Description du sujet de thèse

Background
Nuclear fission power is expected to play a key role in space exploration in the coming years. Two main generic fission reactor concepts are typically considered for nuclear space power: Nuclear Thermal Propulsion (NTP) and Nuclear Electric Propulsion (NEP). In the NTP concept, a working fluid—usually liquid hydrogen—is heated to a high temperature in a nuclear reactor and then expands through a rocket nozzle to generate thrust. This approach offers the advantage of high thrust but also presents significant challenges related to fuel reliability, reactor testing, and operational safety. In contrast, the NEP concept operates on a fundamentally different principle: nuclear thermal energy is converted into electricity, which then powers an electric propulsion system. NEP offers higher reliability and simplified testing and safety procedures compared to NTP, but at the cost of lower thrust.

Since 2018, the Reactor Physics team at LPSC Grenoble has been working on the design of micro-reactors for NEP and the development of numerical tools tailored for modeling these specific nuclear energy systems. The team is currently investigating two reactor concepts within the framework of the RocketRoll project [1], funded by the European Space Agency (ESA): Molten Salt Reactors (MSRs) and Heat Pipe Reactors (HPRs). To support these studies, the team is using two computational tools: PRESTO and NepFOAM. PRESTO (NEP Rocket Design Tool) is an optimization tool designed to identify the optimal design parameters of a given reactor concept, including its geometry, dimensions, materials, and technologies. Its development is currently being continued in collaboration with CNES (French Space Agency) through a co-funded PhD project.

To perform detailed reactor analyses, the team uses a multi-physics tool called NepFOAM [2][3] that numerically couples OpenFOAM and SERPENT 2. OpenFOAM is a C++ toolbox for developing numerical solvers for continuum mechanics problems, including Computational Fluid Dynamics (CFD), using the Finite Volume Method (FVM). SERPENT 2 is a multipurpose, three-dimensional Monte Carlo particle transport code for neutron simulations. This multi-physics tool consists of three interconnected modules: neutronics, thermal-hydraulics, and thermal-mechanics. Various physical phenomena, scalar fields, and vector fields are used to couple these three modules, making this coupling an essential aspect of reactor modeling, particularly important when using a liquid fuel, as in Molten Salt Reactors (MSRs). During numerical simulations, these three modules are executed sequentially for each reactor region. For transient simulations involving neutron transport calculations using the Monte Carlo Quasi-static method, the multi-physics tool employs two different time steps. A smaller time step is used for integrating the thermal-hydraulic balance equations and the neutron flux amplitude, while a larger time step is used for calculating the neutron flux shape with SERPENT 2.

Space Molten Salt Reactors (SMSRs)
Among the various nuclear reactor concepts that could be considered for Nuclear Electric Propulsion (NEP), Molten Salt Reactors (MSRs) offer intrinsic advantages due to their unique design characteristics. In particular, an MSR enables the development of a core design with relatively high power density and operating temperature, small fuel pressure and temperature gradients, and relatively simple reactivity control systems. These features are crucial for the success of an NEP design, as they are key to improving the overall performance and reliability of a nuclear reactor in a space mission.

Indeed, studies conducted during the RocketRoll project indicate that achieving an NEP specific power (i.e., the electric power output divided by the total NEP mass) higher than 50 We/kg will be necessary to make these systems cost-effective compared to chemical rockets, considering current and near-term technologies [1]. MSRs also enable the development of innovative systems [4] for reactor startup, reactivity control, and handling of specific accidental scenarios such as hot reentry. Additionally, this reactor concept can be adapted for other space missions, including surface power applications on the Moon or Mars.

Nevertheless, designing a space-based MSR presents significant technical challenges. The most critical among these include selecting reactor materials with adequate corrosion resistance to molten salts, handling gaseous fission products present in the molten fuel salt, operating fluid systems in microgravity conditions, and developing a reliable reactor startup procedure. Other key technical challenges—common to all NEP concepts—concern addressing nuclear safety issues, such as hot reactor reentry and launch accidents, coupling the reactor with the power conversion system, designing an adequate radiator system, and performing necessary nuclear tests.

Work carried out in the framework of a previous PhD thesis identified two promising MSR designs: (a) a high-power (1 MWth) MSR with a fast neutron spectrum and (b) a medium-power MSR using a thermal spectrum. During that PhD project, only the conceptual design of the fast-spectrum MSR was developed [5]. The analysis began with PRESTO to determine the reactor's principal characteristics, followed by detailed steady-state and transient studies using NepFOAM.

PhD project objective
The objective of this PhD project is to develop the conceptual design of a Nuclear Electric Propulsion (NEP) reactor based on a Molten Salt Reactor (MSR) using a thermal neutron spectrum. This study will define the main characteristics of the reactor design and its expected performance. Additionally, key parameters and performance metrics of other NEP subsystems, such as the power conversion system and radiator, will be determined. The reactor startup procedures will also be investigated. The conclusions and recommendations from this study will help define the tests and experiments needed to increase the Technology Readiness Level (TRL) of the concept.

