The innovative CANDLE (Constant Axial shape of Neutron flux, nuclide densities and power shape During Life of Energy producing reactor) burnup strategy has been successfully applied to both fast and thermal reactors. In particular for thermal reactor applications, CANDLE block-type high temperature gas-cooled reactors (HTGRs) with either uranium or thorium fuel cycle had been proposed and investigated for their simple and safe reactor operation, and the ease of designing a long life reactor. Small sized long life CANDLE HTGRs with thorium fuel shows superior burnup performance than the ones with uranium fuel but their axial power peaks are relatively higher which may not be advantageous during a depressurization accident. In this work, we proposed and investigated the use of Pa-231 mixed homogeneously in the (Th-232/U-233)O_2 fuel kernel of the TRISO particles to obtain lower axial power peaks. Addition of Pa-231 decreases the required amount of natural gadolinium burnable poison in the fresh fuel for establishing a valid CANDLE HTGR design since Pa-231 has a large thermal absorption cross section. Besides the role as a burnable poison nuclide, Pa-231 also serves as a fertile nuclide during the CANDLE burning since Pa-231 is finally transmuted to a fissile U-233 nuclide. A promising analysis result shows that for U-233 enrichment of 15 w/o and Pa-231 addition of 7.50 w/o, the axial power peak is decreased from 5.9 to 3.6 W/cm^3 while the averaged burnup is increased from 138 to 149 GWd/t. This extends the core life time about 16 %, i.e. from 35 to 41 years with CANDLE active core height of 800 cm and reactor thermal power of 30 MWth.
