A perturbation-based acceleration for Monte Carlo – Thermal Hydraulics Picard iterations. Part I: Theory and application to extruded BWR unit-cell.

A novel acceleration scheme for coupled Monte Carlo (MC)- sub-channel Thermal Hydraulics (TH) Picard iterations (PI) is here presented. This scheme, named GTF-based acceleration, uses a deterministic, perturbation-based prediction step that produces an improved initial guess of the power, and the corresponding temperature and density profiles. The improved temperature and density profiles are used within the next MC calculation, therefore accelerating the convergence of the whole algorithm. The prediction step relies on the use of the generalized transfer functions (GTF) method for the calculation of the cross sections’ spatial distribution. The GTF allows to capture non-local effects arising in 3D cross sections generation. In this paper, the GTF-based acceleration is applied to an extruded Boiling Water Reactor (BWR) unit-cell with axial reflectors. A speedup factor between 2.1 and 12.4 was observed with respect to the standard PI. Application of the scheme to three-dimensional problems is presented in Part II of this article.