Not Rocket Science, just Nuclear Rocket Science

Launchpad and Background

It is recognized by many international space actors that the deployment and usage of nuclear reactors could open up an immense range of new space applications from surface fission power to in-space propulsion for transportation, orbit logistics and transfer operations as well as deep space scientific exploration. With regards to propulsion, one key rationale is that compared to e.g., solar based systems, which become non-operational for longer distances due to the reduced solar radiation, nuclear based vehicles could in theory enable significantly lower masses combined with substantially higher power. In terms of performance metrics, this means that nuclear systems could facilitate much higher specific impulses, in particular with nuclear electric propulsion (NEP), and/or substantially higher thrusts, in particular with nuclear thermal propulsion (NTP). Some of the main benefits of this would be larger payloads, reduced transport/travel durations and/or energy/electricity/heat supply sources for very long periods. It is for these reasons that major space players such as the US, Russia and nowadays also China have undertaken extensive R&D programs on nuclear reactors for space applications. Recognizing the strategic potential of nuclear for advanced propulsion systems, the European Space Agency (ESA) has also launched several feasibility studies in very recent years. The aim of these studies is to assess the potential of NEP/NTP vehicles for in-space propulsion as well as to identify needs with regards to technology readiness, safety requirements and availability of European infrastructures to deploy such technology. And in this context, given that many of the reactor concepts foreseen for space have strong synergies with systems currently being developed for earth applications, such as Small modular reactors (SMR) and especially microreactors (mR), it is of key importance to strengthen nuclear research towards space applications and to consolidate in this framework, the collaboration with the space research and industrial communities.

Loading and Powering up towards Space 

At PSI, research at the centre for nuclear engineering and science (NES) is primarily focused on reactor systems for terrestrial applications. This is also the case for the Laboratory for Reactor Physics and Thermal-Hydraulics (LRT) which conducts computational and experimental research with a focus on the safety of existing and future commercial nuclear reactor systems. Nevertheless, activities related to space nuclear reactors (SNR) have been carried out in the past by LRT researchers. Explorative studies were conducted on SNR concepts for long-term surface fission power and kinetics modelling of Uranium gas cores, among others. A solid experience was also built over the years with regards to experiments and instrumentation for heat pipe flow behaviour, something of key relevance for NEP concepts. On this basis and to complement its main research programs related to terrestrial nuclear power, the LRT launched the EPSILON program (Expertise Program in Switzerland for In-space transportation and Logistics Operations based on Nuclear) in 2022. The aim of EPSILON is to serve as consolidated Swiss framework for the coordination of research, education and support projects related specifically to nuclear space applications. Given that international studies conducted so far have pointed out the availability of advanced models and simulation (M&S) tools as key priority for SNR design as well as for operational and safety evaluations, one of the first EPSILON activities was to launch research towards the development of an integrated computational framework for SNR analyses. 

During 2023, preparatory steps were undertaken through masters projects. First, neutronic studies were conducted to evaluate the capabilities of the Serpent monte-Carlo transport code for simulations of KRUSTY type microreactor cores. In this context, several neutronic quantities related to core operation and safety were computed, including reactivity, delayed neutron fractions, control rod worths, reactivity coefficients as well as 3-D power peaking factors. Sensitivity studies upon modelling approximations such as reflector gap were in this context also performed. Moreover, shielding calculations were conducted to evaluate the neutron and gamma flux attenuation within the shielding structure during flight conditions. For both the core- and shielding simulations, the results were compared to reference solutions published in period 2015-2019 by Los Alamos National Laboratory (LANL). Overall, it was found the LANL results could be reproduced in a quite satisfactory manner both qualitatively and quantitatively (see Fig. 1). 

Fig. 1 Comparison with LANL Solutions of Serpent simulations for KRUSTY reactor design -
Control rod reactivity worth (Left) and neutron flux from core to upper shielding structure (Right)

Secondly, the modelling of heat transfer and cooling of microreactors in the OpenFOAM framework was considered. An OpenFOAM solver previously developed for very high temperature gas cooled reactors (VHTRs) was tested for heat pipe cooled systems. As part of this, several modelling aspects were studied with regards to their influence on the predicted temperatures, including gap resistance at the interface between fuel and heat pipes. Also, the effects from a single pipe failure on the heat pipe temperatures was also evaluated (see Fig. 2). Overall, the results obtained from these thermal analyses were found to reflect the expected behaviour, providing confidence that the VHTR solver could be used as starting point for multi-physics analyses of heat pipe cooled reactors.

