6 ms·

For the USN (and other military's) ones at least there are many other differing issues and design goals that are divergent enough from civilian usage that it's

by xoa 5y ago

For the USN (and other military's) ones at least there are many other differing issues and design goals that are divergent enough from civilian usage that it's definitely more then just cost. For example on the most basic issue of fuel, the A4W reactors (found on current USN super carriers) run on highly enriched U235, designed around 93% and as high as 97%. In other words, the fuel is flat out weapons grade by itself. A typical civilian plant is more like 3-5%. Very high enrichment allows more density and very long times between refueling, they can go something like 10 years vs 1-2 years for civilian. But obviously that would be a big proliferation concern even ignoring cost, nor are there many places to get that kind of fuel. For the military that is irrelevant, it's not a limiting problem since the vessels these reactors live on tend to also carry literal nuclear weapons. But it means it won't just transfer to other usage. There are plenty of other differences in naval reactors, like they also formulate with high burn up fuels (metal ceramics, u-al/u-zr) rather then uranium dioxide. Military vessels also have ample trained personnel to throw at maintenance and operations, they can make tradeoffs for things that are more finicky but provide higher performance. They're anything but "self-contained" really, and even the Navy wanted to simplify that. A major goal for the new A1B reactor in the Gerald R. Ford-class supercarriers, as well as normal stuff like "more power, weighs less" was to cut the number of people needed to run the reactors and propulsion. I mean, yeah, all this certainly does add to the cost too. But it's not just about the cost, or rather the design goals and missions are divergent enough that they necessitate costs for military reactors that would be a waste, dangerous, or both elsewhere. Where the Naval reactors might well carry over to I think would be future space usage, a lot of what the navy is worried about with sending a reactor out on a carrier or sub for years seems to overlap with challenges and goals faced by a reactor on a spaceship sent to the outer solar system.

