A twin assemblies benchmark for coupling behavior and kinetics parameters calculations

Abstract

Traditionally, stochastic analysis of reactor kinetics behavior had always been a prohibitively expensive problem, in terms of the necessary calculation time. Recent advances in computational methods, however, have made it possible for several widely used Monte Carlo codes to incorporate methods that enable the study and simulation of such topics, with one of them being reactor space–time kinetics or STK, which until recently was limited to deterministic codes only. The present paper uses 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 system’s behavior in terms of its neutronic coupling and its kinetic parameters, namely the neutron propagation probability, generation time, lifetime and delayed neutron fraction. The TFM method is applied to a coupled core configuration with the purpose of generating a numerical benchmark. The Serpent 2 Monte Carlo code is used for the stochastic part of the calculation. The geometry consists of two fuel assemblies placed in a light water tank, with a water blade of varying width between them. The process consists of adjusting the width (thus the separation) of the two assemblies and then extracting TFM, flux and fission rate results. The blade width ranges between 0 cm and 20 cm. The system behavior, as well as the coupling effects between the two assemblies, are then analyzed and discussed. By obtaining the system’s eigenvalues from the transient fission matrices, the degree of coupling can be assessed. As the assemblies move further apart, the system slowly transitions from a single core, to two only loosely coupled ones. This study enables to produce a benchmark to compare future calculations schemes against and predefine an innovative way of designing high dominant ratio configurations. An actual experimental program could be led in ad hoc zero power reactor able to perform them, such as the KUCA reactor of Kyoto University, where similar (in their philosophy) experiments were done in the 90’s.