Fig. 2 Assessment of OpenFOAM Framework for Thermal Analyses of KRYSTUY Space Reactor Designs 1) Reactor Geometry 2) Computational meshing 3) Effects of Gap Resistance on Fuel/heat Pipe Temperatures; 4) Effect of Heat Pipe Failure on Temperatures

Lifting off to open^SPACE with the MARVIS project

To consolidate the LRT activities on nuclear space applications, a proposal towards the development of a European open source computational and experimental platform for SNR evaluations was submitted early 2023 for the first MARVIS call (Multidisciplinary Advanced Research Ventures in Space) of the Swiss National Science Foundation (SNSF). Referred to as the open^SPACE platform, the underlying principles are to combine fully open source 1) solvers and associated models for advanced nuclear reactor core analyses; 2) solvers and associated models for dynamical coupling between reactor- and balance-of-vehicle (BOV) systems (e.g. power conversion-, heat rejection- and/or electric propulsion systems); 3) experiments and instrumentation (E&I) databases for validation purposes (see Fig. 3).

Fig. 3 Conceptual Overview of the PSI open^SPACE Platform

Eventually, the open^SPACE proposal was among the two projects selected by SNSF and approved for funding by the Swiss Secretariat for Research and Innovation (SERI). Established in the form of a consortium project involving PSI, ETHZ and the Lausanne based company ALMATECH (ALM) specialised in space and naval engineering, the MARVIS project has two main goals. The first goal is to conduct research aimed at designing, testing, verifying and releasing a first operational version of the platform combining M&S tools and E&I data for space nuclear reactors. The second goal is to connect nuclear researchers with space industry actors and on this basis contribute to the consolidation of EPSILON as a coordinated Swiss “network of excellence” for support to the Swiss space office and/or ESA on initiatives related to nuclear space applications.

Now since the ecosystem of space operations and/or missions that could potentially use NEP/NTP systems (either as accelerator or as enabler) is very large and since the spectrum of potential reactor concepts is also very wide, the specific objective outlined for MARVIS was to start the open^SPACE developments with focus on NEP vehicles and with Sodium Heat Pipe (SHP) cooled nuclear reactors. This selection of SHP-based NEP designs was made because such systems 1) allow one to leverage past experience as well as available expertise of LRT researchers more effectively; 2) offer strong synergies with surface fission power concepts as well as terrestrial microreactors; 3) were assessed as having reached among highest degree of maturity based on US and Chinese studies reported so far in the open literature; 4) could enable missions of potentially high commercial interest to the Swiss space industry such as, for instance, small reactor driven tugs for deep space exploration. A research plan consisting of 4 inter-connected work packages (WP), each one to be conducted via a dedicated PhD thesis, was proposed. The four WPs (see Fig. 4) consist of 1) development of integrated neutronic computational schemes and 3-D solvers for high-fidelity steady-state, transient and shielding calculations; 2) development of multi-physics coupling schemes for reactor neutronics/thermofluid core simulations and for dynamical coupling with BOV systems; 3) experimental characterization of SHP flow behaviour in steady-state and transient conditions using high resolution instrumentation; 4) development of a CFD- as well as experimentally informed advanced 1-D code for simulations of SHP multi-phase flow behaviour. On October 1, 2024 the MARVIS/open^SPACE project was kicked off with the arrival of all four PhD students at PSI.

Fig. 4. MARVIS/open^SPACE Project Scope

Future Stages of Exploration

The MARVIS/open^SPACE research activities will be focused on a specific NEP design for which a suitable mission will moreover need to be determined. Although this will enable a key step forward within the European landscape of space related nuclear applications, it will be of critical importance that open^SPACE be gradually extended to other systems and different missions. All of this will potentially require new solvers, models and experimental data. One intention is therefore to establish collaborative projects with other European nuclear reactor researchers in order to broaden the range of users and/or contribute to further developments/enhancements of the platform. Another intention is to strengthen the collaboration with ESA and with the Swiss space industry through complementary projects dedicated to scientific challenges not directly addressed in MARVIS but assessed as key, both to consolidate the open^SPACE developments as well as to contribute in advancing and maturing European technology in this area.