Evaluation of pebble-bed homogenized cross sections in HTGR fuel cycle simulations

In the operation of multi-pass pebble-bed High Temperature Gas-cooled Reactors (HTGRs), the reactor core consists of different batches of fuel pebbles with different compositions. Calculating pebble-bed homogenized cross sections is a basic procedure in pebble-bed fuel cycle simulations by deterministic codes. VSOP code calculates the pebble-bed homogenized cross sections by constructing an averaged-fuel-density pebble and performing in-line lattice computations. In this paper, a new approach is proposed to evaluate pebble-bed homogenized cross sections, which has been implemented in the calculation scheme of the PANGU (HTGR Physical Analysis of General Utility) code. The proposed approach considers non-flat flux distribution among different pebbles and preserves neutron reaction rate. Numerical tests suggest that the proposed approach is advantaged to treat the pebble bed HTGR fuel cycles with multi-type fuels, such as thorium-uranium (Th-U) fuel cycles.