In-core fuel management optimization of pebble-bed reactors

A reduction of the power peak in the core of High Temperature pebble-bed reactors is attractive to decrease the maximum fuel temperature and to increase fuel performance. A calculation procedure was developed, which combines fuel depletion, neutronics and thermal–hydraulics to investigate the impact of a certain (re)loading scheme for the pebble-bed type HTR. The procedure has been applied to a model of the Pebble Bed Modular Reactor (400 MW) design.This paper shows that an important reduction in axial power peaking can be achieved by adopting a multi-pass recycling scheme for the pebbles. By dividing the core into several radial fuel zones in combination with multi-pass recycling the power profile can be flattened in the radial direction. For a core with two fuel zones the impact on the keff and maximum power density as a function of the zone size has been investigated. A heuristic method has been used to find the optimal pebble loading pattern for several (re)loading schemes. Using this method a reduction of the maximum power density from 10.0 to 8.2 MW/m3 has been achieved for a core with three radial fuel zones.The effects of the improved power profiles on the fuel temperature during normal operation and a Depressurized Loss Of Coolant (DLOFC) accident have been analyzed. It was found that the optimized power profile results in a reduction of the maximum fuel temperature of 80 °C and 300 °C for normal operation and DLOFC conditions, respectively.