Rolls-Royce Ramps Up Microreactor Work For Lunar Exploration

As new forms of nuclear energy increasingly feature in future space propulsion and power options, Rolls-Royce is accelerating plans to leverage decades of know-how in the nuclear submarine sector to develop next-generation microreactors for a wide range of space exploration roles.

Nuclear power is already used in space exploration in the form of radio-isotope thermoelectric generators, which release energy over time through radioactive decay. However, the higher power and efficiency demands of more ambitious missions to the Moon, Mars and beyond are driving studies of fission reactors to generate thermal energy for both propulsion and exploration power.

• Brayton cycle adopted for scalability
• Reactor targeted for lunar use in 2030

Conventional nuclear reactors on Earth heat water into steam,  which then drives a turbine and generates electricity. For space applications, particularly power generation, Rolls-Royce’s Novel Nuclear group is evaluating a microreactor that avoids the complexities of using water and instead relies on gas as its working fluid.

The concept combines a reactor with a Brayton-cycle heat engine that converts the heat energy of fission into electricity in a similar way to a gas turbine. However, unlike an open Brayton-cycle gas turbine—which draws in air from the atmosphere, compresses it, heats it and expels it back to the atmosphere—the Rolls design uses a closed Brayton cycle, in which the working gas stays inside the engine. Heat is introduced and expelled with a heat exchanger.

“We’ve assessed a lot of different power conversion options, and for a system producing around 40 kW and higher, the Brayton cycle offers the most scalability for the least mass,” says Katie Jarman, assistant chief engineer for Rolls-Royce Space Programs.

Rolls’ rapid progress in space nuclear activity follows the 2022 award of a modest £249,000 ($313,000) UK Space Agency study contract and the subsequent 2023 award of £2.9 million to deliver an initial demonstration of a UK lunar modular nuclear reactor. The Lunar Surface Nuclear Power Contract and associated funding under Phase 1 of the International Bilateral Fund, cosupported by the Australian Space Agency, was followed in April this year by an additional £1.18 million.

Awarded to Rolls-Royce Submarines and U.S. nuclear reactor specialist BWXT Advanced Technologies, the latest contract calls for further development of space microreactors for exploration missions. Virginia-based BWXT also is partnering with Lockheed Martin on developing a nuclear thermal rocket for the NASA-DARPA Demonstration Rocket for Agile Cislunar Operations program.

In addition, BWXT is collaborating with Aerojet Rocketdyne, Northrop Grumman, Rolls-Royce Liberty Works and Torch Technologies under the U.S. Defense Department Strategic Capabilities Office’s Project Pele. Targeting the development of a full-scale transportable microreactor, Project Pele aims to deliver a prototype to Idaho National Laboratory for testing this year.

In April, Rolls announced another related contract: a $1 million award from NASA to LibertyWorks for the preliminary design of an advanced closed Brayton-cycle converter for next-generation space-based nuclear microreactors.

In the UK, testing is underway at Rolls-Royce’s Bristol, England, facility, where a former helicopter turboshaft test rig is being used to evaluate the Brayton cycle. “This is generating data for our understanding of how you close a gas turbine system,” Jarman says. “Instead of using the gas turbine to provide propulsion, you’re using it to generate electricity—and then recirculating a fluid back through a heater to regenerate more electricity.”

The tests, which include turbomachinery from an off-the-shelf automotive racing turbine engine, are to continue through this year as part of efforts to reinforce design codes and methodology as well as to build confidence in the company’s computational design capability.

“From a specific design perspective, we’ll work on things like which working fluid we want in our system,” Jarman says. “It will help us define which gas we need to go through the core and the gas turbine. As we develop our understanding, we’ll put more prototypic turbomachinery in that testbed. We’re targeting the NASA 40-kW surface power requirement, but we think that over the course of our design program, and as our understanding matures and our analysis methods get better, we can push that power up as we get more confidence in how the technology is going to work.”

Rolls-Royce is also collaborating with the University of Oxford on heat exchanger and heat transfer technology. “That’s aimed at understanding more about miniaturized heat exchangers,” Jarman says. The studies have included preliminary heat transfer concepts, ranging from heat pipes to separate heat exchangers that use a gas different from the primary working fluid.

“The key is being able to design them as efficiently as possible, which makes it easier to test the redesigns,” she adds. Bangor University, Loughborough University, the Nuclear Advanced Manufacturing Research Center and the Welding Institute have also been involved in the project.

Studies have shown that heat pipes—a system traditionally used in air conditioning that comprises a hollow cylinder filled with a vaporized fluid—will likely not feature in the microreactor. “But we learned a whole bunch of stuff in that process, mostly to do with our computational capability and the confidence that we’ve got in our modeling and design work,” Jarman says. “And that will be directly read across to our gas architecture.”

Rolls-Royce is also continuing nuclear fuel manufacturing trials. The microreactor will use a more energy-dense, high-assay, low-enriched uranium fuel in which the uranium is enriched up to 20%, compared with the 5%-plus level of commercial nuclear reactors or the 90% weapons-grade material used in earlier nuclear space projects, such as the U.S. Rover-Nerva reactors.

“We also did a lot of work last year on making sure that any integration challenges and demonstrations that we are presenting can be informed through making big models,” Jarman says. The company’s latest miniature microreactor model is “a very small version of a much bigger full-scale demonstrator, which is intended to derisk some of the interfaces and work through the challenges around putting this all together,” she adds.

The basic architecture of the microreactor incorporates rods of uranium fuel housed in a graphite monolith. Once fission begins, heat from the monolith is transferred to a series of gas channels arranged around the reactor, governed as a whole by rotating neutron-absorbing control drums. Heated gas from the reactor is then fed into a central plenum that mixes it to a uniform temperature before passing it into the adjacent section that houses the turbomachinery power-conversion system. There, the hot, pressurized gas spins turbines that are connected to electrical generators. The gas is then cooled by radiators before it is passed back through channels into the reactor core section to be reheated.

Although the interface between the two main reactor core and turbomachinery sections provides the option for separate loops with different gases, Rolls is focusing on a simpler, lighter-weight single-gas loop design. “We’re still investigating the pros and cons of exactly what is used to transfer the heat from the core,” Jarman says.

For the working fluid, Rolls is investigating helium xenon and nitrogen as its main options. Helium xenon is attractive because of its favorable heat transfer properties, but the company has more experience and tooling for handling nitrogen.

“That’s one of the really key decisions that we need to make because both could work and both have pros and cons,” Jarman explains. “It all depends on where we want to put the risk in our program.”

Ultimately, the answer will be driven by whichever solution best meets the small space and weight requirements of the space mission. The current design envisages a unit measuring about 3 m (10 ft.) in length and 1 m (3 ft.) wide.

“We’re working through that in a lot more detail, and that will be one of the really interesting investigations to come out of the international bilateral,” Jarman says. “This will have implications on the further commercialization of the technology, depending on how ambitious and how novel you want key technology to be.”