Design of innovative zero power reactor configurations for modeling coupling/decoupling phenomena : Application to kinetics transients

The study of space-time neutronic behavior of a nuclear reactor is the subject of both computational and experimental fields. Future and existing experimental facilities are called to answer questions on issues such as core stability and response to perturbations, that are commonly seen in large power reactors and are related to spatial decoupling. Calculations with accurate neutron kinetics models are required to characterize such decoupling effects in nuclear systems, both at small and large scales. The end goal of this work is to propose an innovative approach to analyze and reproduce such spatial effects in smaller ZPR (Zero Power Reactor) configurations. This is achieved by using the dominance ratio or eigenvalue separation as design criteria and connecting them to the characteristics of the system.The methodology followed here relies on two main approaches. A hybrid stochastic – deterministic method based on the Transient Fission Matrix (TFM) model, implemented in the Serpent 2 Monte Carlo code and a deterministic calculation scheme based on Kobayashi’s multipoint kinetics model, running on the ERANOS 2.4 system of codes. The two approaches complement each other well, each covering the other’s limitations and enable the evaluation of both complex geometries and the design of decoupled high dominance ratio configurations. At their core, both methods track the neutron population across the system and relate it to its kinetic behavior. This allows one to access higher source distribution modes related to the flux harmonics, whose study is key to understanding nuclear reactor spatial effects.The TFM model, permitting one-dimensional analysis with a fine nodal mesh, is an ideal tool for determining the dominance ratio and getting a detailed look at how prompt and delayed neutrons propagate in a geometry. On the other hand, the deterministic calculation scheme based on Kobayashi’s model allows for a lower calculation time and memory requirements, while enabling the study of three-dimensional coupling effects at the cost of a reduced number of nodes.A fast/thermal coupled core ZPR concept, developed at CEA Cadarache in the context of the ZEPHYR versatile facility, unfortunately frozen at the moment, is analyzed using the above methodology. This complicated geometry offers a good way to both test the validity of the models and gain an understanding of the associated coupling effects. Additionally, a simple coupled fuel assemblies benchmark problem was developed, for testing coupled core systems calculations, models, and methodologies. The system’s kinetic behavior is analyzed in response to geometry and material changes. The distance between the assemblies is changed, fuels of different reactivities are used, control rods are introduced to parts of the system and finally, different levels of boron dilution in the moderator are tested. This study enables us to better understand how the coupling is affected by various parameters and it deals with commonly encountered scenarios, in both experimental programs and power reactor operation.Finally, the developed methodology is applied towards producing high dominance ratio configurations in the VENUS-F zero power reactor of the SCK-CEN, in support of a larger experimental campaign between CEA and SCK-CEN, part of which is focused around developing a better understanding of space-time kinetics effects. This is done through a progressive optimization process that aims to gradually redesign the VENUS-F core, while keeping certain considerations in mind.