Oak Ridge fuel powering NASA Rover on Mars trek

The world will be watching as NASA’s Perseverance Rover makes its final descent into Jezero Crater on the surface of Mars Thursday — powered by Oak Ridge fuel.

Mars 2020 is the first NASA mission that uses plutonium-238 produced at the Department of Energy’s Oak Ridge National Laboratory.

Pu-238 — encased in iridium-alloy cladding and insulated by carbon-bonded carbon fiber — is at the heart of the general purpose heat source module that fuels Perseverance’s multi-mission radioisotope thermoelectric generator. ORNL contributed both of these protective layers as well.

As the material decays, the heat released is converted to electricity, charging the Rover’s batteries and powering the onboard advanced imaging and sensor systems.

“Mars 2020 will be the first NASA mission that uses ORNL-produced plutonium-238,” Alan Icenhour, associate laboratory director for nuclear science and engineering at ORNL, stated in a news release.

“This accomplishment represents countless hours of work by dedicated ORNL staff, and it’s rewarding to see this work come to fruition. Helping NASA in its mission to Mars is a significant moment in the history of the lab,” Icenhour said.

"Our part is a grain of sand in this gigantic effort, but it fits perfectly and helps fulfill the great mission of exploring space. And my girls love it — it's been great to bring this experience home to my children and see them excited about it,” Nidia Gallego, Carbon Materials Technology group leader at ORNL Materials Science and Technology Division, stated on ORNL's website.

"We play a small part, but I have great pride in our work. Some missions take years to reach their destination; with the Mars mission, it’s really rewarding that we’ll get to see it launch and, several months later, see it land and start to see the fruits of our work,” George Ulrich, Radioisotope Power Systems program manager, Alloy Behavior and Design group leader Materials Science and Technology Division, stated on the website.

Some of ORNL’s Pu-238 was combined with Los Alamos (N.M.) National Laboratory's existing supply for the Mars 2020 mission. The overall mission includes the Perseverance Rover. ORNL has contributed additional elements to Perseverance, including producing the Rover’s iridium alloy clad vent sets, which are virtually indestructible metal cups that contain the Pu-238 fuel, and the carbon-bonded carbon fiber insulation that surrounds the fuel cladding.

The Perseverance mission continues a 50-year legacy of lab’s contributions to deep-space exploration, including technologies for the Voyager I and II, Cassini and Mars Curiosity missions. ORNL-produced Pu-238 also will power NASA’s 2027 Dragonfly mission to explore Titan, Saturn’s largest moon.

Pu-238 is ideal for deep-space travel because of its long half-life of nearly 88 years, but it has been in short supply, an ORNL news release stated. Previously, the U.S. stockpile consisted primarily of Pu-238 produced at the Savannah River (S.C.) nuclear plant in the late 1980s, which has since been decaying away. But U.S. production of Pu-238 ended more than 30 years ago.

However, 2015 was a milestone for ORNL — the first new stateside production of Pu-238 in nearly three decades. Since then, the lab has been consistently increasing its Pu-238 production capabilities, aiming to produce 1.5 kilograms per year by 2026, the news release stated.

“We have a 50-year history of irradiating targets and producing radioisotopes,” ORNL’s Robert Wham, Pu-238 Supply Program manager, stated in the news release. “Having the resources we have right here makes ORNL well suited to produce the nation’s supply of Pu-238. We have a lot of scientists and engineers all across the lab involved with this effort, and it’s very exciting to them to be contributing to space exploration.”

It’s not an easy process, and ORNL, DOE’s Office of Nuclear Energy and NASA have invested time, money, research and bright minds into improving it. ORNL receives neptunium-237 feedstock from Idaho National Laboratory, which stores the nation’s inventory. Once at ORNL, the neptunium oxide is mixed with aluminum and pressed into pellets. Next, the pellets are put into tubes and irradiated in ORNL’s High Flux Isotope Reactor (HFIR), which causes the neptunium to transmute into Pu-238.

The pellets are moved to shielded hot cells in ORNL’s Radiochemical Engineering Development Center. There, the Pu-238 is separated from the neptunium through a series of chemical processes, converted to an oxide powder and then shipped to Los Alamos for fabrication into ceramic pellets for the thermoelectric generator. Leftover neptunium is recycled to make more Pu-238.

HFIR can irradiate up to 6,800 grams of neptunium per year in batches that stay in the reactor for two to three months. INL’s Advanced Test Reactor is also irradiating small quantities of neptunium on a limited basis and is slated for a makeover next year that will increase its capability to produce Pu-238 as well.

The program has installed automated systems to press and measure the Np-237 target pellets. Wham said in the news release that automating the entire process has allowed the lab to increase the production of pellets significantly, helping to more than triple the output of Pu-238.

Source: The Oak Ridger

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Published February 17, 2021