Mostly about the ocean it would typically fall into in the most common failure shortly after launch scenario.

Nuclear testing got to carefully select locations to minimize harm, uncontrolled reentry of nuclear rockets has significantly less control.

It’s not the 99 rockets falling into the ocean shortly after launch that are problematic, it’s the 1 that almost works but then lands in the city we should be concerned with.

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Not falling on a city, but debris from Russia's launch last week was seen and heard in Melbourne, Australia: https://www.abc.net.au/news/2023-08-08/space-junk-leaves-tra...

That's pretty close for what would be high level nuclear material.

We might see more obvious direct human impact from debris landed in cities, but nuclear debris falling into the ocean or land that isn't densely populated by humans still has plenty of bad side effects.

Sealed fuel capsule at the bottom of the ocean should not be a big deal - ideally you would fish it out and re-launch it. This has already been done with an RTG before.

Nuclear weapons testing was mostly designed to maximize power output by burning up as much fissile material as possible in one big explosion. Conversely, it was meant to minimize fallout. A broken nuclear reactor (no nuclear explosion) would be the opposite: lots and lots of fallout (broken reactor bits) scattered over a wide area. The radioactivity from a broken nuclear rocket could be more dangerous than that from a great many bombs.

One solution would be have a kill-switch that detonates the reactor fuel as a nuclear bomb (assuming it could be configured in such a way; this is probably more difficult than I imagine). There would be major political problems with this approach though, as you're then shooting nuclear missiles.

Even as a layman in these things, I know that a nuclear reactor is far, far from a nuclear bomb and isn't easily made one. You can't 'configure' a reactor to become a bomb.

A better way would just be shielding, make sure it stays in 1 piece through reentry and impact. And still doesn't leak radiation then.

Can’t we just send up the propulsion system and the fuel separately? So like, ya, either could fail, but one is just some fancy equipment being g destroyed and the other is…unert uranium or plutonium coming down, needing to be cleaned up but not in a particular dangerous mode. Then once in orbit and the risk of crashing into earth is much less, put them together, fire up the reactor and go?

I suspect that the fuel separate from the reactor is still a major risk.

It can’t be configured that way. That’s simply not how reactors or bombs work.

Reactors that have not yet been started do not contain that much radioactivity actually, nuclear detonation creates a lot of gunk during the energetic phase due to neutron activation and similar processes.

Also you can presumably shield reactors to survive reentry or even ship the fuel in multiple such reentry proof containers. And of course longer term build these in space, given nuclear propulsion is basically in space only anyway given its characteristics.