Leakage correction in group-constant generation for pebble-bed HTGRs

Neutronics design of commercial reactors currently utilizes a procedure linking lattice physics and core physics calculations together, where broad-group constants are generated by fine-group lattice calculations and are then used by downstream whole-core diffusion calculations. Among this procedure, the leakage effect in group-constant generation contributes a non-negligible source of errors. In pebble-bed high temperature gas-cooled reactors (HTGRs), this effect becomes even important because the neutron diffusion length is relatively large compared with the size of pebble lattice. Traditional neutronics analysis codes for pebble-bed HTGRs need repeatedly re-call lattice calculations with the leakage information obtained from core diffusion calculations. In this paper, existing methods for leakage correction are reviewed, and a new method is proposed to correct broad-group constants by using buckled fine-group spectrum. The new method eliminates the need of online coupling between lattice physics and core physics calculations, and thus makes it possible to use the tabular group-constant library as is done in light water reactors (LWRs). Numerical tests show that the proposed method are promising for leakage correction in pebble-bed HTGR analyses.