Burnup instabilities in the full-core HTR model simulation

[Display omitted] •We performed full-core burnup calculation coupled with Monte Carlo code.•Depletion instabilities have been detected for HTR system at high burnup.•We assess the stability of time step models in application to core calculation.•Discussion of the modeling factors related to burnup core simulation is presented. The phenomenon of numerical instabilities present in the Monte Carlo burnup calculations has been shown and explained by many authors using models of LWR, often simplified. Some theoretical considerations about origins of oscillations are very general, however it may be difficult to apply it easily to other models as a prediction of stability. Physics of HTR core differs significantly from the properties of light water system and the reliable extrapolation of the current numerical results is not possible. Moreover, most of the works concerning HTR burnup calculations put no emphasis on the spatial stability of the simulation and apply very long time steps. The awareness in this field of research seems to be not sufficient. In this paper, we focus on the demonstration of depletion instabilities in the simulations of HTR core dedicated for deep burnup of plutonium and minor actinides. We apply various methodology of time step implemented in advanced Continuous Energy Monte Carlo burnup code MCB version 5. Stability analysis is very rare for the full core calculations and the awareness of the oscillation’s problem is obligatory for the reliable modeling of a fuel cycle. In the summary of this work we systematize and discuss factors related to the stability of depletion and review available solutions.