nickelpro 5y ago
Can't put a traditional naval reactor or anything that resembles one on a spacecraft. 1) If it explodes on takeoff you're fucked 2) Naval reactors (and all reactors) require massive heatsinks to dissipate entropy. There's no ocean in space. Space-based nuclear energy is all based around RTGs, reactors have no place in space
- discordance 5y ago
  Except for the sun
- marcinzm 5y ago
  The soviets launched a bunch of reactors (not RTGs) into space. https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor https://en.wikipedia.org/wiki/BES-5 https://en.wikipedia.org/wiki/BES-5
- acidburnNSA 5y ago
  Almost all space reactor concepts have fresh fuel at takeoff and then start-up once they're launched. You can hold fresh nuclear fuel in your hand with very little hazard. It's only once you start splitting atoms that the radiation levels get high. So you launch the fresh and mostly inert reactor to avoid this risk. RTGs, on the other hand, are radioactive from the get-go, but are usually quite small. The USA did run a reactor in space (SNAP-10A) and the Soviets did a few dozen.
  - nickelpro 5y ago
    SNAP was a scaled up RTG, as are the soviet examples. You can call these reactors if you want but then we're just playing a semantics game. I tried to head this off by saying "traditional naval reactor or anything that resembles one". Also the problem isn't running the reactor on the launch pad, the problem is if the launch vessel explodes and the fuel load gets spread out over your launch area. The enriched uranium used in naval-style propulsion is absolutely not "safe to hold in your hand" and the weight requirements for using natural uranium, which is safe, would be prohibitive for use in space.
    - cameldrv 5y ago
      The odd numbered SNAPs were RTGs. The Even numbered ones were reactors. SNAP-10A was the only reactor from that program to fly.
    - acidburnNSA 5y ago
wpietri 5y ago
This is the kind of comment that keeps me coming back to HN. Thoughtful, nuanced, and full of information from a field I know very little about. Thanks!
- anon_tor_12345 5y ago
  On the other hand this is the kind of comment that keeps me away from HN. Full of self-congratulatory fawning. Thanks!
jabl 5y ago
Except for the USN and RN (which uses US-derived designs), other navies using nuclear propulsion don't use such highly enriched fuel. Russia AFAIK uses somewhere around 30-40%, which is still classified as HEU, so problematic for civilian use, but most likely not directly usable in a bomb. France runs on 7% enriched UO2 (although using plate-based 'caramel' fuel rather than cylindrical pellets in rods like typical civilian nuclear fuel). This requires them to refuel every 10 years rather than having life-of-ship reactors like the latest generation US submarine reactors, but OTOH French law requires reactors to be defueled and inspected every 10 years anyway. As for space usage, launch weight restrictions make LWR style reactors impractical. Look at something like the NASA Kilopower as an example of what a (very small) space-based power reactor might look like. For nuclear propulsion like a nuclear thermal rocket, that's again a different kind of reactor pretty different from both LWR's and Kilopower.
- xoa 5y ago
  >As for space usage, launch weight restrictions make LWR style reactors impractical. OK, so this is a reply to both you and @nickelpro (your comment is newer but also higher), who wrote: >Space-based nuclear energy is all based around RTGs, reactors have no place in space You both seem to have an image in your heads regarding future long duration deep space vehicles (I explicitly mentioned "outer solar system") here that is a mixture of old space assembly and soft science-fiction, wherein industrial capacity is all terrestrial and any ship is built entirely on Earth, launched and off it goes. All-in-one. Even SpaceX with its use of pure chemical rockets to Mars and terrestrial construction plans to break with that: in-orbit refueling is an absolutely key part. And for going farther then that (and as Starship and successors/competitors kick starts a new era of space economics and industry) the clear and necessary next step will be in-space assembly (be it in LEO or a Lagrange point dock or whatever ends up being most practical at a given time). In the same way we don't expect our ships to somehow be built hundreds of miles inland and then make their way to the ocean or fit entirely on a single semitruck, stay indefinitely on what can be launched out of Earth's atmosphere makes no sense either. The important aspects are all at cross purposes. Aerodynamic considerations are a waste in vacuum and constrict design in very important ways. Engines to get out of a strong gravity well need high thrust, whereas for long distances in space one really wants very high ISP. A torch drive that can do both necessarily bears a striking resemblance to a high energy weapon system to whatever happens to be facing the business end of it, and all known practical models (nuclear salt water, thermonuclear pulse) are ludicrously polluting. And outer solar system ships will need strong variable electric sources with high power/mass too despite solar being entirely impractical. RTGs won't cut it. So sure I don't think we'll ever see one launch off Earth's surface (I hope not anyway, if humanity is willing to light one of those off here it means we're facing a threat big enough that trashing our home is considered worth it). But that's a-ok, because what we'll do is built empty reactors, or reactor components, and launch those separately from fuel, and put it all together in space. Or for that matter far enough down the road maybe we build that stuff on the moon or in the asteroids or who knows. It obviously wouldn't be a copy/paste, but to the extent that USN reactor designs will get used outside of the military that's where I see it making sense.

by xoa 5y ago

nickelpro 5y ago

Can't put a traditional naval reactor or anything that resembles one on a spacecraft. 1) If it explodes on takeoff you're fucked 2) Naval reactors (and all reactors) require massive heatsinks to dissipate entropy. There's no ocean in space. Space-based nuclear energy is all based around RTGs, reactors have no place in space

discordance 5y ago
Except for the sun
marcinzm 5y ago
The soviets launched a bunch of reactors (not RTGs) into space. https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor https://en.wikipedia.org/wiki/BES-5 https://en.wikipedia.org/wiki/BES-5
acidburnNSA 5y ago
Almost all space reactor concepts have fresh fuel at takeoff and then start-up once they're launched. You can hold fresh nuclear fuel in your hand with very little hazard. It's only once you start splitting atoms that the radiation levels get high. So you launch the fresh and mostly inert reactor to avoid this risk. RTGs, on the other hand, are radioactive from the get-go, but are usually quite small. The USA did run a reactor in space (SNAP-10A) and the Soviets did a few dozen.
- nickelpro 5y ago
  SNAP was a scaled up RTG, as are the soviet examples. You can call these reactors if you want but then we're just playing a semantics game. I tried to head this off by saying "traditional naval reactor or anything that resembles one". Also the problem isn't running the reactor on the launch pad, the problem is if the launch vessel explodes and the fuel load gets spread out over your launch area. The enriched uranium used in naval-style propulsion is absolutely not "safe to hold in your hand" and the weight requirements for using natural uranium, which is safe, would be prohibitive for use in space.
  - cameldrv 5y ago
    The odd numbered SNAPs were RTGs. The Even numbered ones were reactors. SNAP-10A was the only reactor from that program to fly.
  - acidburnNSA 5y ago

