On 22 December, the Department of Energy announced in a press release that for the first time since 1988 the US produced plutonium-238. The demonstration batch of 50 grams created by scientists at Oak Ridge National Laboratory represents an important step toward producing the plutonium necessary to power NASA’s future deep space exploration missions. This production of 238Pu also involves Idaho National Laboratory, which provides the necessary neptunium-237, as well as Los Alamos National Laboratory, which will store the plutonium oxide produced by Oak Ridge.

Plutonium-238 is used in radioisotope power systems (RPSs), devices that convert heat from radioactive decay into electricity to power space probes and rovers that cannot acquire enough energy from solar panels alone. NASA’s current RPS is the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which uses approximately 4 kilograms of 238Pu to produce about 110 watts of power. The Curiosity rover on Mars uses an MMRTG, and another MMRTG is set aside for the Mars 2020 rover.

The Mars Curiosity rover relies on plutonium-238 for power. (Photo credit - NASA)

RPSs have been integral to NASA’s deep space exploration efforts for more than 50 years—powering iconic exploration efforts ranging from the Voyager missions to the recent New Horizons mission to Pluto—and they are expected to be an essential component of NASA’s planetary science missions for the coming decades. Indeed, one of the ten broad conclusions of the recent Nuclear Power Assessment Study conducted by researchers at the John Hopkins University Applied Physics Laboratory is that “[the need] for nuclear power systems is expected to extend for at least one more decade past that covered by the current decadal surveys.” That unavoidable need for 238Pu to support critical science missions led the Obama administration and NASA to broker an agreement with DOE to resume production of 238Pu using NASA funds starting in 2012. To date, NASA has spent more than $200 million to support this restart.

The US currently has 35 kg of 238Pu reserved for civil space purposes, with an unspecified additional amount reserved for national security purposes. Only about 17 kg of the 35 kg is of sufficient quality for use in an RPS. However, the remaining material could perhaps still be used if blended with newly produced 238Pu. The press release indicates that DOE hopes to scale up production initially to 0.4 kg per year and eventually to an average of 1.5 kg per year. According to a statement by a DOE official during a presentation to NASA’s Outer Planets Assessment Group, DOE expects to reach an output of 0.4 kg per year by 2019 and of 1.5 kg per year by the mid 2020s, based on current budget projections.

Plutonium oxide pellet. (Photo credit - DOE)

The committee report for the fiscal year 2016 House Commerce, Justice, and Science Appropriations Act provides guidance for $20 million from the amount made available for the Planetary Science Technology program to be used for NASA to work with DOE to “domestically produce between 3.3 and 11 pounds [1.50 and 5.00 kg] of plutonium-238 annually beginning in fiscal year 2016.” The corresponding Senate committee report is silent on the subject. The joint explanatory statement that accompanied the final fiscal year 2016 appropriations law lowers the funding amount to $15 million but retains the rest of the language.

A Stirling alternative to MMRTGs?

A potential alternative to the MMRTG is the Advanced Stirling Radioisotope Generator (ASRG), a yet-to-be-fully-developed RPS that could produce more power using about a fourth as much 238Pu. Proponents of the technology point to its ability to reduce demand on the 238Pu stockpile and decrease urgency of restarting production as rationale for ASRG development. In addition, a 2009 National Academies study on RPSs recommended that NASA complete development of an ASRG “with all deliberate speed.” Similarly, the 2013–22 planetary science decadal survey study committee argued that the “highest priority for near-term multi-mission technology investment is for the completion and validation of the ASRG.”

However, after investing about $270 million into ASRG development from 2008 to 2013, NASA canceled the ASRG program in 2013, citing budget constraints and a desire to spare programs such as the small- to medium-sized Discovery- and New Frontiers–class missions. Nevertheless, NASA has not completely given up on Stirling technology, recently issuing a request for information on use of Stirling devices for space power generation.

Ohio delegation pushes for details on potential use of Stirling generators

Unsurprisingly, multiple members of the Ohio delegation desire to know more about the potential use of advanced Stirling generators for space exploration. Ohio is home to NASA’s Glenn Research Center—the center responsible for RPS development—as well as Sunpower, a contractor involved in the ASRG program. In July 2015, Senator Rob Portman (R-OH) and Sen. Sherrod Brown (D-OH) introduced the Efficient Space Exploration Act in the Senate, and Representative Steve Stivers (R-OH), Rep. Bill Johnson (R-OH), and Rep. Marcia Fudge (D-OH) introduced the companion bill in the House.

Both of those bills require the White House Office of Science and Technology Policy (OSTP) and NASA to submit a report to Congress that assesses NASA’s projected mission requirements for RPS material, plans for use of “advanced thermal conversion technology, such as advanced thermocouples and Stirling generators and converters,” schedule risks associated with potential delays in domestic production of 238Pu, and the costs of producing such material. The bill also directs the OSTP and NASA to explain how NASA has “implemented or rejected” recommendations from the 2009 National Academies RPS study.

That language is included in both NASA authorization bills considered by the House last year (H.R. 810 and H.R. 2039) except for a provision in the Ohio bills that specifies that the report “identify the steps the Administrator will take to preserve taxpayer investment to date in Advanced Stirling Convertor technology.” The Senate has yet to begin consideration of a NASA authorization bill.

Originally published at FYI: The AIP Bulletin of Science Policy News.