A Hybrid Fission/Fusion Reactor Could be the Best way to get Through the ice on Europa

In the coming years, NASA and the European Space Agency (ESA) will send two robotic missions to explore Jupiter’s icy moon Europa. These are none other than NASA’s Europa Clipper and the ESA’s Jupiter Icy Moons Explorer (JUICE), which will launch in 2024, and 2023 (respectively). Once they arrive by the 2030s, they will study Europa’s surface with a series of flybys to determine if its interior ocean could support life. These will be the first astrobiology missions to an icy moon in the outer Solar System, collectively known as “Ocean Worlds.”

One of the many challenges for these missions is how to mine through the thick icy crusts and obtain samples from the interior ocean for analysis. According to a proposal by Dr. Theresa Benyo (a physicist and the principal investigator of the lattice confinement fusion project at NASA’s Glenn Research Center), a possible solution is to use a special reactor that relies on fission and fusion reactions. This proposal was selected for Phase I development by the NASA Innovative Advanced Concepts (NIAC) program, which includes a $12,500 grant.

The list of Ocean Worlds is long and varied, ranging from Ceres in the Main Asteroid Belt, the moons of Jupiter (Callisto, Ganymede, and Europa), Saturn (Titan, Enceladus, and Dione), Neptune’s largest moon (Triton), and Pluto and other bodies in the Kuiper Belt. These worlds are all believed to have interior oceans heated by tidal flexing due to gravitational interaction with their parent body or (in the case of Ceres and Pluto) the decay of radioactive elements. Further evidence of these oceans and activity includes surface plumes and striated features indicating exchanges between the surface and interior.

The main challenge for exploring the interiors of these worlds is the thickness of their ice sheets, which can be up to 40 km (25 mi) deep. In Europa’s case, different models have yielded estimates of between 15 and 25 km (10 and 15 mi). In addition, the proposed probe will need to contend with hydrostatic ice with varying compositions (such as ammonia and silicate rock) at different depths, pressures, temperatures, and densities. It will also have to contend with water pressure, maintain communications with the surface, and return samples to the surface.

NASA has explored the possibility of using a heating or boring probe to pass through the icy sheet to access the interior ocean. In particular, researchers have proposed using a nuclear-powered probe that would rely on radioactive decay to generate heat and melt through the surface ice. However, a team of NASA researchers led by Dr. Benyo has proposed a new method that would rely on something other than conventional radioactive isotopes – plutonium-238 or enriched uranium-235. Instead, their method would involve triggering nuclear fusion reactions between the atoms of a solid metal.

Their method, known as Lattice Confinement Fusion, was described in two papers published in the April 2020 issue of Physical Review C, titled “Nuclear fusion reactions in deuterated metals” and “Novel nuclear reactions observed in bremsstrahlung-irradiated deuterated metals.” As they explain, fusion reactions are favored when it comes to space exploration because they generate enormous amounts of energy without creating long-lasting radioactive byproducts. However, conventional fusion reactions are difficult to achieve and sustain because they rely on extreme temperatures.

Convention fusion methods generally come down to inertial or magnetic confinement to overcome the strong electrostatic repulsion between positively charged nuclei. With inertial confinement, fuels like deuterium or tritium (hydrogen-2 or -3) are compressed to extreme pressures (for nanoseconds) where fusion can occur. In magnetic confinement (tokamak reactors), the fuel is heated until it reaches temperatures in excess of what occurs at the center of the Sun – 15 million °C (27 million °F) – to achieve nuclear fusion.

In contrast, the method Dr. Benyo and her colleagues envision a metal lattice loaded with deuterium fuel at ambient temperatures. Within this lattice, an energetic environment is created such that individual atoms achieve equivalent fusion-level kinetic energies. This is accomplished by packing the lattices with deuterium at densities one billion times greater than in tokamak reactors, where a neutron source accelerates deuterium atoms (deuterons) to the point that they collide with neighboring deuterons, causing fusion reactions. For their experiments,

Dr. Benyo and her colleagues exposed deuterons to a 2.9+MeV energetic X-ray beam, creating energetic neutrons and protons. As she explained in a recent interview with Universe Today:

“The lattice of metal atoms is in kind of like a cube for formation, and the deuterium gas nestles into the vacant, open areas. And what is beneficial with the metal is that it has a lot of electrons. And in our theory, electrons are a very critical component of enabling DD fusion. Because the deuterons, which are the nucleus of a deuterium atom, are positively charged just like a proton. A neutron is neutral, so the neutron can go wherever it may, please. So the electrons are negatively charged. So if you have a screen in between two positive charges, that negative screen will make the other positively charged particle on the other side look neutral to itself. So it reduces that repelling action that it wants to take with two positive charges.”

