Radioisotopes power reliable nuclear batteries for nation's needs

At the bottom of the ocean. In a remote desert. On another planet.

These are some of the places the U.S. government can use reliable, long-lived, durable nuclear batteries - batteries the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) is working to develop.

Brad Johnson leads ORNL's Nuclear Battery Initiative, working with state-of-the-art research tools and a multidisciplinary team of experts from across the lab. The team is pioneering a transformative approach to developing and deploying nuclear batteries for both national priorities and global challenges - underscoring ORNL's commitment to innovating new technology for energy and national security.

"The promise and hope of a nuclear battery is long duration power supplies," Johnson said. "Nuclear batteries are a proven and safe technology that can provide this kind of power, and we are developing advanced technologies to improve their performance and safety."

The team's work not only advances technologies to ensure secure, affordable, reliable energy, but it also fuels the nation's economic growth and energy independence - key national priorities.

Finding the right isotope

Nuclear batteries produce power from the radioactive decay of an isotope, making them ideal for remote locations where stable power is needed for years, or even decades, without human interference.

Not every radioisotope is suitable for nuclear batteries, Johnson said, but several are. Ideally, nuclear batteries should use an isotope with a half-life of between 10 and 100 years - the amount of time it takes for 50 percent of the isotope to decay away. With a shorter half-life, too much of the isotope decays away before it's ever used; with a longer half-life, too much of the isotope is required to make enough energy to power a nuclear battery.

Ideal candidates also would emit alpha and beta particles, which travel a shorter range than the more powerful electromagnetic gamma rays or neutrons. This makes it easier to convert their decay energy into useful electricity.

One example is plutonium-238. ORNL produces Pu-238 for DOE under a contract with NASA. NASA uses Pu-238 as fuel in its multimission radioisotope thermoelectric generators (RTG), the power systems that allows deep space vehicles such as the Mars rovers to operate reliably for multiple years, as they're designed to do.

The radioisotope thermoelectric generator for Perseverance, the most recent Mars rover, uses about 10.6 pounds of Pu-238, converting heat from its natural radioactive decay into electricity to charge the rover's two primary batteries and keep the rover's tools and systems at their correct operating temperatures. Its expected lifetime is 14 years - in theory, plenty of power to carry Perseverance through its stated mission of three Earth years on the Red Planet, and a prime example of translating cutting-edge research into real-world applications.

But future planned missions have different challenges. For example, NASA's effort to get a sustained presence on the moon has been hindered by the two-week lunar night, which requires keeping equipment warm while the moon is turned away from the sun - and makes using solar power on the moon impractical.

Meeting multiple needs

The US currently recognizes the need for long-duration, resilient energy sources, Johnson said - not only for space travel, but for national security efforts, such as supporting military operations in harsh and remote environments. Advanced radioisotope power systems, including both radioisotope thermal generators and radioisotope heater units, offer long-lasting, low-maintenance power sources that allows the U.S. to secure critical infrastructure and addresses emerging threats, even in the most challenging conditions.

Johnson and his team are looking at isotopes that could be viable, as well as ways to produce them, including fusion-based neutron generation. They've identified nine candidates to power radioisotope systems and proposed three of them as the best options - based not only on their properties, but also on how feasible it would be to produce them on the scale needed.

That's one of four primary challenges to overcome to successfully develop a new nuclear battery, Johnson said.

First, the radioisotope chosen to fuel it must be able to be produced easily, efficiently and on a large scale.

"Nuclear battery systems need an abundant supply of radioisotope fuel, and that's expensive and difficult to make," Johnson said.

Second, new technology must improve energy conversion. Currently thermoelectric technology is only 5 percent to 8 percent efficient. Increasing that to between 10 percent and 15 percent would cut fuel costs in half.

"Even modest energy conservation improvements will result in significant cost savings," Johnson said.

Third, new technology must manage thermal energy more effectively, through insulation and other design improvements, so that no energy escapes and is wasted.

And last, power control and power management systems must be improved.

Advanced manufacturing has the potential to address these challenges, he said. 3D printing techniques and the development of spherical powders that let solids behave like liquids offer vast possibilities. Design engineering allows for developing devices and systems that compensate for the limitations of certain materials and thus remain efficient.

Advanced thermoelectric materials, improved radioisotope heat sources and containers, engineered safety systems and new ways of modeling will help overcome barriers - which historically have slowed the creation of new nuclear battery technology programs.

"ORNL is fully equipped to solve these challenges - I would probably even say uniquely equipped, as far as institutions go," Johnson said. "We have the integrated capabilities and the technical leadership needed to innovate and create solutions for success."

Because of the multidisciplinary technologies involved with creating a nuclear battery, he calls the nuclear battery program an "All-ORNL opportunity" and says every one of the lab's directorates can contribute to addressing this national priority - driving innovation, powering economic growth and safeguarding the nation's future.

"There are significant national needs for nuclear batteries," Johnson said. "Our goal is to create a longstanding nuclear battery program that can meet those needs."

UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. DOE's Office of Science is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science. - Kristi L. Bumpus