The European PULSAR research project, conducted between 2022 and 2024, has disclosed the conceptual design for a plutonium-238 (Pu-238)-fuelled radioisotope power system (RPS) for lunar space missions. PULSAR is funded by the Euratom Research & Training Programme. It seeks to overcome obstacles to the development of Radioisotope Power Systems (RPS), a key enabling technology for exploration of deep space, where solar energy is insufficient to deliver power to spacecrafts.
The PULSAR consortium is led by Tractebel, a Brussels-based international nuclear engineering consultancy that is part of the ENGIE group. It includes Belgian Nuclear Research Centre – SCK CEN; French Alternative Energies & Atomic Energy Commission (CEA); the European Commission’s Joint Research Centre (JRC), Airbus Defence & Space; French aerospace company Ariane Group; a world leader in Stirling engines research – Université de Bourgogne-Franche-Comté and its affiliated entity, Université de Franche-Comté; and two consultancies with expertise in innovation and technology transfer (INCOTEC) and collaborative research management (ARTTIC).
The project, which was completed at the end of 2024, delivered significant outcomes, including:
- A conceptual RPS design tailored for lunar applications.
- A feasibility study for Pu-238 production in Europe.
- A market analysis exploring the potential of dynamic power systems beyond space applications.
The PULSAR consortium’s RPS is designed to support a lunar rover or cargo carrier with 100-500 watts. It incorporates safety measures for launch from the Guyana Space Centre and features two Stirling engines powered by a centrally located Pu-238 heat source. The modular design ensures resilience against motor failure, with an expected thermo-electrical conversion efficiency of 20%.
Tractebel nuclear experts conducted comprehensive engineering studies, including structural integrity checks, radiation dose assessments, thermal analysis, and mechanical assembly development. The team developed a 3D mechanical and thermal model to simulate lunar conditions, providing a foundation for future design iterations and higher Technical Readiness Levels (TRL). This work lays the groundwork for Europe’s participation in the upcoming Argonaut lunar lander mission.
Tractebel Space Product Manager Brieuc Spindler said the PULSAR consortium’s achievements “will help position Europe as an autonomous global leader in space nuclear technologies”. European research projects are “focused on advancing nuclear technologies for space exploration, including RPS and radioisotope production, electric propulsion, and fission surface systems”. He added: “By leveraging our nuclear expertise, we are pushing the boundaries of space exploration and enabling Europe to lead in this final frontier.”
Euratom gave details of three work packages in development of the conceptual design.
Work Package 1 of the PULSAR project addressed key challenges in establishing European production of Pu-238. The project progressed from a feasibility study, confirming the realistic potential for Pu-238 production in Europe, towards proof of concept developed.
Two irradiation scenarios using channels at BR2 reactor in SCK CEN, Belgium have been proposed by SCK CEN, with estimated production rates of 378 and 418 g of Pu-238 per year if enough neptunium-237 is available. At SCK CEN, the conventional oxalate conversion route was reproduced at the laboratory scale. Solvent extraction in a continuous process was identified as the preferred method for separating neptunium and plutonium from fission products and each other.
The synthesis and characterisation of PuO2 pellets with representative microstructure were explored. The process at JRC facilities successfully produced PuO2 from reactor-grade plutonium with tailored microstructure and density, closely mimicking results from US studies. The feasibility of using laser welding techniques for the iridium encapsulation was studied and tested at JRC laboratories. Due to material shortages, the limited welding tests were inconclusive.
Additionally, the work package focused on the safety aspects of handling Pu-238, including processing it in gram quantities in a nuclear laboratory addressed by JRC, and identifying the necessary framework for conducting safety assessments, licensing, and regulation of the PULSAR heat source addressed by Tractebel.
Work package 2 sought to establish a conceptual design and study of the energy production unit of the PULSAR, including the heat source and Stirling converter. The study covered both static and dynamic systems. The different systems were then compared by means of performance indicators, such as efficiencies and Technology Readiness Levels (TRL), alongside an evaluation of the presence and status of European R&D within the respective domains. This analysis confirmed that the dynamic Stirling engine could offer the best compromise in terms of efficiency for the limited thermal power expected for the heat source.
Next, a computer-assisted design of a heat source was built based on the NASA’s General Purpose Heat Source (GPHS). Thermal-mechanical calculations for nominal and accidental calculations were performed to map the temperatures in the module (figure). The computed temperature was always below critical values, including in accidental calculations.
In parallel, a state-of-the art review provided a comprehensive summary of the evolution, operation, and understanding of the free piston Stirling Engine for aerospace application. First, the features of a first thermodynamic cycle were assessed about an optimal operating point estimated by CEA. To reach the expected performances, a preliminary Stirling converter technology and design was developed. The preliminary mechanical design was then established and will need to be reviewed as the geometries of the interfaces with the hot and cold sources were not consolidated.
Finally, the design of the PULSAR Stirling engine was optimised, with a focus on achieving the targeted electrical power output while ensuring that the engine operates efficiently and has a prolonged lifespan
Work package 3 aimed to develop the design of the Radioisotope Power System assembly. Early in the project, potential missions were selected, and the specifications were defined for electrical power needs, available geometry, thermal environment, mechanical constraints, etc. (Airbus). Two missions were considered most suitable for the integration of a first European RPS: a continuous power supply for a lunar rover mission, or for a lunar cargo carrier.
In parallel, nuclear safety requirements for launch were defined, considering the perspective of launching the RPS from the Guyana Space Centre (Ariane Group). Demonstrating compliance with these safety requirements will be part of the licensing process.
Based on these requirements, a conceptual RPS design was developed featuring two Stirling engines facing each other with a centrally positioned heat source. This configuration aligns with the objectives of modularity and resilience to motor failure while maintaining a simple geometry. Most of the heat flux from the heat source is transferred to the engines, and converted to electricity with an expected efficiency of around 20%. The low-quality heat remaining at the end of the cycle is removed at the engine heat sink and evacuated by the RPS radiators.
Engineering studies and design reviews with partners were conducted to further validate the concept and consolidate design choices. This process included sizing and orienting the radiating surfaces, verifying structural integrity against anticipated loading patterns, assessing the dose rate from the radioactive source, and developing mechanical assembly principles (Tractebel studies). System integration on the host spacecraft was performed to evaluate the adequacy of power production over day-night cycles on the Moon in different scenarios (Airbus). A compliance check of the heat source and RPS design was conducted with the safety objectives and the launch authorisation process (Ariane Group).
A 3D mechanical model was developed at the end of the project. The assembly comprises independent sub-assemblies for the heat source and the Stirling converters, allowing for the final integration of the source shortly before launch. The model will serve as the basis for further design development, extending the scope and level of details of the engineering files, studying transient modes, failure modes, etc. to reach the necessary higher Technical Readiness Levels.
