2 ms·

- "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

- "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.

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.