Subgroup decomposition method in diffusion theory

A new method for solving the fine-group diffusion equation is developed by estimating the in-scattering and fission source terms with consistent coarse-group diffusion solutions iteratively. This is achieved by extending the subgroup decomposition (SGD) method to diffusion theory initially developed in neutron transport theory. Additionally, a new stabilizing scheme for on-the-fly cross section recondensation based on local fixed source calculations is developed in the subgroup decomposition framework. The method is derived in general geometry and tested in a 1D benchmark problem characteristic of boiling water reactors. It is shown that the SGD method reproduces the standard fine-group results with 3-4 times faster computational speed.