3 ms·

I'm aware of that. But your commercial power plant example is also far from optimized for the requirements of a space power plant. ((edit): Your jet engine ana

I'm aware of that. But your commercial power plant example is also far from optimized for the requirements of a space power plant. ((edit): Your jet engine analogy is apt: a turbojet is a highly miniaturized version of the same type of turbine used in gas power plants. It'd be quite a mistake to look at a power plant turbine and analogize from that that jet airplanes are impossible, because, well, just look how huge those turbines are!)

ceejayoz 2y ago
An in-space power plant isn't the proposal, though. It would be on Mars, and have similar requirements for containment, cooling, maintenance, etc. We are unlikely to want to spew radioactive hydrogen all over the landscape.
- perihelions 2y ago
  It's a fascinating topic, isn't it? The hydrogen exhaust in nuclear thermal rockets isn't actually radioactive (well, in practice it was, because the rockets they actually built had a tendency to fall apart and eject bits of their nuclear fuel in the exhaust, but on paper that shouldn't happen). But the power-plant versions are closed gas loops anyway—you're driving the hydrogen or helium through a turbine, it's a sealed system. I don't think you'd choose to build any serious containment building on Mars. The baseline risk of death on Mars is extremely high; nuclear accident risks don't appreciably move the needle. My understanding is what you'd actually do is put a biological shield—maybe a pile of Martian rock—on the line-of-sight between the reactor and the astronaut base, and, simply, never walk that way. Cooling is substantially easier on Mars than in space, since there's an atmosphere that functions as a heat sink. The space version (i.e., in the nuclear electric propulsion context) has a more difficult challenge, with radiation as the sole heat dissipation mechanism. You're right that a space reactor would have to be designed for zero maintenance, else it'd be a non-starter.
PaulHoule 2y ago
In theory you can build a low capital cost and compact nuclear reactor that operates at high temperature with a closed-cycle gas turbine powerset https://ntrs.nasa.gov/api/citations/20140016755/downloads/20140016755.pdf https://ntrs.nasa.gov/api/citations/20140016755/downloads/20... It could be a molten salt reactor or a liquid metal fast breeder reactor or a high temperature gas cooled reactor (carbide fuel in prismatic or pebble form) It could be highly competitive with solar as a carbon free energy source for terrestrial use but it is not a bird in the hand. (I think the robots in Gundam use a power plant like that which is why Zakus blow up when you hit them) I imagine the reactor, powerset and all, would be packed up into one Starship load but would have some civil works (cooling system) assembled on site. In another comment I point out Casey is accounting for 1 Starship launch of powerplant for 1 Starship load of colonists so this could scale OK from that point of view. Presumably they use a closed fuel cycle which is more like https://www.youtube.com/watch?v=KEfhx5ovuYk https://www.youtube.com/watch?v=KEfhx5ovuYk than Sellafield.
- perihelions 2y ago
  - "but it is not a bird in the hand." Kind of amusing to read this in the context of a "city on Mars" discussion :) - "Presumably they use a closed fuel cycle which is more like" I doubt this. Nuclear fuel reprocessing is a very invasive thing—a difficult industrial process that's hard to get working even on Earth, let alone resource-constrained environments like a putative Mars settlement. I doubt metal-fuel reprocessing like what you linked with the EBR changes the equation much. Reliability and maintenance would be the top drivers here. You'd end up with something like the nuclear submarine solution: tiny, self-contained systems that (in the sub case) are just replaced once every 30 years for refueling. This drives you to choose 90% HEU as your fuel. Beyond what submarines have as their requirements, you're, in addition, critically constrained on mass. That, I think, strongly drives you to unmoderated fast reactors: small, dense cores with lightweight coolants like sodium. From memory, I believe all of the 30+ space reactors that were actually built were HEU fast reactors with Na or NaK coolant. (There's actually droplets of radioactive NaK in Earth orbit right now, according to Wikipedia, because one of them leaked). (This is not my field of expertise; I just spend a lot of time reading NTRS).
  - PaulHoule 2y ago
    It's hard to say. HEU cores can go to a high burnup in a fast reactor (and leave very little actinide behind) but the burnup is limited because U235 depletion will drop reactivity. If you are breeding new fuel that reduces the reactivity drop. Modern FBR cores can get maybe 30% burnup of fertile material without reprocessing. Rather than the low-latency reprocessing cycles that people thought about 1950-1970 martians might not think of reprocessing for 20 years or so. If they really need to ship 1000+ Starship sized reactors they are going to have a different attitude about reliability (e.g. a 5% failure rate that don't make a mess is no problem) I could be wrong but I think space colonists would be fanatical about a circular economy. If they had 1.5 million people on the ground they could not count on getting a new reactor for every citizen every 25 years. I've done some modelling of an asteroid colony where I'd expect people to not waste a wisp of carbon dioxide, for instance, martians on the other hand would have no problem venting it to the atmosphere because they will source it from the atmosphere.

