Abstract :
Burnup calculations have been performed on a standard HTR fuel pebble (fuel zone with radius of 2.5 cm surrounded with a 0.5 cm thick graphite layer) and burnable poison particles (BPPs) containing B4C made of pure 10B or containing Gd2O3 made of natural Gd. Two types of fuel were considered: UO2 fuel made of 8% enriched uranium and PuO2 fuel made of plutonium from LWR spent fuel. The radius of the BPP and the number of particles per fuel pebble were varied to find the flattest reactivity-to-time curve. For the UO2 fuel, the reactivity swing is lowest (around 2%) for BPPs made of B4C with radius of 75 μm. In this case around 1070 BPPs per fuel pebble are needed. For the PuO2 fuel to get a reactivity swing below 4%, the optimal radius of the BPP is the same, but the number of particles per fuel pebble should be around 1600. The optimal radius of the Gd2O3 particles in the UO2 fuel is about 10 times that of the B4C particles. The reactivity swing is around 3% when each fuel pebble contains only 9 BPPs with radius of 840 μm. The results of the Gd particles illustrate nicely the usage of black burnable poison particles introduced by Van Dam [Ann. Nuclear Energy 27 (2000) 733].
