An optimal loading principle of burnable poisons for an OTTO refueling scheme in pebble bed HTGR cores

Abstract

An optimal loading principle of burnable poisons (BPs) is proposed to eliminate the problem of an excessively high power peaking factor in once-through-then-out (OTTO) pebble bed HTGR cores. The effectiveness of the principle is confirmed through neutronic and thermal hydraulic calculations for an HTGR core with thermal power of 600 MWt. Spherical BP particles are distributed uniformly together with TRISO-coated fuel particle inside the free fuel zones of fuel pebbles to maintain constant k ∞ during burnup. The applicability of BP materials, such as B 4C, Gd 2O 3, Sm 2O 3, Eu 2O 3, Er 2O 3, CdO, and HfO 2, is investigated. Because complete BP depletion at the target burnup is required to avoid reactivity loss and extra enrichment, the four BP materials considered as applicable candidates are B 4C, Gd 2O 3, Er 2O 3, and CdO. The complete BP depletion is demonstrated by a burning fraction at the target burnup. The power density is excessively high at the core top and the power peaking factor is 4.44 when no BP is added to the fuel pebbles. Optimal BP loading reduces the power peaking factor from 4.44 to about 1.7. Because of the power peaking factor reduction, the maximum fuel temperatures are lower than the maximum permissible values of 1250 °C for normal operation and 1600 °C during a depressurization accident.