Nuclear Science and Engineering / Volume 158 / Number 3 / March 2008 / Pages 264-271
A burnable poison (BP) loading principle has been proposed for once-through-then-out refueling of a high-temperature gas-cooled reactor (HTGR) core with pebble fuel. The principle holds that an axial core power peaking factor can be minimized when k of the fuel pebbles is kept constant during their axial movement from the top to the bottom of the core by adding BP. This principle has been confirmed numerically using B4C with 10B enrichment of 90% and Gd2O3 with natural content as BP. Spherical particles of B4C and Gd2O3 are distributed uniformly in the fuel pebble. The respective optimal radius and number of BP particles are 90 m and 1650 for B4C and 950 m and 16 for Gd2O3. Through addition of B4C and Gd2O3, the power peaking factors are reduced from 4.4 to 1.61 and 1.64, respectively. Burnup reactivity swings are reduced from 38% to about 2% in both BP loadings. Because of reduction of the power peaking factors, the maximum fuel temperatures are respectively lower than the maximum permissible values of 1250 and 1600°C for normal operation and depressurization accident.