[Arrezz]

How big is the added risk of toxic waste? I'm not sure how much waste is produced in relation to propulsion energy given but it must be quite small? And I imagine that during travel in space you could just dump that waste out into space considering the vastness of it all.

[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

[api]

I don't know for certain but I'm fairly sure that the idea is that you don't activate the reactor until it's in space. Before a reactor is turned on the fuels are less radioactive. It's once you turn it on that radioactivity increases dramatically and you get all the nasty decay products and such.

So not zero but not as much as you might think.

Personally I don't like the idea. Environmental concerns are real, but those aside it's likely more expensive than multiple refueling flights with big conventional rockets. These would be expendable and very costly to research, develop, fuel, and launch, whereas for the same cost you could probably put stages in orbit and send fuel up to them with reusable tankers. Like hydrogen this is another example of NASA chasing the sexiness of high performance in a pure sense (high iSP etc.) without doing a total cost analysis.

In general SpaceX and Blue Origin have the right approach.

[Robotbeat]

Why is this downvoted? This is informative and (in my opinion) basically correct.

And while I agree that in the near term, refueling via chemical rockets is a far cheaper (and even higher performance) way of solving this problem, I do support the research because someday we'll want to go even beyond refueling of chemical rockets. When you get REALLY high transfer times between Earth and Mars, the higher Isp makes a significant difference.

To explain: Conventionally, it takes about 6-8 months to get to Mars. Nuclear thermal rockets can shorten this time for the same mass in LEO to like 3 or 4 months. HOWEVER, agreeing with what api said, you can get the same exact speedup by using refueling with conventional rockets (and aerocapture/braking/direct-entry). It increases the required mass in LEO, but if you have cheap (especially reusable) rockets, then cost to launch more mass to LEO is not a major factor compared to the cost of a nuclear thermal rocket. And this is exactly what SpaceX has proposed: (see slides 19 through 22) http://www.spacex.com/sites/spacex/files/making_life_multipl...

But the Isp (exhaust velocity) advantage is maintained. The rocket equation is exponential: mass full = (empty mass)*e^((mission delta-v)/(exhaust velocity))

So eventually, when mission delta v is much higher than exhaust velocity, the mass ratio explodes. So a factor of 2 improvement in Isp is worth the extra cost, even if you have reusable rockets. The exponential curve eventually beats even the cheap, brute-force approach, if you want transfer times of on the order of 1 month.

It's also the kind of work NASA should be doing. Private industry is doing a really good job reducing the cost to orbit, so NASA can focus on these longer-term problems.

[baybal2]

> Why is this downvoted? This is informative and (in my opinion) basically correct.

I once wrote that Chernobyl had no chance to explode in a nuclear explosion in rebuke to some guy called Moxie Marlinspike. I had -4 for the next few days on all my posts, and somebody even bothered to find my work email, and futilely tried to troll me and my colleagues into deleting my rebuke for a week.

"That" demographic is definitely there, and working in a "tech" occupation does not preclude a person from being a part to it these days.

[SketchySeaBeast]

> "The massive amount of energy produced by these reactors could be used to sustain human outposts on other worlds and cut the travel time to Mars in half.

>“Many space exploration problems require that high-density power be available at all times, and there is a class of such problems for which nuclear power is the preferred—if not the only— option,”

It seems that nuclear reactors has more utility than simple power to weight ratio.

[mlindner]

Nuclear reactors will be needed if you're going much further out than Earth/Mars. Solar power falls off rapidly as you go further out.

[logfromblammo]

The risk is that in a catastrophic launch failure (read: exploded rocket), the radioactive materials could be dispersed downrange.

The solution--if that's really a problem--is to use the same escape systems used for crewed launches to eject the nuclear fuel with a parachute and emergency beacon, and keep it all inside a durable shielded container until the craft needs to start up the nuclear engine.

[GlenTheMachine]

Effectively, they already do that. Although the United States doesn't launch reactors, we do on occasion launch radioisotope thermoelectric generators (RTGs). These use a core of sub-critical plutonium surrounded by thermocouples, which turn the heat into electricity. These are used for probes going to the outer solar system, where solar panels aren't effective enough.

Anyway - there is certainly a concern with the plutonium in RTGs being dispersed by a launch failure. The engineering that goes into designing the protective system for RTGs is extensive; they each have their own miniature heat shield, and are surrounded by iridium and carbon blocks. Tests show that they can indeed survive the explosion of the launch vehicle.

[m4rtink]

IIRC long ago a US RTG ended up in the ocean due to launch failure, only to be recovered and sussessfully re-launched on a new satellite.

These things are tough! And also expensive, so you might as well reuse them once they shrug off the rocket exploding under them.

[mlindner]

Except that's not a big problem. A non-activated reactor just contains enriched uranium. Uranium is dug out of the ground and you can buy it on amazon and chemically concentrate it yourself. It's safe to hold and handle (wash your hands afterwards so you don't eat particles) and store in your house even. (In the US this is all legal.)

Reactors only become dangerous after you activate them and short lived isotopes are created that also happen to be types that are bioavailable, like cesium-137 and strontium-90 which the body will take up and store inside the body.

[yellowapple]

> Uranium is dug out of the ground and you can buy it on amazon and chemically concentrate it yourself.

Well now I know what I'm putting in all my nieces' and nephews' stockings this year: https://www.amazon.com/Images-SI-Uranium-Ore/dp/B000796XXM/

[logfromblammo]

Reactor fuels or RTG cores are a bit more dangerous than ore.

A properly designed reactor requires the fuel to be in the core to sustain a chain reaction, and neutron activation of other elements in the reactor does not occur until the reaction has started. Thus, a rocket explosion would not cause a criticality event. The worst that would happen would be dispersion of nuclear fuel to a place where someone might handle it without its transport-safety shielding. Which still wouldn't be that bad.

[scohesc]

I would imagine the engine would be fairly inert if it's not activated until in orbit.

I could see issues if the craft all of a sudden loses its orbit with an radioactive engine burning up in the atmosphere spewing radiation (although I'm sure we get bombarded with way more from the sun potentially?)

Maybe if during launch something goes catastrophically wrong and blows up mid-air like a bomb of sorts?