ORNL extracts in-demand isotope from plutonium leftovers

Newswise – A new method developed at Oak Ridge National Laboratory proves that the junk from one effort is the precious isotope of another.

One of the by-products of the Department of Energy’s national plutonium-238 production program is Promethium-147, a rare isotope used in nuclear batteries to measure material thickness.

It is difficult and costly to dispose of waste containing radioactive elements left over after irradiating Neptunium-237 targets in the High Flux Isotope Reactor, a user facility of the DOE Office of Science, to make Pu-238 for space exploration to manufacture. Last year, a new ORNL project for the DOE isotope program began to break down Pm-147 from the fission products that are left over when Pu-238 is separated from the target.

The main goal of these efforts is to restore domestic production of Pm-147, which is in short supply. This has a side effect: it reduces the levels of radioactive elements in the waste so that they can be disposed of safely and cheaply both now and in the future.

“In recovering a valuable product that the DOE Isotope Program wanted, we recognized that we could reduce our disposal costs,” said Richard Mayes, ORNL, group leader, Emerging Isotope Research. “There are some synergies.”

Pm-147 is not the only valuable metal in the Pu-238 production waste stream of interest to the DOE isotope program, but it is the first Mayes team to be successfully mined. ORNL’s Susan Hogle wrote the original proposal to mine Pm-147 from Pu-238 waste and provides computational assistance in detailing potential isotopes in the solution. Mayes worked with Lætitia Delmau, interim group leader for the Radiochemical Separations Science Group at ORNL, to develop a process to extract and purify it.

“We are currently the only manufacturer of Pm-147 in the US and there is a market for it,” Mayes said. Research shows that it could have additional uses in medical imaging and as a radioisotope to generate electricity for space probes.

While the Pu-238 supply program is funded by NASA and administered by the DOE Nuclear Power Bureau, the recovery of Pm-147 is funded by the DOE Isotope Program at the DOE Science Bureau.

ORNL is also researching for the DOE isotope program, in which neodymium-146 is irradiated in the high flux isotope reactor (HFIR) to produce Pm-147. This process would provide a new alternative, strengthen the domestic supply chain for this important isotope, and reduce US reliance on a single foreign supply.

The Mayes team’s efforts are tied to the Pu-238 production schedule, which currently has between two and four processing cycles per year. They estimate that each batch contains approximately hundreds of Pm-147 curies. Working in shifts for several days, the team used hot cells at ORNL’s Radiochemical Engineering Development Center (REDC) to isolate a concentrated batch of Pm-147 from solution of Pu-238 by-products.

This batch is then transferred to a shielded cave – a smaller type of hot cell – at the REDC, where it is cleaned and prepared for shipping to customers of the DOE Isotope Program. The process takes a few months and around 20 people.

The team will show next year whether the recovery of Pm-147 from Pu-238 by-products is sustainable. In this case, the project should grow alongside the Pu-238 program. The lab’s goal is to produce enough Pu-238 to produce 1.5 kilograms of plutonium oxide annually to meet NASA’s space travel requirements. Plutonia made by ORNL is on board the Perseverance rover, which is now exploring Mars.

Delmau, who specializes in radiochemical separations, said Pm-147 was one of about a dozen elements left after processing Pu-238, and isolating individual elements would require work.

“Separating lanthanides from one another and from trivalent actinides has been a challenge for decades,” she said. “When elements have the same chemical properties, it can be very difficult to choose one.”

Currently, Pm-147 is the only isotope recovered from the waste stream. However, an important side effect of the project could ultimately be a significant reduction in the amount of transuranic material that the laboratory must have. Mayes said successful efforts with Pm-147 could lead to the extraction of other materials at ORNL.

“We have a waste stream that contains valuable products – products that could have many uses,” he said. “Our national laboratories have the resources and experience to isolate them safely and efficiently.”

UT-Battelle administers ORNL for the Department of Energy’s Office of Science, the greatest proponent of basic science in the United States. The Office of Science is working to address some of the most pressing challenges of our time. More information is available at energy.gov/science.

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