ceejayoz 2y ago

An in-space power plant isn't the proposal, though. It would be on Mars, and have similar requirements for containment, cooling, maintenance, etc. We are unlikely to want to spew radioactive hydrogen all over the landscape.

perihelions 2y ago
It's a fascinating topic, isn't it? The hydrogen exhaust in nuclear thermal rockets isn't actually radioactive (well, in practice it was, because the rockets they actually built had a tendency to fall apart and eject bits of their nuclear fuel in the exhaust, but on paper that shouldn't happen). But the power-plant versions are closed gas loops anyway—you're driving the hydrogen or helium through a turbine, it's a sealed system. I don't think you'd choose to build any serious containment building on Mars. The baseline risk of death on Mars is extremely high; nuclear accident risks don't appreciably move the needle. My understanding is what you'd actually do is put a biological shield—maybe a pile of Martian rock—on the line-of-sight between the reactor and the astronaut base, and, simply, never walk that way. Cooling is substantially easier on Mars than in space, since there's an atmosphere that functions as a heat sink. The space version (i.e., in the nuclear electric propulsion context) has a more difficult challenge, with radiation as the sole heat dissipation mechanism. You're right that a space reactor would have to be designed for zero maintenance, else it'd be a non-starter.

PaulHoule 2y ago

In theory you can build a low capital cost and compact nuclear reactor that operates at high temperature with a closed-cycle gas turbine powerset https://ntrs.nasa.gov/api/citations/20140016755/downloads/20140016755.pdf https://ntrs.nasa.gov/api/citations/20140016755/downloads/20... It could be a molten salt reactor or a liquid metal fast breeder reactor or a high temperature gas cooled reactor (carbide fuel in prismatic or pebble form) It could be highly competitive with solar as a carbon free energy source for terrestrial use but it is not a bird in the hand. (I think the robots in Gundam use a power plant like that which is why Zakus blow up when you hit them) I imagine the reactor, powerset and all, would be packed up into one Starship load but would have some civil works (cooling system) assembled on site. In another comment I point out Casey is accounting for 1 Starship launch of powerplant for 1 Starship load of colonists so this could scale OK from that point of view. Presumably they use a closed fuel cycle which is more like https://www.youtube.com/watch?v=KEfhx5ovuYk https://www.youtube.com/watch?v=KEfhx5ovuYk than Sellafield.

perihelions 2y ago
- "but it is not a bird in the hand." Kind of amusing to read this in the context of a "city on Mars" discussion :) - "Presumably they use a closed fuel cycle which is more like" I doubt this. Nuclear fuel reprocessing is a very invasive thing—a difficult industrial process that's hard to get working even on Earth, let alone resource-constrained environments like a putative Mars settlement. I doubt metal-fuel reprocessing like what you linked with the EBR changes the equation much. Reliability and maintenance would be the top drivers here. You'd end up with something like the nuclear submarine solution: tiny, self-contained systems that (in the sub case) are just replaced once every 30 years for refueling. This drives you to choose 90% HEU as your fuel. Beyond what submarines have as their requirements, you're, in addition, critically constrained on mass. That, I think, strongly drives you to unmoderated fast reactors: small, dense cores with lightweight coolants like sodium. From memory, I believe all of the 30+ space reactors that were actually built were HEU fast reactors with Na or NaK coolant. (There's actually droplets of radioactive NaK in Earth orbit right now, according to Wikipedia, because one of them leaked). (This is not my field of expertise; I just spend a lot of time reading NTRS).
- PaulHoule 2y ago
  It's hard to say. HEU cores can go to a high burnup in a fast reactor (and leave very little actinide behind) but the burnup is limited because U235 depletion will drop reactivity. If you are breeding new fuel that reduces the reactivity drop. Modern FBR cores can get maybe 30% burnup of fertile material without reprocessing. Rather than the low-latency reprocessing cycles that people thought about 1950-1970 martians might not think of reprocessing for 20 years or so. If they really need to ship 1000+ Starship sized reactors they are going to have a different attitude about reliability (e.g. a 5% failure rate that don't make a mess is no problem) I could be wrong but I think space colonists would be fanatical about a circular economy. If they had 1.5 million people on the ground they could not count on getting a new reactor for every citizen every 25 years. I've done some modelling of an asteroid colony where I'd expect people to not waste a wisp of carbon dioxide, for instance, martians on the other hand would have no problem venting it to the atmosphere because they will source it from the atmosphere.