Depleted Uranium Batteries Could Turn Waste into Power

It seems like something right out of Doc Brown’s garage in Back to the Future: a rechargeable battery made with depleted uranium. But that’s exactly what researchers in Japan have built.

Scientists at the Japan Atomic Energy Agency (JAEA) say their uranium battery could help renewable energy sources such as wind and solar farms to provide a stable energy output by serving as a potential alternative to large-capacity lithium-ion batteries. They also say their battery is the first of its kind in the world and they have verified its charging and discharging performance. While still in the early stages of development, the technology could turn nuclear waste into a resource.

Japan’s nuclear power industry has left a legacy of 16,000 tonnes of depleted uranium, which is created as a byproduct of nuclear fuel production. In the United States, by comparison, the Department of Energy stores about 750,000 tonnes.

For uranium to be used as a fuel in reactors, it goes through an enrichment process that yields both enriched uranium and depleted uranium, which has lower amounts of the fissile isotope U-235. Depleted uranium is weakly radioactive; however, its chemical toxicity is similar to that of natural uranium, and it can cause kidney damagein high doses. Still, depleted uranium has found many uses due to its density, including in armor-piercing rounds and radiation shielding for medical applications. Now, the JAEA researchers say depleted uranium also could be used in batteries.

The JAEA announced the development in March, and provided a rough outline of the battery technology. While the agency declined to say when or where a paper with more details on the technology would be published, they confirmed some aspects. Specifically, the prototype is a flow battery, a form of energy storage that has been proposed for intermittent renewables like wind and solar. Flow batteries store energy in two tanks of liquid electrolyte solutions, one positively charged and the other negatively charged, with larger tanks providing more capacity.

The solutions are pumped into the battery stack, where they react at the electrodes and generate electricity. In the prototype, uranium was used as the active material for the negative electrode, and iron as the active material for the positive electrode. The battery’s electrolyte solution is a mix of organic solvent and a salt containing both positive and negative ions that is liquid at temperatures under 100 °C.

Uranium Battery Prototype Development

Rechargeable uranium batteries had been proposed roughly 25 years ago by Yoshinobu Shiokawa of Tohoku University and Hajimu Yamana and Hirotake Moriyama of Kyoto University. According to Kazuki Ouchi, an assistant principal researcher at JAEA, including iron in the design was a key part of the new prototype. Specifically, using iron ions with different states of oxidation helped stabilize the electrolytic solution. Using iron as an electrolyte was a key part of the invention. By combining an iron electrolyte with a uranium electrolyte, JAEA researchers achieved a voltage of 1.3 volts in the prototype design.

The single-cell voltage of the prototype was 1.3 volts, which is close to that of a common 1.5-volt alkaline battery, and it was able to light up a small LED. The battery was charged and discharged 10 times, during which time its performance was almost unchanged, indicating relatively stable cycling, the researchers say.

During the charge and discharge cycle, the color of the uranium electrolyte solution changed from green to purple and back to green, reflecting its different oxidation states.

Asked about the safety issues surrounding the use of depleted uranium, the team suggested concerns could be addressed with proper shielding.

“The radioactivity of uranium in the prototype battery in this research is not a problem due to the lab-scale size (about 3 milliliters of electrolytic solution). Uranium has relatively low but not negligible radioactivity,” says Ouchi. The team plans to evaluate the amount of radioactivity given off by the battery and study shielding structures for larger capacity designs.

The research team is now developing flow cells including electrodes for a higher-capacity battery. The larger design would use 650 tonnes of uranium and have a capacity of 30,000 kilowatt-hours, roughly equivalent to the daily electricity supplied to 3,000 households in Japan.

“The concept of a uranium battery may offer interesting insights as a test case into the performance of non-aqueous electrolytes but it has significant issues including uranium safety and weight,” says David Howey, a professor of engineering science at the University of Oxford who specializes in battery technology but was not involved in the research.

The battery would also struggle against existing technologies. “Existing technologies for stationary storage, such as Li-ion batteries and flow batteries, have had many years of development and scale-up, and therefore are far more cost-competitive than new technologies because of Wright’s law,” says Howey, referring to the phenomenon of costs falling as production rises.

“Any new technology has to offer a path to extremely low costs as production scales—and in this case it’s not obvious what this path would be and whether it would be acceptable environmentally and politically,” Howey adds.

Ouchi says that in countries with nuclear power generation as a base for their energy policies, the amount of depleted uranium is expected to increase in the future as the amount of electricity generated increases, which would provide a regular source of the material for this type of battery.