Monte Carlo power iteration: Entropy and spatial correlations

Michel Nowak (1) , Jilang Miao (2) , Eric Dumonteil (1, 3) , Benoit C. Forget (4) , Anthony Onillon (3) , Kord S Smith (2) , Andrea Zoia (1)

1 CEA-DES (ex-DEN) - CEA-Direction des Energies (ex-Direction de l'Energie Nucléaire)
2 Department of Nuclear Science and Engineering
3 IRSN - Institut de Radioprotection et de Sûreté Nucléaire
4 Department of Nuclear Science and Engineering

The behaviour of Monte Carlo criticality simulations is often assessed by examining the convergence of the so-called entropy function. In this work, we shall show that the entropy function may lead to a misleading interpretation, and that potential issues occur when spatial correlations induced by fission events are important. Additional information can be extracted from the analysis of the higher-order moments of the entropy function, or from the center of mass of the neutron population. Within the framework of a simplified model based on branching processes, we will relate the behaviour of the spatial fluctuations of the fission chains to the key parameters of the simulated system, namely, the number of particles per generation, the reactor size and the migration area. Numerical simulations of a fuel rod and of a whole core suggest that the obtained results are quite general and hold true also for real-world applications.

Keywords: Entropy; Clustering; Power iteration; OpenMC; Tripoli-4 R

Domains: Nuclear Theory [nucl-th]; Nuclear Experiment [nucl-ex]

Michel Nowak, Jilang Miao, Eric Dumonteil, Benoit C. Forget, Anthony Onillon, et al.. Monte Carlo power iteration: Entropy and spatial correlations. Annals of Nuclear Energy, 2016, 94, pp.856-868. ⟨10.1016/j.anucene.2016.05.002⟩. ⟨cea-02486077⟩