TRIGON: A Monte-Carlo graded composition scheme for multiphysics reactor calculations

Abstract

Typically, Monte-Carlo neutronic calculations of nuclear reactors use a coarse mesh of discrete zones, each with homogeneous material properties. Computation resources limit the spatial resolution, along with the safety parameters’ estimates’ accuracy. As a result, the chosen safety margins for values such as the point power peaking factor need to be larger, resulting in a restricted reactor performance.We present here the TRIGON method that allows neutronic analysis for problems with graded material characteristics, increasing the overall accuracy with little computational penalty, and by using standard Monte-Carlo tools. We use a non-rectangular mesh, along with a specialized tallying scheme to effectively model piecewise-linearly varying materials. We demonstrate the method’s applicability in multiphysics problem solutions by calculating a reactor with a coolant temperature gradient, as well as in a continuous fuel burnup problem. We focus on MTR type research reactor fuel, but the method may suit power reactors just as well.