wpietri 5y ago

This is the kind of comment that keeps me coming back to HN. Thoughtful, nuanced, and full of information from a field I know very little about. Thanks!

anon_tor_12345 5y ago
On the other hand this is the kind of comment that keeps me away from HN. Full of self-congratulatory fawning. Thanks!

jabl 5y ago

Except for the USN and RN (which uses US-derived designs), other navies using nuclear propulsion don't use such highly enriched fuel. Russia AFAIK uses somewhere around 30-40%, which is still classified as HEU, so problematic for civilian use, but most likely not directly usable in a bomb. France runs on 7% enriched UO2 (although using plate-based 'caramel' fuel rather than cylindrical pellets in rods like typical civilian nuclear fuel). This requires them to refuel every 10 years rather than having life-of-ship reactors like the latest generation US submarine reactors, but OTOH French law requires reactors to be defueled and inspected every 10 years anyway. As for space usage, launch weight restrictions make LWR style reactors impractical. Look at something like the NASA Kilopower as an example of what a (very small) space-based power reactor might look like. For nuclear propulsion like a nuclear thermal rocket, that's again a different kind of reactor pretty different from both LWR's and Kilopower.

xoa 5y ago
>As for space usage, launch weight restrictions make LWR style reactors impractical. OK, so this is a reply to both you and @nickelpro (your comment is newer but also higher), who wrote: >Space-based nuclear energy is all based around RTGs, reactors have no place in space You both seem to have an image in your heads regarding future long duration deep space vehicles (I explicitly mentioned "outer solar system") here that is a mixture of old space assembly and soft science-fiction, wherein industrial capacity is all terrestrial and any ship is built entirely on Earth, launched and off it goes. All-in-one. Even SpaceX with its use of pure chemical rockets to Mars and terrestrial construction plans to break with that: in-orbit refueling is an absolutely key part. And for going farther then that (and as Starship and successors/competitors kick starts a new era of space economics and industry) the clear and necessary next step will be in-space assembly (be it in LEO or a Lagrange point dock or whatever ends up being most practical at a given time). In the same way we don't expect our ships to somehow be built hundreds of miles inland and then make their way to the ocean or fit entirely on a single semitruck, stay indefinitely on what can be launched out of Earth's atmosphere makes no sense either. The important aspects are all at cross purposes. Aerodynamic considerations are a waste in vacuum and constrict design in very important ways. Engines to get out of a strong gravity well need high thrust, whereas for long distances in space one really wants very high ISP. A torch drive that can do both necessarily bears a striking resemblance to a high energy weapon system to whatever happens to be facing the business end of it, and all known practical models (nuclear salt water, thermonuclear pulse) are ludicrously polluting. And outer solar system ships will need strong variable electric sources with high power/mass too despite solar being entirely impractical. RTGs won't cut it. So sure I don't think we'll ever see one launch off Earth's surface (I hope not anyway, if humanity is willing to light one of those off here it means we're facing a threat big enough that trashing our home is considered worth it). But that's a-ok, because what we'll do is built empty reactors, or reactor components, and launch those separately from fuel, and put it all together in space. Or for that matter far enough down the road maybe we build that stuff on the moon or in the asteroids or who knows. It obviously wouldn't be a copy/paste, but to the extent that USN reactor designs will get used outside of the military that's where I see it making sense.