PNNL

Abstract

When performing a Monte Carlo criticality calculation, it is essential to ensure that the fission source is adequately sampled. This is particularly important when only a few, user selected source points are used to initiate a calculation. For many problems, however, this approach can result in extremely slow source convergence rates that may not be easily observed by the user. For example, problems with fissionable material regions that are extremely long and narrow, or problems with multiple fissionable material regions separated by large distances can present serious source convergence challenges. In general, results from calculations with poorly converged fission sources will be incorrect, so it is important to recognize when this occurs. One way to better understand how the source points are created from generation to generation is to use the KCODE particle plotting technique available in the MCNP1 Visual Editor.2 By plotting the source generation points for different cycles, the propagation of particles can be observed to help verify that the fissionable material regions are being adequately sampled. In this paper we discuss the generation of particle density plots in the MCNP Visual Editor and present several examples of their usefulness.