At a hearing last week, members of the House Science Committee heard testimony about the status of domestic plutonium-238 supplies, an essential fuel for many robotic science missions. A Government Accountability Office report released at the same time recommends that the Department of Energy improve its long-term planning for producing Pu-238 and associated power systems.
The Space Subcommittee of the House Science Committee held a hearing on Oct. 4 to receive updates on the reconstitution of domestic plutonium-238 (Pu-238) production. The Department of Energy manufactures the radioisotope for NASA, which employs it as a source of heat and electricity on robotic science missions where other power sources cannot be used.
Congress has taken a significant interest in radioisotope power systems (RPSs), directing NASA and the White House to study the risks and requirements associated with them in the NASA authorization legislation that it passed in March. At last week’s hearing, Subcommittee Chair Brian Babin (R-TX) expressed continued interest in such questions as whether NASA’s estimates of its needs are accurate, whether a lack of Pu-238 supplies is restricting mission planning, whether the RPS designs currently in use are appropriate, and whether DOE’s work to ramp up Pu-238 production will meet NASA’s needs.
Some of these questions are also addressed in a new Government Accountability Office report, the release of which coincided with the hearing. While raising no alarms, GAO does suggest that there are gaps in DOE’s planning of its Pu-238 program and that coordination with NASA could be improved.
Participants probe limits of Pu-238 supplies
The U.S. originally began producing Pu-238 in the 1960s and ceased production in 1988 with the closure of DOE’s Savannah River Site. After that, the government relied on inventories and, until 2009, it also purchased supplies from Russia. In 2011, the U.S. began working to resume domestic production. DOE completed its first quantities of Pu-238 in late 2015 and has now produced 100 grams.
Currently, the total inventory of Pu-238 available to NASA is 35 kilograms, of which 17 kilograms meets specifications for use on space missions. The remainder would have to be blended with new supply to be rendered usable. For context, the recently completed Cassini mission to Saturn, which launched in 1997, used 33 kilograms. DOE estimates that it will be able to increase production to 1.5 kilograms per year by 2025.
Shelby Oakley, director for acquisition and sourcing management at GAO, testified that in the recent past radioisotope availability has narrowed the scope of NASA’s mission planning. She noted that the agency did not offer an RPS as an option on its third New Frontiers mission, which ultimately became the OSIRIS-Rex asteroid sample return mission. Ralph McNutt, a member of the Cassini team who has long experience in the RPS field, pointed out in his written statement that what is now called the Parker Solar Probe had been reformulated to eliminate its need for an RPS.
David Schurr, deputy director of NASA’s Planetary Science Division, said that budgetary restrictions rather than RPS availability is currently the limiting factor in planning future RPS-enabled missions. He reported that NASA plans to include an RPS aboard the Mars 2020 rover and that an RPS is included as an option for the New Frontiers mission to be selected in 2019 with a launch target of 2025. He also pointed out that solar power systems have now improved to the point that they can be used instead of RPSs on missions as far from the sun as Jupiter or even, “for fairly limited missions,” Saturn.
Schurr testified that current inventory will be sufficient to meet mission needs through 2025 and that blending it with new supplies would make it enough to meet all anticipated needs through 2030.
Committee members inquired whether NASA might need to stretch its supply and if it could obtain additional supplies from other sources. Asked by Babin whether Pu-238 might be used on crewed exploration missions, Schurr said it is not under active consideration and that using it for that purpose would require an increase in production.
Subcommittee Ranking Member Ami Bera (D-CA) and Rep. Dana Rohrabacher (R-CA) both asked whether Russia might supply Pu-238 again. Schurr and McNutt replied there are no negotiations under way. McNutt told Rohrabacher that, when the decision was made to discontinue purchases, the Russians suggested that “if we wanted to fund a plant in Russia that they would be interested in taking our money and producing plutonium for us. It was not going to be cheap.”
GAO recommends DOE shore up Pu-238 production plans
Oakley affirmed that DOE’s planned pace of Pu-238 production will meet NASA’s near-term needs. However, she said that the new GAO report “tries to convey the fact that if this new supply of Pu-238 isn’t established and the goals aren’t met by DOE, then [Pu-238 supply] could become a limiting factor again in the future.”
She said that DOE’s supply project had in the past been “managed in a very segmented, short-term approach” due to uncertainties in funding. This contributed to GAO’s recommendations that DOE “develop an implementation plan with milestones and interim steps” and better assess and track risks to production. She reported that the absence of such planning had made project timeframes and costs difficult to predict, and that, in addition to schedule slippage, the total cost of reconstituting the program had risen from an estimate of between $85 million and $125 million to “upwards of about $235 million.” She said it is still difficult to determine if current estimates represent “a realistic accounting of the risks.”
Tracey Bishop, the deputy assistant secretary for nuclear infrastructure programs at DOE, testified that the Pu-238 project had received increased appropriations for fiscal year 2017, allowing DOE “to further reduce risk and accelerate schedule.” She reported that Pu-238 production is proceeding apace and that newly produced fuel clads would be used on the Mars 2020 rover. She also reported that the department is continuing to develop methods of optimizing the production process.
Debate ongoing over radioisotope power system designs
A related problem discussed at the hearing is whether the RPS technology currently used for converting heat from Pu-238 into electricity is adequate. At the moment, NASA only employs Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs), but was also actively pursuing a technology called the Advanced Stirling Radioisotope Generator (ASRG) until the program was cancelled in 2013.
Schurr said that ASRGs would require only a quarter of the supply of Pu-238 as an MMRTG to produce the same amount of electricity. However, he explained that ASRGs employ moving parts, which substantially increases the risk of the generator failing during the mission. He said that, although the ASRG program has been cancelled, NASA is still considering the technology.
McNutt testified that there remains a great deal of debate in the RPS community over the practicality of ASRGs, and in his written testimony he called the program an “unfortunate debacle.” He also argued that current reliance on MMRTGs is the result of a search for a single technology for all uses. He counseled against such an approach, lamenting the loss of capability provided by the “long-lived, high efficiency” General Purpose Heat Source (GPHS) RTG used on Cassini and the New Horizons mission to Pluto and the Kuiper Belt. While the technology cannot be used in environments with an atmosphere, he suggested the technology “should be seriously reconsidered for reestablishment” as a backup for deep space missions.