US restarts production of plutonium-238 to power space missions

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Pellets of PU-238 are used to power deep-space probes(Credit: US Department of Energy)

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In an effort to avert an outer space energy crisis, the US Department of Energy's Oak Ridge National Laboratory (ORNL) has restarted production of plutonium-238 (PU-238) after almost 30 years. The civilian stockpile of the plutonium isotope used to power the radiothermal generators (RTG) that make electricity for US deep space probes has dwindled to only 35 kg (77 lb), so the first 50 g (1.7 oz) of plutonium oxide produced by the laboratory marks a major turnaround in American space capabilities.

PU-238 is an unstable isotope of plutonium with a half life of 87.7 years. As it decays into uranium-234, each gram produces about 0.5 watts of thermal power, which allows small RTGs to power spacecraft far from the Sun or on the surface of planets without the need for solar panels.

It's a system that's been very successful since the Apollo program, but since the Savannah River Plant in South Carolina stopped producing PU-238 in 1988, NASA has been relying on a stockpile of the element purchased from Russia in 1993. Only half of what remains meets specifications, so ORNL says that there's only enough left to power two or three NASA missions. Though PU-238 is produced in nuclear reactors, extracting it requires a prohibitively expensive isotopic separation process, so special production lines are needed.

To remedy this, ORNL is developing a new production line under a US$15 million program. The process involves taking neptunium-237 (NP-237) feedstock from the Idaho National Laboratory, converting it to neptunium oxide, then mixing it with aluminum, and pressing it into high-density pellets. These are then bombarded with radiation in ORNL's High Flux Isotope Reactor to turn the NP-237 into NP-238. This, in turn, decays quickly into PU-238.

The newly made PU-238 is then dissolved, the plutonium is separated from the left-over neptunium, oxidized, and sent to the Los Alamos National Laboratory for final processing into pellets. Meanwhile, the remaining neptunium is recycled to create more plutonium. One bonus is that the new plutonium can be mixed with the existing substandard stock to bring it up to par.

Now that the first plutonium has been produced, ORNL says that the next step in the program will be to analyze the sample to determine its purity and amount of PU-238. The laboratory will then analyze how efficient the production methods are and then scale up the process.

According to ORNL, the laboratory will soon be making 300 to 400 g (10.5 to 14.1 oz) per year, which will increase to an average of 1.5 kg (3.3 lb) per year.

"Once we automate and scale up the process, the nation will have a long-range capability to produce radioisotope power systems such as those used by NASA for deep space exploration," says Bob Wham, project leader for ORNL's Nuclear Security and Isotope Technology Division.

The video below discusses the new ORNL PU-238 production program.

Source: ORNL

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