Uncertainty underprediction in coupled time-dependent Monte Carlo simulations with Serpent 2

This paper studies the possibility to use the traditional batch-based estimate for statistical uncertainty in uncoupled and coupled transient calculations with the Monte Carlo code Serpent 2. Such a study is needed due to a fundamental difference in the way neutrons are divided into batches between the criticality source calculations and these time-dependent calculations as well as new batch-to-batch correlations arising from the coupled solution. The uncertainty estimate given by Serpent 2 is compared to the true uncertainty calculated from several independent simulations to calculate the uncertainty underprediction factor. The uncoupled transients were calculated for the Flattop (fast spectrum) and STACY-30 (thermal spectrum) experiments. The results show a nice agreement with the batch-wise uncertainty estimate and the true uncertainty. The case chosen for the coupled transient is a very short prompt super-critical power peak in a PWR pin cell disregarding the effect of delayed neutrons. The results show that a too small population size leads to underestimation of the uncertainty by the traditional batch-wise estimate throughout the whole transient, most likely due to undersampling effects. An interesting effect is observed in the time frame of strong coupling between fission power and temperature solution: Whereas the uncertainty prediction of the batch-wise estimate is very accurate before this period of strong coupling, the batch-wise estimate actually overestimates the uncertainty in the fission power at this period. Finally, the relation between the relative standard deviation in fission power and the fuel behavior solution was studied during the coupled transient. The main results from this part indicate, as one could expect, that the relative standard deviation in the neutronics solution was not transmitted into the fuel behavior solution in a simple manner, but through the corresponding physics governing the fuel behavior.