What kinds of nuclear reactors are suitable for operations in space? (Beyond thermionic and RTG)

Question

If we want a nuclear power plant in space with a capacity similar to that of a conventional water boiling turbine nuclear power plant on Earth today, what kinds of fundamental design changes would have to be made? The largest reactor in operation delivers about 6-7 GW. How could that be replicated in space, either free floating or on a surface?

How do thermionic emission reactors, like the Russian TOPAZ, scale up to Gigawatts? I suppose that radio thermal generators are out of the question on that scale, if for no other reasons the cost of creating and handling plutonium-238. Is water boiling the big future for nuclear power also in space?

I want to emphasize operations in space, not manufacturing. Assume manufacturing on Earth and launched, at most piecemeal for simple docking assembly. I want to ask about what kind of BIG nuclear reactor actually would work in space, rather than how it would get there.

To clarify, I am not asking about nuclear thermal propulsion systems, but reactors optimized for the production of electric power.

Answer 1

The question is how to convert the heat from a fission reactor into electricity. Most systems that have been either built (Topaz) or proposed (SP-100) rely on thermionic conversion. This is the same approach used by RTGs. It's not very efficient, 5% to maybe 15% on a good day, but thermionic conversion is extremely reliable and requires no moving parts. The Voyager spacecraft are both still today living off of thermionic conversion, almost 40 years after launch. There is no problem with scaling up this conversion approach. For larger reactors, regardless of the conversion approach, you may need to use heat pipes to get the heat efficiently from the reactor to the converter.

If you're ok with moving parts (keep in mind that these things would usually be expected to work at least a decade or two with no maintenance), then Stirling engines are the conversion technology of choice. The fluid would be helium, always in the gas state. (Superheated water is a nasty fluid to use as it likes to destroy, over long periods of time, all materials it comes in contact with.) Efficiencies of 40% to maybe 50% can be achieved.

There was a program to develop an RTG replacement that used a Stirling converter for much more efficient use of the expensive and hard-to-get Plutonium-238. However that program (ASRG) was cancelled a few years ago due to cost overruns and not achieving the technical objectives. Still, with sufficient perseverance, a Stirling engine would be the most sensible conversion technology in the long run for space nuclear power.

The Brayton Cycle has also been proposed. Here is a good paper on the work on space Stirling and Brayton engines.

Answer 2

At this time thermoelectric and thermionic systems are the only types which have been used in space. However, there are a few other reactor types which have been proposed for use in orbit for the purpose of generating power.

Thermophotovoltaic systems would essentially use solar cells optimized for the thermal spectrum of the reactor rather than the sun. The radiation hardening of these systems is an obstacle, as is operating them at high temperatures.

The Rankine Cycle typically used by reactor power has very low temperature cold sinks, which is going to make radiating waste heat away very difficult thanks to the Stefan-Boltzmann law. However, NASA's project Prometheus proposal featured a fission generator which generated power via the Brayton Cycle. The Brayton Cycle can be used with no intermediate cooling stages or heat exchangers, which is very attractive for conserving mass in spacecraft. Using gaseous helium as the coolant is also a good idea for saving mass.

While highly speculative, fission fragment reactors may be attractive in the future due to their high efficiencies.

Fusion reactors at this point are a bit too speculative for me to comment on regarding practicality, though in principle aneutronic fusion could be a very attractive option due to the large amount of energy which can be extracted via direct energy conversion rather than thermodynamic processes.

Answer 3

Nuclear rocket engines preliminary designs are already there for space operations. There are mainly three types of nuclear fission reactors are proposed by engineers. They are 1. Solid core Nuclear thermal fission reactor engine 2. Liquid core Nuclear Thermal fission reactor engine 3. Gas core Nuclear Thermal fission reactor engine

Fusion reactor engines are also considered for designs. However these engines are still in design level. These reactor engines works as same as normal rocket engine where the propellant will absorb energy released from nuclear fusion or fission reactors. The main practical difficulties in these engines is mainly carrying out controlled fission or fusion reaction at the core of the engine. In future these kind of reactors can be developed.