High-power electricity direct from radiation is the vision of Rads to Watts

What may sound like a cross between science fiction and a reality TV competition is actually a serious engineering effort to capture the energy from nuclear radiation and convert it to electricity directly using advanced charge-carrying materials.

“One of the exciting things about this is that it's not just a typical phased structure,” DARPA Defense Sciences Office (DSO) program manager Tabitha Dodson told Nuclear News. “People are going to have to outperform the other performers to proceed at the downselect. I think it’s going to be a very fun part of this program.”

From ideas to action: Last summer, NN wrote about DARPA’s request for information on potential radiovoltaic power systems (DARPA wants to bypass the thermal middleman in nuclear power systems). At the time, Dodson said responses would show if there was “sufficient scientific rationale” to make a case for DARPA investment.

Ideas flooded in—DARPA received over 60 responses proposing radiovoltaics that benefit from recent materials advances in efficiency and radiation tolerance.

Now, DARPA has taken the next step. On June 20, the agency released its program solicitation for a 30-month program called Rads to Watts. Multiple performers are expected to design unit cells, meet milestones, and exceed program parameters to prove out different concepts for the direct conversion of nuclear radiation energy into electricity.

Abstracts are due July 10 and proposals are due August 20. Work is expected to begin in December and be divided into three periods: a base period of 14 months, an option period 1 from months 16–24, and an option period 2 (or “bonus” period) from months 25–30.

The big picture: Space is one domain where radiovoltaics could reshape what’s possible. In space, “there are regions where solar arrays simply don't work,” Dodson said, and then there are places where the use of solar arrays is limited, including “the more radioactive parts of Earth's orbit where solar arrays don't last for longer than on the order of months.”

Dodson is partial to space applications—her work at DARPA has included a key role in establishing and then serving as program manager of the nuclear thermal propulsion rocket program DRACO (Demonstration Rocket for Agile Cislunar Operations)—but some radiovoltaics applications are closer to home.

“The vision for this at the agency level is that it could apply to any domain or really any mission that needs power for which the mission doesn't have a logistics supply chain to get batteries or fuel to that location, and/or a place where there's no sunlight—somewhere that a solar array just won't work—so places like the bottom of the ocean, or in the Arctic, in particular in the winter when it's dark all the time and it's really hard to get to.”

While the applications could be diverse, so too could the radiation sources be. “The vision is that this could apply to any radiation source, across the spectrum from a naturally decaying radioisotope all the way to a reactor which is emitting neutrons and gammas,” Dodson said. Radiovoltaic radioisotope power sources represent the “near-term and simpler” option, Dodson said.

“The novel thing about this program is that we're looking at high power,” Dodson said. “We define that as greater than a kilowatt. Someday in the future, the vision is that maybe it's applied to megawatts, but we at least want it to be proven that we can develop a unit cell to survive the flux and also the fluence [flux multiplied by the duration of time it's operating] such that the unit cell can be built into a large array.”

Designing for diverse missions: Performers can design radiovoltaics for a range of mission-relevant solutions. As the solicitation puts it: “By allowing performers to choose different combinations of power density and lifetime for a unit cell, DARPA intends to characterize the performance limits of radiovoltaics derived with different materials, mechanisms, and architectures through this program.”

Proposals must explain how a team expects their candidate design to fit a particular mission. The solicitation includes some examples: “durations of days at 3,000 W to support terrestrial operations; durations of weeks or months at 2,000 W to support underwater or Arctic operations; or durations of years at 1,000 W to support various space operations.”

To evaluate and compare the performance of diverse designs, DARPA will use a “figure of merit” to represent “multiple opportunities across a range of trades between power density and resistance to cumulative radiation dose.”

Specifically, the figure of merit “will be the power density multiplied by operating time of the solution” and is intended to allow for trade-offs between power density and the lifetime of a system. Proposers can and must define their own objective operating time, power density, and radiation source and particle type.

Current limitations: “When you look at the state of the art, today's radiovoltaics—a lot of them—are very inspired from photovoltaics, which are your traditional P-N–junction semiconductors,” Dodson said. Low-power radiovoltaics are in limited service today but are challenged by radiation-induced material damage under high energy exposure.

For deployments in space, radiovoltaics envisioned by the DARPA program would not only be more resilient than solar arrays, they also don’t require a broad, flat surface exposed to a sun, but “can be accordioned or folded into all kinds of different geometries, such that they’re much more compact to be high power,” Dodson explained.

The radioisotope power systems currently used for long-term NASA science missions typically use decaying plutonium-238 to generate heat, which is then used to generate electricity. Radioisotope thermoelectric generators (RTGs) can generate 100s of watts with 50–100 kg devices, with efficiency losses inherent to a thermal energy conversion system. “Radiovoltaics could provide a 10- to 100-fold improvement in output power per unit mass as compared to current state-of-the-art RTGs,” according to the Rads to Watts solicitation.

Testing the tech: “Right out of the gate, phase 1, we want people to get radiation degradation data on what we’re calling a unit cell,” Dodson said. A unit cell, defined as the simplest representative geometry of a radiovoltaic, must have a source region, a charge generation region, and a charge collection region. While one unit cell might produce power on the milliwatt level, a high-power scaled array of unit cells could produce power at kilowatt levels.

The unit cell will be exposed to radiation not only from a team’s chosen radioisotope source, but “also we're going to use a linac to dose the material to levels that will prove it can withstand high-flux radiation,” Dodson said. “Exposing the materials to high levels of radiation, higher than what you would typically be exposing materials to for space testing, is going to be our number-one focus.”

The linac electron beam will ensure all submissions are tested against the same radiation dose, with the sample’s degradation measured as the change in power density before and after radiation dosing. The solicitation specifies that unit cells should show about 0 percent degradation at the 15-month downselect. At the 24-month downselect, after exposure to a fluence that is 1,000 times greater, the unit cells should show less than 20 percent degradation.

Those that make it to the bonus period would be asked to integrate their unit cells into the design of a high-power system, “which we can then give to a transition partner or follow-on program which would then go build that high-powered system,” Dodson said.