Work organization
The PhD work will be organized according to the following four phases:

Phase 1: Definition of the Main Parameters of Thermal MSR Concepts (~6 months)
The starting point will be to identify a few (likely two or three) promising thermal MSR concepts based on analyses performed with PRESTO. These concepts will likely differ in the materials used for the moderator and reflector, the type of molten fuel salt and cladding, and the dimensions of the core components. During this phase the candidate will starting getting familiar with OpenFOAM and Serpent.

Phase 2: Detailed Multiphysics Studies on MSR Concepts (~18 months)
The thermal MSR configurations selected in Phase 1 will be used to develop detailed multiphysics models in NepFOAM, focusing primarily on the components in the fuel circuit. These models will support a more refined design process, helping to identify design limits arising from thermal and neutronic considerations, optimal operating conditions, and necessary reactor design modifications. The analysis will include both steady-state and transient simulations. Steady-state calculations will determine the reactor's operating parameters, while transient simulations will assess specific accident scenarios. Burnup calculations will also be performed to design the reactivity control system, and criticality calculations will analyze hypothetical water submersion accidents. Additionally, this phase will consider the proper handling of fission gases and the reactor startup procedure.

Phase 3: NEP Performance Analysis (~6 months)
The detailed reactor analysis results from Phase 2 will be used in Phase 3 to evaluate the overall performance of the NEP system, considering the reactor, shielding, power conversion, thermal radiator, and electric propulsion subsystems. In particular, the achievable specific power, total NEP mass, and principal dimensions of the NEP subsystems will be assessed. This phase will involve updating PRESTO with the results from Phase 2 and running optimization analyses. The key outcomes will include the optimal parameters and performance metrics for all NEP subsystems.

Phase 4: Conclusions, Recommendations, and Reporting (~6 months)
In the final phase of the project, the results from Phases 2 and 3 will be analyzed in relation to potential mission requirements for a NEP system. These missions could include orbital transfers from LEO to GEO, cargo missions to the Moon and Mars, and deep-space exploration. The findings will also be used to propose necessary experiments for increasing the Technology Readiness Level (TRL) of the concept. During this phase, the PhD manuscript and dissertation will be prepared.

References:
[1] P. Rubiolo, N. Capellan, S. Lorenzi, R. Boccelli, A. D'Ottavio, A. Peressotti, F. Romei, A. Cuenca, A. Barco, A. Herasimenka, A. Abdin, A. Hein, “A Comprehensive Methodology for Designing a Nuclear Electric Propulsion (NEP) Concept”, 75th International Astronautical Congress (IAC), Milan, Italy, 14-18 October 2024, (2024).
[2] M. Tano, “Developement of multi-physical multiscale models for molten salts at high temperature and their experimental validation”, PhD Thesis, Université Grenoble Alpes, I-MEP2 (November 2018).
[3] J.A. Blanco, “Neutronic, thermohydraulic and thermomechanical coupling for the modeling of criticality accidents in nuclear systems”, PhD Thesis, Université Grenoble Alpes, I-MEP2 (December 2020).
[4] P. Rubiolo, F. Quinteros, N. Capellan, M. Marone, J. Giraud, F. Szmandiuk, “Molten Salt Reactor Concepts for Advanced Nuclear Electric Propulsion (NEP) Systems”, 75th International Astronautical Congress (IAC), Milan, Italy, 14-18 October 2024, (2024).
[5] F. Quinteros, P. Rubiolo, V. Ghetta, J. Giraud, N. Capellan, “Design of a Fast Molten Salt Reactor for Space Nuclear Electric Propulsion”, Nuclear Science and Engineering, 197, 1-16 (2023). 10.1080/00295639.2023.2167470.

Contexte de travail

Working environment:
The Laboratoire de Physique Subatomique et de Cosmologie (LPSC) in Grenoble (http://lpsc.in2p3.fr) is a joint research unit affiliated with CNRS-IN2P3, Université Grenoble Alpes (UGA), and Grenoble INP. The laboratory hosts an average of approximately 230 staff members. The PhD candidate will be assigned to the Reactor Physics group at LPSC, which is composed of 10 staff members. The candidate will work within at the FEST (Fluids Experiments and Simulations in Temperature) experimental Platform and will report directly to the head of the group. The thesis will be supervised by Pablo RUBIOLO (Director, 60%) and co-supervised by Nicolas CAPELLAN (40%). We plan to develop this PhD project in close collaboration with Framatome, specifically with the space power group.

Additional information:

Recommended technical background and skills for candidates:
(i) Fluid mechanics and heat transfer, (ii) Neutronics, (iii) Nuclear reactor design and safety, (iv) Programming in Python and C++, v) knowledge on Computational Fluid Dynamics codes (Fluent, OpenFOAM, etc.) and neutronics Monte Carlo codes (e.g. Serpent, MCNP, etc.) is a plus. During the project, the student will present their work at least once at an international conference in the field. The candidate is also expected to publish at least one paper in an international journal.

Language proficiency:
• English proficiency: Advanced level (C1)
• French proficiency (optional): Beginner level (A1)

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.