SBU, and WASB levels. However, for Th/U MOX level, it increases in the range of thorium content from 0 to 30% and decreases when it is larger than 30% because of the better performance of U-233 than Pu-239 in thermal reactors.

The performance difference of the four spatial separation levels is synthetically evaluated bythe levelized lifetime cost method.The fuel cycle cost of the Th/U MOX, SBT, SBU, and WASB changes with the same trend as the effective enrichment of U-235 or the initial inventory of U-235 in the reactor cores because the latter determines 70% of the total cost.
