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.
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
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.
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.
[1] https://en.wikipedia.org/wiki/SNAP-10A
Highly enriched uranium is still barely radioactive, very similar to natural uranium. The half-life of U-235 is 703 million years. As half-life approaches infinity, atoms approach stability. The dose rate of holding navy nuclear fuel is modest compared to the hazards of fission products.
The on-contact for a HEU billet is over 10mrem/hour. You and I have very different ideas about nuclear safety apparently and I presume you haven't worked professionally in the field with that attitude.
According to that Wikipedia article, SNAP 10A was centered on a device that created and maintained a controlled, sustained nuclear fission reaction. I would call that a "nuclear reactor" even if no attempt were made to harvest the power. I think that the rest of the world is probably with me on this. The Chicago Pile 1 is widely regarded as the first nuclear reactor, and nobody particularly cares how it generated electricity. What they care about is that it demonstrated a controlled, sustained nuclear fission reaction.
You are getting the absolute basics wrong, so you have no standing to question the OP's atittude.
Nuclear reactor runs a nuclear chain reaction, hence the name, RTG does not. There is no scope for debate here. The difference is night and day and is obvious if you look at fuel, power to weight ratio, or do physics 101.
RTGs run on decay heat and use plutonium 238, they cant be turned off, their power slowly drops off over decades. Reactors use U235, have active control and starting/stop procedure and 10-100x higher power to weight
That said, it seems like, all by itself, conversion efficiency is a tricky measure of the usefulness of a design for spaceflight purposes. Wouldn't it be more useful to consider the total cost to deliver a given energy production capacity to space? In that case, rocket fuel itself, and the tyranny of the rocket equation, becomes a major consideration. If an efficiency gain comes at the cost of increasing the weight of the energy generation system in some way, then perhaps it doesn't end up being a net win over the less efficient design.
By comparison the voyager probe RTG used ~1/10th the fuel for a little over 1/10th the power. https://en.wikipedia.org/wiki/MHW-RTG So the only advantage was cheaper fuel.
The same reactor couod be hooked up to a 20% efficient stirling engine to keep it low maintenance, to a >50% efficient convined cycle or have 0% electricity output and be used for heat or water desalination.
By comparison SL-1 a 400kw thermal design so 4x power used a 12,000 kg pressure vessel including shielding. https://en.wikipedia.org/wiki/SL-1
It is true that RTGs are the only type of nuclear power used in space now, but that is more to do with type of craft we send to space rather than practical limitation of nuclear power in space.
A large spacecraft will have enough surface area mount enough radiative heatsinks to dissipate the heat from a nuclear reactor. Designs exist that have the math worked out for this since the 50s.
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.
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.
Also high thrust engines if you can get them can make use of the Oberhausen effect & some maneuvers, like specific orbit captures or crewed radiation belt transits need them as well.
Still no problem to build that thing in space if you can pull it off. :)
Are you saying you don't think a nuclear reactor powered spacecraft will ever be launched into space on a chemical rocket in the forseeable future, or that you don't think nuclear powered rockets will ever be used to get from ground to orbit?
You're probably aware, but nuclear power in space has not been limited to plutonium powered RTGs that output a few hundred watts.
https://en.wikipedia.org/wiki/US-A
https://en.wikipedia.org/wiki/BES-5
https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor
One has to design the whole reactor on paper before building it and get it approved by the Nuclear Regulatory Commission (NRC). If, when you build it, you find you need to make changes to the design that are above a certain threshold, you have to recertify (not sure of the details. Can't find a good link about this). Imagine trying to build something as complicated as a nuclear reactor and you can't make iterative improvements. NuScales design approval process, the approval to be able to build the first reactor, cost $500 million dollars, took 2 million man hours, included over 2 million pages of documents, and after submitting in Jan of 2017 did not get approval until around 4 years later in August of 2020[1]. This is just to be able to build the first design. No wonder nuclear power has seen no progress in the last 50 years.
Fortunately this company did persevere and now is planning to build the first power plant in Utah, hoping to be operational in 2030. The people/governments stating that climate change is a crisis, and I do believe there is way too much CO2 in the atmosphere, should be fast tracking this approved tech with as much money as usable to build thousands of these reactors as quickly as possible.
This isn’t a defense of the current regulatory process. The time frames, in particular, sound pretty egregious (4 years to approve a prototype, and 2m pages of documentation feels like a lot, too, but again I don’t know how these things are designed). But I would be extremely uncomfortable with lax regulatory oversight, given that nuclear accidents have permanent, irreversible impacts on society and geography.
Now, small-scale reactors may be a different beast entirely. If the quantity of materials is pretty much guaranteed not to have potential to cause problems for anyone but the operators for a short period of time…. Then there certainly seems to be a case for a shorter regulatory cycle. But I would be shocked if scale isn’t already taken into account for the current regulatory burden.
[1] https://www.nwfc.gov.au/observations-and-recommendations/cha...
Actually building the power plant with the reactor at a specific location is a different problem.
“SMR - Small Module Nuclear Reactors — Gas Cooled” => https://youtu.be/TYnqJ4VnRM8
- Fort St. Vrain, US[1]. High temperature gas cooled reactor. Operated for 12 years. Corrosion problems. Converted to natural gas.
- AVR reactor, Germany.[2] Pebble bed reactor. Had a pebble jam. Not repairable. Most fuel removed. Pressure vessel remains on site, with hope of full decommissioning in a century or so.
There's a small reactor of this design working at a university in China, and a medium sized one one (200MW electrical output) is supposed to come on line this year. We'll see how that works out.
Boring old boiling water and pressurized water reactors have simplicity in the radioactive part, and water is easy to handle. Designs that involve moving pellets or chemical processing of radioactive fluids add much complexity to a system that is very hard if not impossible to repair. The track record of such reactors is not good.
[1] https://en.wikipedia.org/wiki/Fort_St._Vrain_Generating_Stat...
Of course it's not as exotic as pebble bed reactor, or helium-cooled uranium-thorium reactor.
Rigorous, yes, but not "very, very difficult to pass". The Navy needs a consistent stream of replacement operators, and their preferred way of getting them is to take reasonably capable volunteers and tutor/coach/remediate as many people as needed once they're in that group.
My experience too. Do you remember the "skyhook"? We'd return to the barracks after class and find that the guy next to you had vanished without a trace.
And the reactor brings itself home for service if needed.
That seems way different than what you'd want in Antarctica.