WASHINGTON — The biggest roadblocks to having transportable nuclear power, according to DARPA program manager Tabitha Dodson, come down to heat and weight.
“There’s an enormous amount of energy inside a nuclear reaction, whether it’s fission or radioisotope,” Dodson told Breaking Defense. “[But] there is a lot of heavy and non-transportable hardware that deal with heat management for both fission and RTG [radioisotope thermal generators] systems. That heat management piece is always what drags down all of the dreams of transportable nuclear power.”
As a way to bypass that roadblock, DARPA funded seven competing teams as part of its Rads to Watts program to refine a range of approaches to radiovoltaics. The objective of the multi-million-dollar program (full figures weren’t disclosed) is a new kind of miniature power cell that can run anything from satellites in space to tactical radios to pacemakers, tough enough to work for years or decades without recharging and in extreme environments that freeze or fry traditional power systems.
“We’re going to go out there and replace the AA battery with something that lasts 30 years,” said Staff Sheehan, founder and CEO of Project Omega, a Rads To Watts contractor. Larger versions could replace diesel generators at forward bases, he told Breaking Defense, while the smallest could fit on a computer chip, acting as a built-in battery.
Radiovoltaic power sources can be so compact because they generate energy on different principles than better-known forms of nuclear power. Nuclear reactors and RTGs — used on space probes like Voyager — run hot while creating power, Dodson explained, so hot they need bulky cooling systems to keep from damaging themselves. Radiovoltaics, by contrast, turn radiation directly into electricity, without having to convert it into heat along the way. Instead, they capture the radioactive particles in a semiconductor, which excites electrons into a usable electric current. It’s the same process by which solar panels, which are photovoltaic devices, capture photons from sunlight.
The tricky part of tapping such intense energies is that they can do real damage to electronic systems, especially at close range and over a long exposure. In fact, the more energy a given kind of particle provides, the faster it can fry the power cell built around it — which is self-defeating.
“If it has too much energy, it’ll destroy your semiconductor,” said Pete Cabuay, co-founder and CEO of City Labs, a program participant. “It’s just laws of physics. … If you have a billion ping pong balls hitting the wall, they’re all going to bounce back. But if now you have a cannonball, you’re going to destroy that wall.”
Historically, this problem confined inventors to using weak particles in low-energy applications, like self-illuminating exit signs.
Rads to Watts is aiming to solve that problem, producing high-powered, long-living devices. To do that, the competing teams span a wide range of approaches, Dodson said. “We have seven prime performers on the program and they all have different ideas,” she said.
Cabuay’s team at City Labs is on the more conservative end of the spectrum, using a tamer radiation source called tritium, which emits a form of beta particles. But to reach the power output required for the competition, the team is packing more tritium into a smaller volume, a task that requires serious engineering and chemistry.
City’s sticking with tritium, Cabuay said, because there’s a long, unhappy history of researchers trying higher-energy radiation sources, only to burn out their device: “The US patent database [is] littered with ideas for radiovoltaics.”
Other teams, however, are all using more intense radiation sources. They’re counting on advances in materials science to design new kinds of semiconductors that can endure higher energies.
“We need to figure out a semiconductor material that can actually withstand intense radiation,” said Justin Kasper, chief of technologies at BWXT, a company that builds nuclear reactors for the Navy. “We want to hammer that thing all the time, so we’re generating a meaningful amount of power.”
BWXT, teamed with Johns Hopkins University, is using alpha particles for their radiation source. Because alpha particles are larger and hit harder than betas, the BWXT-Johns Hopkins team is exploring a wide range of new and more resilient semiconductor materials.
There are “billions of possible permutations,” Kasper explained, so they use AI to simulate different crystal structures and pick out the best candidates for physical tests.
Avalanche Energy is also using alpha particles as its source, though it’s taking a different approach. “It’s a two-step process,” lead physicist Daniel Velazquez explained in an interview.
Instead of toughening up its semiconductor to absorb the alphas directly, Avalanche protects it with “an absorbing layer of liquid metal,” he told Breaking Defense. The alphas hit the metal and convert their intensely concentrated energy to much more manageable electrons, which the semiconductor then turns into electrical power.
Another team, led by Morgan State University in Maryland and partnered with Sheehan’s Project Omega, is trying to get the best of both worlds. Like City Labs, this team is using beta particles, but it’s a higher-energy kind of beta, emitted by strontium-90 instead of tritium. Like the BWXT-Johns Hopkins team, it’s relying on novel materials to hold up under higher-energy radiation sources — in this case with Northrop Grumman providing the AI simulations of candidate materials.
All these competing radiovoltaics have already demonstrated power outputs of at least 10 watts per kilogram, Dodson said. That’s two to three times as efficient as traditional RTGs, she said, but it’s just a start.
“We’re pretty confident that we could, you know, get much greater than 10 watts per kg,” she said, “somewhere between 10 and 100 watts.”
The teams will finalize their prototype power cells over 15 months, into next year, Dodson said. Then the best designs get to proceed to a nine-month endurance test to see if they really do hold up under their own internal radiation and external, environmental pressures. After that, she said, the aim is to have at least one candidate ready to transition to large-scale deployment with the military.