Fission matrix homogenization and iterative convergence in RAPID

The fission matrix based radiation transport code RAPID is able to perform high fidelity and fast 3D, pin-wise whole core calculations. The code can quickly estimate the system fission matrix by combining pre-calculated database fission matrices, and solving for the system fission matrix provides the multiplication factor and a detailed fission distribution. In this work, RAPID acceleration and numerical techniques will be examined. These are the fission matrix collapsing or homogenization options, choice of the power iteration tolerance and estimation of iterative error. In old RAPID calculations, in order to achieve a fast power iteration convergence of the whole core fission matrix, the pin-wise fission matrix is collapsed into an assembly-wise one following a 2-D pin-wise core slice calculation. In addition to the radial collapsing from pin to assembly, other radial and axial collapsing options are tested to find out the best balance between accuracy and speedup. Applying the fission matrix collapsing technique, RAPID requires at least two eigenvalue calculations. Different tolerances of the 2-D and 3-D power iteration calculations are combined to investigate the convergence. A more sophisticated method to estimate the iterative error is provided to determine the convergence. Overall, a combination of all acceleration techniques speeds up the RAPID calculation on BEAVRS benchmark problem by three times, with only a negligible loss of accuracy.