NUMERICAL BENCHMARK OF STRONGLY TO LOOSELY COUPLED ASSEMBLIES USING THE TRANSIENT FISSION MATRIX METHOD

K Routsonis,J Tommasi,P Blaise,M Margulis

doi:10.1051/epjconf/202124706038

Abstract

Advances in computational methods have given rise to the study and simulation of different aspects of reactor behavior. As such, topics associated with high computational costs become feasible candidates for further investigation and one of them is reactor space-time kinetics (STK). Until recently, STK simulation and point kinetics approximation were limited to deterministic codes, with Monte Carlo codes being too costly to start with. However, recent developments in this area have allowed the use of certain methods in stochastic codes. One such technique is based on the Transient Fission Matrix (TFM) model, a hybrid method that uses a system response obtained through Monte Carlo and stored in fission and time matrices as input for deterministic calculations. The result enables a view of the STK in terms of neutron propagation probability and propagation time across the system. The TFM method was applied to a simple coupled core configuration to generate a numerical benchmark. The Serpent 2 Monte Carlo code was used for the stochastic part of the calculation. The configuration consists of two fuel assemblies placed in a light water tank, with a water blade of varying width between them. TFM, flux and fission results were obtained for varying water blade widths, ranging between 0 cm and 20 cm. The data is then used to analyze the behavior of the system, as well as the effects of the coupling between the two assemblies. As the assemblies move further apart, the system slowly transitions from two tightly coupled assemblies that essentially form a single core, to two almost independent cores. This study enables to produce a benchmark for future calculations and predefine an innovative way of designing high dominant ratio configurations, required for tackling Monte Carlo residual problems. An actual experimental program could be led in ad hoc zero power reactor (ZPR), such as the KUCA reactor of Kyoto University.

Highlights

Until recently, most widely used stochastic transport codes lacked native support for reactor kinetics calculations
With initial versions of such modules becoming available, research into the field or reactor space-time kinetics will likely experience an increase in the near future
A detailed numerical benchmark based on an UOX – UOX assembly coupled system was developed in order to help in the comprehensive study of reactor kinetics effects by offering a common ground for the application and testing of calculation schemes

Summary

Introduction

Most widely used stochastic transport codes lacked native support for reactor kinetics calculations. With initial versions of such modules becoming available, research into the field or reactor space-time kinetics will likely experience an increase in the near future. As part of the benchmark, the full set of fission-to-fission probability TFM matrices is available for prompt and delayed neutrons, alongside the average neutron fission-to-fission time matrices for the prompt-to-prompt case, as well as flux results. The system utilizes PWR-style fuel assemblies, with reactor grade UOX fuel (UOX – UOX system) and a second version is currently being studied, where one assembly will use MOX fuel (UOX – MOX), in order to investigate the effect between two differently reactive fuels. The results are obtained with the Serpent 2 Monte Carlo code [5]

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Results

Conclusion