U.S. Plutonium Stockpile Good for Two More Nuclear Batteries after Mars 2020

by

WASHINGTON — Even if NASA and the Department of Energy cannot get plutonium-238 production in full swing by 2021 as planned, there is enough of the nuclear material in the U.S. stockpile to fuel three of the same kind of the nuclear batteries used by the Curiosity rover now exploring Mars, a DOE official told outer planets scientists Feb. 20.

One of those batteries, known as a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), is reserved for the Mars 2020 rover: the only nuclear-powered mission NASA has committed to, for now. Based heavily on the design for the 2-year-old Curiosity, Mars 2020 will use a single MMRTG, which requires about 4 kilograms of plutonium-238 to produce 110 watts of electricity.

That leaves another two MMRTGs worth of plutonium-238 for some other NASA mission, Alice Caponiti, DOE’s director of space and defense power systems, said in a Feb. 20 presentation to the NASA-chartered Outer Planets Assessment Group in Mountain View, California.

The two extras could be ready by late 2021, Caponiti said, giving rise to the possibility that future New Frontiers and Discovery missions — NASA programs in which mission concepts led by single principal investigators compete for funding — could be nuclear powered.

NASA has ruled out nuclear power for the next Discovery mission, which will be selected this summer and launch by Dec. 31, 2021. For the next New Frontiers mission, “we have not made a decision whether radioisotope power supplies will be offered,” NASA spokesman Dwayne Brown wrote in a Feb. 20 email. Competition for that mission will begin in the government’s 2016 fiscal year, which starts this Oct. 1. It would notionally launch around 2021, creating a tight timetable for MMRTG delivery.

Plutonium-238 pellet
Plutonium-238 pellet. Credit: U.S. Department of Energy

Plutonium-238 powers spacecraft, landers and rovers in places where solar power is not practical, such as the outer reaches of the solar system and shadowed craters. Radioisotope power supplies such as MMRTGs convert heat emitted by decaying pellets of plutonium-238 into electricity. There is no nuclear reaction.

Plutonium-238 is refined and stored at the Department of Energy — or will be, once NASA-funded repairs to DOE equipment are completed and DOE resumes irradiation of neptunium-237 targets that was suspended in the United States back in 1988.

In 2012, after a few false starts, the Obama administration got Congress to go along with a plutonium-238 restart, under the condition that NASA pay to repair aging DOE infrastructure at the Los Alamos National Laboratory in Los Alamos, New Mexico. When the repairs are complete, the Energy Department will start producing 1.5 kilograms of plutonium-238 a year.

“The question is when that starts,” Caponiti told outer-planets scientists. She said that even if production does not immediately ramp up to 1.5 kilograms a year when the new equipment comes online, “something less than the full production rate” could still support NASA’s needs.

This is because plutonium-238 decays over time (its half-life is just under 90 years), meaning the longer the fuel is stored, the more energy it loses. Of the 35 kilograms reserved in the U.S. stockpile for civil space programs, 17 kilograms meet DOE’s minimum required energy levels. The other 18 kilograms do not, but could be refreshed by an infusion of newly refined plutonium-238.

Even if production gets off to a slow start, any new plutonium-238 helps and “is going to have an immediate effect on missions,” for the better, Caponiti said.

Also at the meeting, Ralph McNutt, a planetary scientist who led a NASA-chartered study of the agency’s future nuclear needs, briefed the group on the results.

The Nuclear Power Assessment Study examined both robotic and crewed mission concepts planned by NASA and the broader space science community over the next 20 years and concluded “nuclear power systems are certainly going to be needed during that time period,” McNutt said.

His presentation marked the first public summary of the study since its November completion. The 185-page report is the product of about six months of work and has not been released because of security concerns, McNutt said.

The big takeaways from the study are that NASA should make plans both to increase its spending on plutonium-238 infrastructure at DOE, which would allow production above the planned 1.5 kilograms a year, and consider an upgrade for the MMRTG, such as the Advanced Stirling Radioisotope Generator the agency canceled in 2013.

That next-generation nuclear battery would have produced about 140 watts of electricity using a quarter of the plutonium an MMRTG needs. While NASA is pressing on with some basic technology development for the plutonium-powered Stirling generator, there are no longer plans to produce a flight unit.
The MMRTG is “all we have at the moment, like it or not,” McNutt said.