NEUTRONIC MODELING OF A FAST CORE WITH MODERATING MATERIALS USING APOLLO3R CODE

M Zajaczkowski,V Pascal,C.De Jean Saint,J.-M Palau,M Margulis,P Blaise

doi:10.1051/epjconf/202124706009

Abstract

In order to improve passive safety of Sodium-cooled Fast Reactors The French Alternative Energies and Atomic Energy Commission (CEA) has proposed a new core design called CADOR - an SFR core with enhanced Doppler reactivity feedback. One of its most important design features is the introduction of solid moderating materials inside each fuel assemblies to slightly decrease the average neutron energy. The article focuses on development and validation of a neutronics calculation scheme able to produce accurate results in case of CADOR and other fast cores with moderating materials. The study uses two different fuel assembly models moderated by metallic beryllium and zirconium hydride (ZrH2) respectively The study includes discussion of neutron scattering treatment and different ways of spatial homogenization and energy condensations. The results indicate that the accurate scattering treatment leads to much better estimation of Doppler constant, especially in case of ZrH2 moderated core. By using combined deterministic-Monte Carlo calculation scheme we are able to quantify the biases on global reactivity, reactivity feedbacks and control rod worth. We demonstrate that spatial homogenization plays a more important role in case of moderated CADOR assemblies and thus preserving certain level of heterogeneity within fuel assemblies can lower the calculation bias significantly.

Highlights

The Sodium-cooled Fast Reactor (SFR) is arguably one of the most developed and researched Generation 4 reactor concepts
In this study we focus on development and validation of a deterministic neutronics calculation scheme for APOLLO3 R code suitable for calculations of CADOR and other cores where additional moderation is used alongside with fast spectrum
We proved that because of the high importance of resonance upscattering a accurate treatment of neutron scattering is strongly advised for fuel temperatures higher than nominal

Summary

Introduction

The Sodium-cooled Fast Reactor (SFR) is arguably one of the most developed and researched Generation 4 reactor concepts. On the other hand addressing safety issues such as positive void coefficient, low Doppler Effect feedback and narrow margin to prompt criticality is a challenging task. In search of safer SFR core design, CEA has launched a “CADOR” research project. The aim is to design a core with more robust passive safety features by increasing the magnitude of Doppler reactivity feedback [1]. One of the project’s key changes compared to a standard SFR is addition of small amounts of neutron moderator inside the core. While the resulting neutron spectrum is still close to the one of a typical SFR, the neutron population in the resonance energy range is visibly increased (Figure 1).

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