Monte Carlo Alpha Iteration Algorithm for a Subcritical System Analysis

Abstract

Theα-kiteration method which searches the fundamental mode alpha-eigenvalue via iterative updates of the fission source distribution has been successfully used for the Monte Carlo (MC) alpha-static calculations of supercritical systems. However, theα-kiteration method for the deep subcritical system analysis suffers from a gigantic number of neutron generations or a huge neutron weight, which leads to an abnormal termination of the MC calculations. In order to stably estimate the prompt neutron decay constant (α) of prompt subcritical systems regardless of subcriticality, we propose a new MC alpha-static calculation method named as theαiteration algorithm. The new method is derived by directly applying the power method for theα-mode eigenvalue equation and its calculation stability is achieved by controlling the number of time source neutrons which are generated in proportion toαdivided by neutron speed in MC neutron transport simulations. The effectiveness of theαiteration algorithm is demonstrated for two-group homogeneous problems with varying the subcriticality by comparisons with analytic solutions. The applicability of the proposed method is evaluated for an experimental benchmark of the thorium-loaded accelerator-driven system.