Op-ed | An argument for space fission reactors

America’s space program has long held a special place in the public’s imagination, but NASA missions are limited by budget constraints. NASA must use its funding wisely to implement balanced, cost-efficient programs to develop enabling technologies, such as technologies to power future NASA missions.
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            This op-ed originally appeared in the Dec. 12, 2017 of SpaceNews magazine.
America’s space program has long held a special place in the public’s imagination, but NASA missions are limited by budget constraints. NASA must use its funding wisely to implement balanced, cost-efficient programs to develop enabling technologies, such as technologies to power future NASA missions. Speaking as the former project manager of three successful missions — Voyager, Galileo, and Cassini — and the canceled Prometheus-Icy Moons Orbiter, I have a unique perspective to share.
Space nuclear power has long been recognized as an enabling technology for space science and exploration. Many of NASA’s greatest achievements have relied on radioisotope nuclear power: Viking, Voyager, Galileo, Cassini, New Horizons, Curiosity. However, in 1988, the United States stopped making the plutonium-238 we need as a heat source in radioisotope thermoelectric generators (RTGs). With existing supplies likely to run out soon, NASA is planning to restart Pu-238 production with a sizable investment (about $100 million per year) that is expected to yield hundreds of grams over the next several years. Unfortunately, that is a very small amount compared to 24 kilograms of Pu-238 flown on Voyager or the 33 kilograms on Cassini.
While NASA’s goal is a production rate of 1.5 kilograms per year, there are several major obstacles — both technical and political — that will not be easy to overcome, even with additional investment in Pu-238 production infrastructure. This, in turn, makes NASA reluctant to consider future Voyager- and Cassini-like missions.
Whatever production rate is achieved, every gram of Pu-238 will have significant value, but even at the most optimistic production levels NASA will not have enough Pu-238 to pursue future outer planet flagship-class missions such as exploring the surface of Titan or melting through the surface ice on Europa and powering a submarine in the ocean below. Clearly, it is risky for NASA to bank solely on Pu-238-supported space nuclear power without a backup.
Beyond RTGs
NASA does not need to be content with simply trying to maintain radioisotope power capability, or to settle for less power for deep space exploration than was available for past missions. NASA can develop a balanced power portfolio that includes space fission power, which offers essentially unlimited power for space exploration.
While space fission won’t compete on a mass basis with radioisotope systems at power levels below 500 watts (the break-even point might be between 500 and 1,500 watts), it will compete on a recurring cost-per-kilowatt basis at even lower power levels.
A small fission system would not only allow NASA to reconsider several-hundred-watt Cassini-like missions, but would also finally enable science missions that need kilowatts of power. A 10-kilowatt-electric (kWe) reactor could enable ambitious nuclear electric propulsion missions to orbit and explore the outermost planets and Kuiper belt objects.
Historically, the biggest obstacle to space fission power has been the perceived development cost, largely because development of advanced nuclear technology and new test facilities can be very expensive and take many years. NASA’s Kilopower project addresses these concerns by showing that a simple space reactor can be developed for a few hundred million dollars and that facilities already exist to support the early development. The key difference between other programs (including the ill-fated Prometheus) and Kilopower is that Kilopower is demonstrating low cost by live testing of actual fission reactors — something that hasn’t been done in the U.S. since the 1960s. Critics had said it was impossible to perform an affordable, simple nuclear-powered test in today’s regulatory environment — but the Demonstration Using Flattop Fission experiment, conducted by Los Alamos National Laboratory in partnership with NASA in 2012, showed that it is possible.
Get KRUSTY
The Kilowatt Reactor Using Stirling Technology (KRUSTY) experiment, scheduled for completion in early 2018, will show that a flight-like space reactor can be designed, fabricated, and tested for only a few tens of millions of dollars. The remaining work to bring the entire system to flight status is significant but mostly nonnuclear (i.e. the remaining technical challenges exist in the power conversion system and spacecraft integration; an actual flight reactor could look and operate essentially the same as KRUSTY).
If KRUSTY is successful, the cost to bring a 1-kWe reactor to flight status should be relatively low: a few hundred million dollars, or well within the budget NASA is willing to spend on a limited amount of radioisotope power.
This is not to say that radioisotope power should be abandoned. It will always be essential for space exploration, and a significant investment in Pu-238 production is well warranted. If space reactors were used for missions that require significantly more than a hundred watts, a kilogram per year of Pu-238 would be sufficient for the smaller missions that benefit the most from the mass and packaging advantages of small RTGs as well as for spacecraft that require Pu-238 for thermal management.
Fission technology can also improve propulsion systems. Second-generation fission power systems could enable ambitious exploration of the outer solar system with 10- to 100-kW nuclear electric propulsion systems. Subsequent generations could then focus on propulsion for human spaceflight missions, where fission might have the greatest impact.
NASA is making a substantial investment in nuclear thermal propulsion, which offers up to double the capability of chemical rockets and which would greatly enhance human exploration. While NASA should be lauded for its bold vision, the leaps required in nuclear development and testing are reminiscent of previously failed programs that took too large a step. Even if ultimately successful, nuclear thermal propulsion benefits may not justify the price tag (tens of billions of dollars), especially if there is continued progress towards lower cost, higher capability launch vehicles.
Multi-megawatt nuclear electric propulsion systems can provide an order of magnitude improvement over chemical systems, thus providing far more potential than nuclear thermal propulsion. Multi-megawatt nuclear electric propulsion systems also have the advantage that the reactor technology can be developed via several incremental, achievable steps, of which Kilopower is the first. The balance of nuclear electric propulsion system development provides additional major challenges — but they are nonnuclear, can also be completed in steps, and build upon current solar electric propulsion efforts.
The primary reason to invest in space fission power is the unique capability it would bring to NASA, with benefits well beyond the deep space missions it will enable. NASA will need space reactors to establish a robust power environment on the moon and Mars, where solar power is limited by day and night cycles, latitude, seasons, dust storms, buildup, degradation, etc. Ultimately, incremental advances could lead to higher power, lower specific-mass nuclear electric propulsion reactors that would revolutionize human travel throughout the solar system.
If the U.S. is serious about continuing to be a leader in space exploration, NASA must rebalance its power portfolio by adequately funding a simple system that establishes fission power in space. This simple system will allow the continuation of Voyager- and Cassini-class missions at the outermost planets and open up subsurface missions at Europa, Enceladus, and Titan. NASA can then use the expertise, technology, and infrastructure outflow from that development to achieve more ambitious capabilities, such as higher power surface reactors for human missions and, eventually, nuclear thermal propulsion and/or nuclear electric propulsion space reactor programs.
Americans have not lost enthusiasm for exploring the solar system and beyond. Now is the time to create the tools that will power that dream.
John Casani is a retired Jet Propulsion Laboratory employee who served as project manager for the Voyager, Galileo and Cassini missions, all of which used RTG power systems.
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