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.
"Mined" from where? How?
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?
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 ...
Theoretically, for reactors we don't have.
There are elements with far more favorable decay paths. Short decay + using that decay too = pretty much a clean nuclear reactor.
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.
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.
>“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.
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.
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.
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.
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.
These things are tough! And also expensive, so you might as well reuse them once they shrug off the rocket exploding under them.
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.
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/
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.
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?