“The electron screening gives rise to what we call a cold plasma,” added Dr. Lawrence Forsley, a nuclear at NASA Glenn and longtime colleague of Dr. Benyo. “And it turns out the electron screening gives you the equivalent of up to 4000 electron volts of equivalent energy, while four KV is 44 million degrees. So by using this technique without using the magnets, but because everyone’s in close proximity, as Teresa points out, you have now overcome the barrier, and you’re now on the way to inducing the fusion reactions.”

This process could allow for fast-fission reactions using lattices built from metals like depleted uranium, thorium, or erbium (Er⁶⁸) in a molten lithium matrix. The team also observed the production of more energetic neutrons, indicating that boosted fusion reactions – aka. screened Oppenheimer-Phillips (O-P) nuclear stripping reactions – also occur in the process. Either fusion process is scalable and could be a pathway to a new type of nuclear-powered spacecraft. According to Dr. Forsely, this includes the Nuclear-Thermal and Nuclear-Electric Propulsion (NTP/NEP) NASA and other space agencies are currently investigating:

“Right now, there’s an effort between NASA, DARPA and the DOD, looking at nuclear thermal propulsion. So the idea is you get twice what we refer to as impulse seconds, maybe three times over chemical rockets, because you can heat fuel and throw it out the back end of the rocket at a higher velocity. There’s another one, which is nuclear electric propulsion. And we currently use this as solar electric propulsion on a variety of spacecraft that are exploring out as far as Jupiter. But if you want to go much further, or you want to go much faster… you can have a compact nuclear source [and] use that to essentially electrify grids and send xenon or mercury out the back end of the spacecraft.”

This type of nuclear process could be part of a Europa Lander, a proposed NASA mission that would build on the research conducted by the Europa Clipper and JUICE. It could also be used to create power systems for long-duration exploration missions, similar to NASA’s Kilopower Reactor Using Stirling Technology (KRUSTY) project. As Dr. Benyo described, nuclear applications become necessary when planning for missions beyond the Earth-Moon system, where solar power becomes less reliable – as illustrated by the recent losses of the Opportunity rover and Insight lander:

“We’ve relied on solar power panels, which has been very beneficial. And it enabled us to do so many more things than we could think of before. But the thing with solar panels is there’s a limit. I mean, look at the Mars Opportunity rover that just shut down. Because there’s so much dust on the solar panels. It can’t charge up the rover anymore. This technology opens up so much, so many more possibilities. So our agency is looking at nuclear power and propulsion and much more.”

A bonus of this new process is the critical role that metal lattice electrons whose negative charges help “screen” positively charged deuterons. According to the theory developed by project theoretical physicist Dr. Vladimir Pines, this screening allows adjacent deuterons to approach one another more closely. This reduces the chance that they will scatter while increasing the likelihood that they will tunnel through the electrostatic barrier and promote fusion reactions. Last but certainly not least, the technology provides assurances for those worried about contaminating extraterrestrial environments.

“That’s one of the advantages of our system is that we don’t have the typical radioactive byproducts as a typical fission reactor would have,” said Dr. Benyo. “So we would be able to protect the biomaterials that we encounter once we bore through that ice crust.”

“Some of the work that we’ve done indicates that when we do this fusion-fast fission of uranium or thorium, we end up not so much with a bimodal distribution,” Dr. Forsley added. “We sort of end up with a lot of ‘neutron-rich daughters’ coming off, but they have very short half-lives, and they emit basically beta rays (electrons), so the shielding requirements should be far less for what we’re doing.”

With more study and development, this technology could someday be used to power missions to multiple “Ocean Worlds,” such as Enceladus, Titan, Triton, and even Pluto and the Kuiper Belt. It could also be the means through which space agencies maintain “sustainable exploration programs” on the Moon, Mars, and beyond. Finally, this proposed method could have applications for life here on Earth, providing a new kind of nuclear energy and medical isotopes for nuclear medicine.

As Leonard Dudzinski, the Chief Technologist for Planetary Science at NASA’s Science Mission Directorate (SMD), said:

“The key to this discovery has been the talented, multi-disciplinary team that NASA Glenn assembled to investigate temperature anomalies and material transmutations that had been observed with highly deuterated metals, We will need that approach to solve significant engineering challenges before a practical application can be designed.”

Further Reading: NASA, NASA Glenn