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| Illustration of a conceptual spacecraft enabled by nuclear thermal propulsion (Credits: NASA) |
This week I attended the Nuclear and Emerging Technologies for Space (NETS) conference. It was a bit of nostalgia for me since I worked briefly on the NASA-led SP-100 program in the mid-1980s. The SP-100 was meant to demonstrate a nuclear fission reactor that could provide 100 kW of electrical power to a spacecraft. It was cancelled before flight hardware was built, in part due to public resistance about launching a nuclear reactor.
Later on at NASA I also worked on the solar dynamic Brayton power system originally planned for the space station; and Stirling microsystems development for multiple applications. Both technologies convert heat to electricity and can use radioisotope or fusion reactor heat sources. Later still, I briefly supported the Jupiter Icy Moon Orbiter program that was slated to use a small nuclear fission reactor for propulsion and power.
None of these systems have flown on any spacecraft. But there is reinvigorated interest in nuclear fission based power and propulsion to expand the capabilities and performance of scientific and human exploration space missions.
So why write about all this in a blog devoted to hydrogen? Because nuclear fission reactors have significant potential implications for hydrogen systems in space as well as on the ground in the energy sector. First, a very quick summary of the primary types of space-based nuclear power and propulsion technologies.
RTGs, NTPs, NEPs, and Fission Surface Power for Space Missions
Radioisotope thermoelectric generators (RTGs) convert the thermal energy from a decaying radioisotope source to electrical energy using an array of thermocouple junctions. There is no nuclear fission involved. RTGs have been used on more than two dozen spacecraft over the past 60 years and enable very long mission durations far from the sun.
The thermocouple junctions in an RTG can be replaced by a Stirling engine for much higher efficiency and higher associated electrical power output. Heat drives the Stirling cycle via a working gas (e.g. helium) resulting in pressure-volume mechanical power that is converted to electricity. Stirling technology is at a high readiness level and will likely start replacing RTGs on some future missions.
Nuclear thermal propulsion (NTP) uses the fission process to heat up a propellant - generally hydrogen - to a very high temperature and then accelerate it through a nozzle to create thrust. Nuclear fission replaces the heat of combustion in traditional chemical propulsion such as rocket stages using hydrogen and oxygen. For missions beyond earth orbit, NTP enables much faster transit times.
Nuclear electric propulsion (NEP) utilizes the fission process to generate electrical energy which then drives an electric propulsion system (e.g. arcjet or ion thruster). Depending on the propulsion system used, the thrust is produced by accelerating hydrogen, xenon, argon, or another fluid. Some of the electrical energy generated also provides primary power to the spacecraft. NEP provides much greater power capacity and higher propulsion efficiency for spacecraft.
Finally, small modular nuclear fission reactors on the moon, Mars, or other celestial bodies can enable extended power intensive surface operations. In addition to mobility and life support functions, these reactors would enable full scale in situ resource utilization such as the production of hydrogen and oxygen from harvested ice.
NIMBY Everywhere
A nontechnical barrier that all nuclear fission systems face, whether in-space or on the ground, is sometimes termed "not in my backyard" (NIMBY). NIMBY comes into play on many topics that impact the public, from landfills to prisons. In the case of nuclear reactors, the relevant "backyard" can be a very large area indeed based on air or sea circulation patterns.
The general public wariness of anything related to nuclear power is not unwarranted. For every well known nuclear related disaster - Three Mile Island, Chernobyl, Fukushima - there are many more "near misses". These almost accidents are caused by operator error, various system failures, fraudulent inspection or maintenance records, and many other factors.
In addition to the public risk perception related to operating nuclear systems, there is also the issue of nuclear waste disposal. NIMBY comes into play even more strongly on this issue. As a result, public acceptance of nuclear fission for any application - space, power plants, or otherwise - is a challenging consideration.
Nuclear Power for Hydrogen Production
Existing nuclear power plants, and the utilities that own them, have a rather intractable economic problem. These plants are no longer cost competitive compared to other power generation options. And in the U.S., for example, many state and regional public utility commissions are balking at passing these higher costs on to the consumer.
But nuclear reactor cores cannot simply be shut down like a fossil fuel power plant. The half-life of the radioactive fuel used in them is unaffected by the whims of political policy or utility economics. Decommissioning a nuclear power plant is a complex and expensive undertaking that requires ongoing site control and monitoring for a very long time.
One potential option is to repurpose some of the existing global nuclear power plant capacity to produce hydrogen via water electrolysis. This approach would contribute to the reduction of greenhouse gases as the generated hydrogen displaces legacy fossil fuels. It also redeploys nuclear power assets that are no longer economically competitive for grid power generation.
Whether these advantages offset the previously mentioned public safety and nuclear waste disposal concerns is a matter of considerable debate. It will require circumspect policy formulation that ideally engages the public at large. And the path forward will undoubtedly vary among nations based on regional energy policy, resources, existing infrastructure, and many other parameters.
