In searching for new energy sources to meet power demands, researchers have found some low-energy devices can be powered using the waste of nuclear reactors. This emerging technology, which is similar to photovoltaic cells, offers unique solutions and can operate for decades under extreme temperatures. 

A recent project at the Woodruff School of Mechanical Engineering at the Georgia Institute of Technology centered around recycling nuclear waste to produce electricity in a process known as betavoltaics. The project—led by Shannon Yee, associate professor at Georgia Tech—found that nuclear waste can create power in a battery, and multiple materials within the waste can be used, from tritium to strontium. 

This betavoltaics process has been developed to energize everything from temperature and imaging sensors used in the body, or down well holes, to strain-measurement devices that can collect data about the conditions of a structure.

The Woodruff project, in collaboration with Stanford University and funded by the Defense Advanced Research Projects Agency (DARPA), followed the premise that betavoltaics can achieve conversion efficiencies that make energy considerably more efficient.

The power amounts are small, so the conversion method is valuable for small devices. The technology can generate about 1 watt of power continuously for as long as 30 years. So the primary use for betavoltaics is for remote and long-term use, said Yee, who also directs the Scalable Thermal Energy Engineering Laboratory.

Although this low-power source can’t be used to energize a community (that would require all the nuclear waste the world has to offer, researchers say), the growth in sensors and wireless devices used in buildings, homes and factories make these small and long-term power sources especially valuable.

At the center of the project is a nuclear battery, which generated electric current from electrons emitted from the radioactive source. Betavoltaic batteries use semiconduction junctions to convert electrons from beta radioactive decay, in which the atoms emit high-energy electrons, called beta particles—into usable current. 

The benign isotopes from nuclear material known as tritium produce a limited amount of radiation. The Woodruff team examined what they found to be another interesting material: strontium. Betavoltaic batteries with strontium have the advantages of high energy density, long service life, strong anti-interference ability, small size, light weight, easy miniaturization and integration.

These batteries operate similarly to the way solar panels work, Yee explained. Solar panels catch photons from the sun, turning them into current, while betavoltaic processes use silicon to capture beta particles emitted from the radioactive gas. Beta particles have high kinetic energy, which interacts with the solid, kicking off more electrons, and those electrons cause a cascade of other electrons.

As the electrons are kicked free from an atom, they leave a hole. In a normal atom or crystal lattice, the negative charge of the electrons is balanced by the positive charge of the atomic nuclei, so the absence of an electron leaves a net positive charge where the hole is.

That is exactly how a photovoltaic cell works, Yee said. A photon comes in, it creates an electron hole pair and then that electron hole pair is separated by an electric field. Because that beta particle is a high-speed electron, it has more mass than a photon. It also has charge and, as a result, creates more electron hole pairs.

Leveraging advances in photovoltaics

From the 1990s into the 2000s, solar cell efficiencies have been improving, and Yee and other researchers have taken advantage of those improvements. 

“We got better at designing and building” similar cells that could act as batteries, he said. That was when Woodruff got its grant from DARPA, and collaborated with Stanford to use strontium-90 as its radioactive source material.

“Strontium is actually a dominant waste product in a lot of our spent nuclear fuel,” he said. Because strontium is denser than tritium, the overall energy density is higher. So strontium is in that nice place where it’s actually a pretty energy-dense system.”

Another advantage to strontium is that after it decays, it becomes a stable isotope, “so you don’t have all of this additional radioactive material afterward,” he said.

Battery design

The next challenge was battery design. One concept Yee envisioned was that rather than building a cell and then putting the isotope on top to power it, they integrated the isotope within the cell, and created a multilayer stack where the radioactive materials were contained within the battery.

They then tested the premise. 

“We just wanted to see how they would perform under a simulated situation,” he said. 

Getting radioactive material in the form of nuclear waste on hand was another challenge. 

“We didn’t want to handle strontium-90,” Yee said. “So what we used instead was a linear accelerator where we accelerate electrons.” In that way they could mimic the same spectrum that would be achieved from strontium-90 beta particles.

The Woodruff team found that they could create a battery that would last 13 years without recharging. The batteries could generate several watts of power throughout that time, without interruption.

Based on these results, “We got excited that maybe there’s something here for the community to really latch onto,” Yee said. 

The research team then led a workshop hosted by the Lawrence Livermore National Laboratory in California around 2017.

However, the energy community had different ideas, Yee said. Another energizing method already in use focused on radioisotope thermoelectric generators (RTGs), amounting to nuclear material that gets exceptionally hot—and that heat is then converted into electricity. Deep-space satellites are powered by RTGs.

To make betavoltaics a commonplace solution for low-power, long-life energy, the development of batteries needs to improve, Yee said. Despite the growth of the solar power industry, “we haven’t seen huge improvements in solar cell efficiencies.” 

While more such cells are being manufactured, the technology hasn’t changed much. 

”We see more solar cells that are being deployed, but we haven’t necessarily seen any new technological advance that would change any with this,” he said.

Nuclear materials are also highly regulated, and there are many constraints related to how companies can ship them, especially across state lines.

Use cases

Despite the challenges, commercial applications of the technology are available, and companies such as City Labs, Miami, have been building solutions. The company’s use-cases center on energizing ultra-low-power sensors for extended times, especially in extreme environments.

“Our goal is to be able to put autonomous sensor devices—whether it’s a medical implant or inside of industrial machinery or downhole oil wells—anywhere where it’s difficult to replace batteries,” said Peter Cabauy, the company’s CEO. “The sweet spot is in the low microwatts.”

Betavoltaic batteries can operate for decades, even under extreme conditions.

“The first batteries we ever made were in 2008, and they’ve been operating for 15 years,” Cabauy said. “They’ve been subjected to all sorts of temperature extremes,” and continue to operate for purposes such as taking a temperature reading every hour or every 10 minutes in a challenging or inaccessible location. The company now has contracts with several space agencies to provide the technology for powering systems in space.

City Labs has customers conducting their own research and development with these long-lived batteries or that are preparing to build the batteries into products.

In the short term, Cabauy predicts the systems will first be found in medical implants and Class III medical devices such as small heart pacemakers or similar devices.

Other use cases include powering sensors that monitor conditions in an oil well. Even in the harshest conditions, the betavoltaic battery would simply continue to power the sensor and spare the users from replacing batteries. By doing that, it’s possible for oil well operators to get a sense of the health of the well and improve safety and efficiency.

Sensors could similarly use the batteries for building intelligence in cases of tracking the conditions of a bridge or building. That could include structural monitoring to understand any changes in the structure over the course of years. 

On a building, the battery-powered sensors could be installed as part of the fabric of the structure, collecting data for months, years or decades until someone opts to retrieve that data to learn information such as what may have caused a crack on a wall, and if the strain conditions have changed. They could also be used to power scientific apparatus in spacecraft, or similar equipment used under the sea, underground or in polar areas.

“I think people will constantly be revisiting [betavoltaics technology],” Yee said. 

And much of that may still take place in academic environments. “We always have students that come through and have new ideas. That’s the wonderful thing about research—you search again,” he said.

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