Weighted delta tracking performance evaluation in the presence of localized heavy absorbers

Particle tracking is the most computational expensive process in a Monte Carlo transport simulation. The default tracking methods used are generally surface tracking and delta tracking, as well as hybrid algorithms which combine the two. Recently, alternative methods have been proposed, such as Weighted Delta Tracking (WDT). It is a modified version of delta tracking where all collisions are accepted as real and the particle weight is modified accordingly, to maintain an unbiased calculation. In this work, WDT was implemented in the Monte Carlo code SCONE. The algorithm was adapted to include scattering reactions as well as absorptions: in case of absorption, the weight is modified to account for the probability of a real collision, while in case of scattering the algorithm performs standard delta tracking. The performance of this method is evaluated on both a thermal and a fast assembly model in the presence of localized heavy absorbers, showing that applying WDT does not bring any notable advantage. In addition, a sensitivity study on the weight control method used, Russian Roulette, is performed, showing that the choice of parameters is especially relevant in the case of the fast spectrum assembly. In the final part of the paper, the performance of WDT is compared to that of a surface tracking - delta tracking switch when implicit capture is applied. Generally, it is concluded that in order to observe a more pronounced benefit from WDT, it should be applied to a model physically dominated by absorption rather than scattering. (authors)