Session: 02-13: Structural Evaluation, Performance Assessment, Multiphysics Coupling - III

Paper Number: 133060

133060 - Neutronics-Mechanics Coupling for Fast Transient Simulation in Molten Salt Reactors 

Abstract: 

Among the wide variety of nuclear reactor technologies, molten salt reactors (MSRs) stand out as their nuclear fuel is dissolved in a liquid molten salt which in most concepts also serves as the coolant. When studying a small or slow reactivity insertion into such a reactor, power increases relatively slowly and the velocity of mechanical waves propagating in the liquid fuel can be assumed infinite [1, 2]. On the contrary, prompt supercritical excursions have much lower characteristic timescales, of the order of 10 ms, which requires the wave velocity to be considered finite and the salt compressible. The presence of gas bubbles in the liquid fuel, as it is envisaged for the MSFR [1], must also be taken into account since it significantly impacts the propagation of mechanical waves. A finite wave velocity means a delay in the expansion of the liquid fuel following an increase of its temperature, i.e. a delay in the negative reactivity feedback, as well as the propagation of strong pressure peaks which might deform and eventually damage the reactor vessel [1–3]. The compressible study of such fast transients is therefore of primary importance for the safety of MSRs.

Although various codes were implemented to simulate fast prompt supercritical excursions in MSRs and did provide valuable insight into the phenomenon, they rely on quite limiting assumptions due to its complex multiphysical nature. For instance, the gas phase can be assumed spatially homogeneous [2], the overflow tank and neutron transport effects can be neglected [3] and to the authors' knowledge the deformation of the vessel due to fluid–structure interaction has not been simulated yet. To relax those hypotheses, the development of a neutronics/multiphase-compressible-thermal-hydraulics/structural-mechanics coupling tool based on the APOLLO3 and EUROPLEXUS codes has begun at CEA. APOLLO3 solves the neutron transport equation with the deterministic S_N method, as well as the delayed neutron precursor balance equations, while EUROPLEXUS solves the compressible Euler equations with the finite volume method. For now neutronics and thermal hydraulics have successfully been coupled, and in this paper the associated coupling strategy will be described and simulation results for verification test cases will be presented.

[1] A. Laureau. “Développement de modèles neutroniques pour le couplage thermohydraulique du MSFR et le calcul de paramètres cinétiques effectifs”. PhD thesis. Université Grenoble Alpes, 2015.

[2] M. Aufiero et al. “Monte Carlo/CFD Coupling for Accurate Modeling of the Delayed Neutron Precursors and Compressibility Effects in Molten Salt Reactors”. Transactions of the American Nuclear Society 116 (2017), pp. 1183–1186.

[3] E. Cervi et al. “Development of a multiphysics model for the study of fuel compressibility effects in the Molten Salt Fast Reactor”. Chemical Engineering Science 193 (2019), pp. 379–393.

Presenting Author: Théo Vidril CEA, Service d'Études Mécaniques et Thermiques

Presenting Author Biography: Théo Vidril graduated in physics from the École Normale Supérieure de Lyon in 2022 and subsequently specialized in nuclear reactor physics at the Institut National des Sciences et Techniques Nucléaires, the school administered by French CEA. Convinced that innovative nuclear reactors have a significant role to play in decarbonizing society, he contributed during two internships to the research in magnetic confinement fusion before joining the molten salt reactor community. As a first-year PhD student at CEA Paris-Saclay, he is currently developing a neutronics–mechanics coupling tool to simulate prompt supercritical transients in liquid-fuel reactors.

Authors:

Théo Vidril CEA, Service d'Études Mécaniques et Thermiques
Stanislas De Lambert CEA, Service d'Études Mécaniques et Thermiques
Nicolas Lelong CEA, Service d'Études Mécaniques et Thermiques
Florence Drui CEA, Service d'Études Mécaniques et Thermiques
Cyril Patricot CEA, Service d'Études des Réacteurs et de Mathématiques Appliquées
Elsa Merle Grenoble INP, CNRS, Laboratoire de Physique Subatomique et de Cosmologie