There may be enough plutonium-238 to meet NASA’s planetary mission requirements through 2030, albeit with little to spare, according to a report prepared for the agency that was released 4 June.
A NASA nuclear power assessment study released on 4 June identified a need for 38 kg of 238Pu for the robotic missions identified in the 2011 planetary science decadal survey. The study said that once the existing inventory of 35 kg of 238Pu is consumed, the Department of Energy will supply an average of 1.5 kg annually to NASA. That amount “may fulfill [science mission directorate] needs, but with little margin,” the report stated. DOE, which last produced the material in 1998, resumed operations in 2013.
For many planetary missions, sunlight is too faint for solar arrays to meet the spacecrafts’ power needs. The radioactive decay of 238Pu supplies heat for radioisotope thermal systems, which provide electric power. The isotope has a half-life of 87.7 years. The study included nine missions identified in the decadal survey that require nuclear power. Of those, the Jupiter Europa Orbiter requires the most 238Pu: 17.6 kg.
On 3 June, NASA issued a separate request for information from suppliers of Stirling cycle engines, with the idea that these could be used for planetary missions and, potentially, for human exploration missions. Stirling technology would convert the decay heat more efficiently than radioisotope power systems (RPS), which produce electricity using bi-metallic thermocouples, and could dramatically reduce the amount of 238Pu required to power planetary spacecraft.
A total of nine NASA planetary and Apollo missions, including the Pioneer and Voyager spacecraft, have been powered by RPS. Most recently, an RPS is providing power for the rover Curiosity during the Martian nighttime. In addition, 238Pu-fueled radioisotope heater units have been used on 10 spacecraft to warm instruments, structures, and other onboard systems.
The space agency’s Human Exploration and Operations Directorate continues to assess fission power systems (FPS) that would be fueled with highly enriched uranium (HEU). DOE intends to “comp” NASA 20 tons of HEU for space applications and for research and medical isotope production reactors. But this no-cost availability is offset by the more intensive security requirements required for bomb-grade uranium relative to guarding 238Pu, which is not suitable for weapons, the report said.
Recent NASA studies determined that a Mars mission would require 35–40 kW of electrical power for crew habitat and for the in situ production of propellant for a return voyage to Earth. Those power levels are well beyond the capacity of RPS. But one or more fission sources producing tens of kilowatts each could potentially suffice, the report said. Both FPS and RPS share the same technology for thermal conversion. Decisions on a power source for a Mars mission are not expected before 2019, the report said.
Despite billions of dollars spent on developing FPS, only one fission-powered mission has ever flown: SNAPSHOT, the power source of which failed 45 days after its 1965 launch. It remains in Earth orbit to this day.
The study, prepared by the Applied Physics Laboratory at Johns Hopkins University, noted that for spacecraft requiring less than 1 kW of electrical power, 238Pu-based RPS systems will remain the preferred technical choice. No currently scheduled planetary missions need that much power, and FPSs are too massive and require too much shielding for all but very large missions.
The highest power use of RPS was on Cassini, which used 23.8 kg of 238Pu in its three generators to produce 889 W of electrical power at the time of its launch in 1997.
Stirling engine-based generating systems have achieved 40% efficiency in the laboratory—compared to the less than 10% conversion of RPS—and could greatly reduce 238Pu requirements for future missions, the report said. NASA cancelled a program in 2013 to develop a Stirling generator for upcoming planetary missions, citing budgetary constraints. But NASA’s Glenn Research Center on 5 June issued a request for information on 100–500 W Stirling devices. The solicitation asks for “robust and reliable Stirling convertor designs that deliver high efficiency, low mass, simple operation, and long life (without maintenance) for deep-space missions lasting 10 years or more.” The agency has no current plans to solicit bids for building or supplying Stirling devices, the document noted.
The assessment report estimates that developing and flying a 1-kW Stirling convertor is roughly $550 million.
Around 300 kg of 238Pu has been produced by DOE over the past 50 years, the report said. DOE has enough feed material (neptunium-237 and potentially americium-241) extracted from spent reactor fuel to provide four times the 140 kg that NASA has consumed over the past half-century. US-made 238Pu has been produced from the irradiation of 237Np targets, at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory and at the Advanced Test Reactor at Idaho National Laboratory.
Stepping up output of 238Pu to the 5 kg annual maximum established in a DOE environmental impact statement would likely require starting up an unused hot cell at the HFIR, at a cost of $100 million. But other changes, such as improvements in processing, target redesign, equipment upgrades, and use of the additional target locations in the Idaho reactor may suffice for smaller production increases.
The study did not include consideration of nuclear thermal propulsion, which the report said would require an order of magnitude greater power and produce temperatures three times higher than required for power systems.
