[Symmetry]

Whenever you're splitting Uranium atoms the results will tend to be radioactive. The results will build up in the fuel over time and eventually make the reactor stop working. Conventional reactors breed a bit of plutonium too as U238 captures neutrons but most aerospace reactors want to be as light as possible and so use highly enriched Uranium. So after your trip the engine will be quite radioactive but, as you point out, there's a lot of space and outside Earth's atmosphere and Van Allen belts it's moderately radioactive anyways.

Thankfully nuclear reactors aren't particularly radioactive until you turn them on, which is a big improvement on the radiothermal generators, RTGs, that we sometimes use in probes headed for the outer solar system where solar panels don't work. It's during launch, before this part gets turned on, that you have a risk of crashing and losing the reactor somewhere on Earth.

[BAReF00t]

Who said anything about uranium?

There are elements with far more favorable decay paths. Short decay + using that decay too = pretty much a clean nuclear reactor.

[Symmetry]

Well, I'm no nuclear engineer but I'd hope that if you could make one of those that was light enough NASA would use that instead.

[greglindahl]

It's called an RTG, and they've been using them for decades.

[Symmetry]

I'm not sure what you're talking about? RTGs don't have short decay paths, they have to have long decay paths to last through a mission. P238 is what we use for most probes and has a half life of 88 years. It decays to U234 which has a half life of 200,000 years, short enough to be dangerous but long enough to almost never go away. RTGs tend to produce on the order of 100 watts of electricity from 500 watts of heat. A good nuclear engine will want to use 100+ megawatts when in use.

And more importantly RTGs don't put out nearly enough heat to make a usable nuclear thermal rocket. The important thing is being able to turn them on when you're doing a burn but then turn them off when you're coasting to your destination then turn them on again to stop there. RTGs can't do that.

[greglindahl]

What was asked for sounded like an RTG to me. I wasn't saying it was a nuclear thermal rocket or that it could be used as one. As you point out, it's a constant power source.

[ianai]

I’m increasingly of the opinion that nuclear for space should be mined and built in space. Just launch the infrastructure needed to bootstrap the process.

[Symmetry]

I'm all for that in the long run, but that's in the long run. Especially with Orion Drives they're too dangerous to fly themselves off the Earth but too heavy to launch on something else so they're entirely impractical right now. But it would be cool if we could be launching them from the Moon in the 2080s.

[Bud]

Um, uh, um, uh...where to start?

"Mined" from where? How?

[ianai]

The moon? Asteroids? Or even just ship up unrefined ore such that a catastrophic failure can’t threaten the population.

[mitchty]

Neither of the first two are options... at all realistically, nither uranium nor thorium exist in anything below a large planetary size body in any quantity: https://www.quora.com/Do-uranium-and-thorium-exist-in-signif...

And shipping up unrefined ore is also a bit of a ludicrous idea for mass reasons and the rocket equation alone. You do realize you can isolate a nuclear reactor core from explosions on rockets right? What catastrophic failures are you attempting to design your solution of avoiding a nuclear reactor around?

[cblades]

What risks make that extraordinary cost worthwhile?

[BAReF00t]

The moon's dust contains huge amounts of helium-3.

Which apparently is an amazing power source.

Also, why fission? We do have working fusion reactors. They are called hydrogen bombs. (The outer part, at least.) As long as you can keep the G forces low ...

[JumpCrisscross]

> Which apparently is an amazing power source

Theoretically, for reactors we don't have.

[joelthelion]

"just"

[steeve]

i was about the type the same thing, have my upvote friend