Nuclear batteries power deep space exploration vehicles. However, the Plutonium-238 (Pu-238) needed to create the batteries costs roughly $8 million per kilogram, and previous U.S. space missions have nearly depleted the world’s supply.

A recent report issued by the Oregon State School of Nuclear Science and Engineering (NSE) for the Center for Space Nuclear Research (CSNR) investigated a targeted delivery system for Pu-238 that could make production more efficient and less cost prohibitive. While Pu-238 is not used for any weapons applications, it must be generated in a nuclear reactor.

“The current method for creating Pu-238 is both expensive and inefficient,” said Jackson Harter, a graduate researcher at NSE. In order to get Pu-238, researchers irradiate Neptunium-237 (237Np) within certain parameters and with enough control to stop the conversion process at the optimum moment, creating as much Pu-238 as possible.

“It’s a lot like developing a photograph in a darkroom,” said Harter. “Stop the chemical process too soon or too late and you don’t get the best results.”

The Department of Energy (DOE) has initiated a new production program but it will supply only 1.5 kgs/year. With funding from National Aeronautics and Space Administration (NASA), CSNR at the Idaho National Laboratory (INL) has designed a method for Pu-238 production that is affordable and can supplement the DOE program.  

At the request of CSNR director Dr. Steven Howe, and under the direction of NSE professor Dr. Andrew Klein, the Oregon State researchers examined the method designed by CSNR with funding from NASA.  The helix-shaped system can be inserted into a reactor, and water pumped through the helix will allow spherical capsules of 237Np to float around the reactor. This creates the best possible environment for its conversion to Pu-238.

“By enclosing the 237Np in the spheres the idea is to reduce loss and increase efficiency in production,” Harter said. “The old method included simply placing a block of neptunium in the reactor vessel and initiating the transformation with little control over the conversion process”

Part of the system analysis included ensuring the capsule material could endure the heat generated during the process, and most importantly keep the 237Np contained.

“A significant aspect of the CSNR concept is to continuously supply target capsules around the nuclear reactor so that the PU-238 can be produced in small, controllable quantities.  This is critical in that handling smaller quantities means much smaller and cheaper facilities,” said Howe.

The research team, including Harter, Philip Belstering and Don Buenaventura, conducted a thermal hydraulic analysis of the system and created a project budget as part of their report to CSNR.

“Our computer simulations show that the system works,” Harter said. “We did the neutronic calculations in MCNPX and the flow simulation in Solid Works. The next step will be to seek the funding to actually build a model and test it, provided CSNR wants to continue with the research.”

The Oregon State research team issued their findings to CSNR in June after presenting the project at the University Engineering Expo, where it won best NSE project.

“The student team at OSU led by Klein has produced a significant design contribution to the CSNR.  Their timely design of the feed system for the capsules answered a critical question for the effort-- could we move a sufficient amount of target material around the reactor to produce the desired amount of product?  Their effort was very well done and provides support for further development of the concept,” said Howe.

The implications of this project extend beyond national budgets, to the far reaches of our solar system. “Because of the rapid decline of sunlight intensity, all space exploration missions going past Mars require the use of a radioisotope power source.  The best radioisotope for these ‘space batteries’ is Plutonium-238,” said Howe. 

Replenishing the supply of Pu-238 in a timely and cost-effective manner is critical to future space exploration projects. Pu-238 has a half-life of approximately 87 years, so once a supply has been rebuilt it will remain useable in storage for a long period of time.

**Top photo: Pu-238 courtesy for DOE Digital Archives. 

**Photo courtesy of Kendon Shirley - Pictured left to right: Jackson Harter, David Stewart-Smith (NSE Advisory Board Chairman), unknown, Don Buenaventura, and Jose Reyes(Professor and CTO of NuScale Power).