Reducing DLOFC fuel temperatures by mixing thorium with LEU in a single-zone six-pass fuel cycle in a PBMR-DPP-400 core

Abstract

Many studies have been done in neutronics and thermal-hydraulic simulation of the standard 6-pass fuel recirculation scheme for the standard 9.6 wt % enriched, 9 g per fuel sphere low-enriched uranium (LEU) fuel in the PBMR-DPP-400, using different versions of the VSOP diffusion codes. Maximum DLOFC temperatures were all below the upper limit of 1600 °C. The DLOFC temperature is highly dependent on the peaking factor of the power; meaning a lower maximum DLOFC temperature can be obtained by supressing the axial power peak and by moving the radial power peak towards the external reflector. In this study the standard 6-pass fuel recirculation was retained. The improvement strategy was thus attempted by means of flattening the axial power profile by mixing substantial amounts of thorium into the LEU fuel. The addition of thorium led to breading of substantial amounts of 233U. This led to slower depletion of enrichment with burn-up, which increased fuel reactivity and power densities near the bottom of the core and thus flattened the axial power profile. The effect was a reduction in maximum DLOFC temperature by 44 °C. The simulations were made using the VSOP-99/05 diffusion code. It was further shown that the results obtained are also applicable to the Chinese HTR-PM and the proposed strategies for further improvement can be expected to produce even much better results in Block Reactors than in Pebble Bed Reactors Reducing DLOFC fuel temperatures by mixing thorium with LEU in a single-zone six-pass fuel cycle in a PBMR-DPP-400 core