When nuclear reactors leak poison gas then something has gone wrong, and it makes international headlines.

Diesel generators are designed to leak poison gas; hundreds of millions of them do this every day.

I see your point, any nuclear excursion tends to get blown out of proportion, but it does have some unique dangers.

In particular, diesel fumes eventually dissipate, whereas radioactive materials will settle and continue to be a problem for decades. Which may not be a problem if it's left alone, but anyone unlucky enough to interact with it could end up with it on their body/clothing unaware that they're absorbing an unsafe dose over a period of days or weeks.

But yes, I agree, we do need to move away from fossil fuel sources one way or another.

Sure you can, you just need to agree on the comparison criteria, like, for example, the total number of illnesses/death from normal (and predicted abnormal) operation of the device. Burning fossil fuels still leads to deaths even if they are spread over a large area and/or time.

I've previously worked on a nuclear licenced site and we were not even allowed to throw smoke alarms in the general rubbish due to there being radioactive material inside.

My concerns are solely with how fuel would be managed, so that it isn't lost, and subsequently broken. One example was scrap workers taking a radioactive source to a scrap yard to be melted down, not realizing what it was.

Meanwhile, radioactive toxic stuff will remain toxic locally.

Thanks to that we are where we are.

Examples at a concentration as dangerous as a reactor melting down?

Hell, https://en.wikipedia.org/wiki/Great_Molasses_Flood killed more than the Fukushima Daiichi meltdown.

It was produced for the benefit of all mankind.

And now that's whose bloodstream it can be found in.

Mainly for two reasons, it's half-life is longer than the lifespan of living things, and people spread it around like they never should have done at all.

That makes it analogous to opening Pandora's box more so than all kinds of dangerous things.

And in any case, this is a piece of military hardware which might plausibly be "reconfigured" be being struck by a rocket. It's not a civilian reactor inside a secure facility.

Your statement is analogous to saying "charcoal grills do not start forest fires". Used correctly, they certainly shouldn't. But they sure as hell can if you kick them over on a dry summer day. Same deal here.

To be glib: Not vs. fucking rockets they aren't. Those designs imagine that the fuel doesn't move and that the coolant is the medium of failure.

What happens when you move the fuel?

It stops reacting. If you blow up a nuclear reactor you've made a nice radiological hazard (though Uranium/Thorium themselves aren't particularly hazardous), but as soon as the fuel stops being contained inside of the appropriate neutron-mediating environment and is scattered, it will become subcritical.

The question of whether or not deploying nuclear reactors in warzones is a good idea is entirely different from the question of whether meltdowns are a realistic possibility. A critical nuclear reaction can be sustained with neutron reflectors and moderators using an amount of fuel that would otherwise be subcritical. Any "reconfiguration" only reduces the criticality factor of such a reactor.

Poisoning us on the long term, and worse - starting a runaway greenhouse heating process which will make the planet uninhabitable in just a few hundred years.

Again, no one seems to be understanding the context here and just wants to jump in with knee-jerk pro-nuclear arguments. The application under discussion is a portable generator to be deployed locally as backup power in presumably-chaotic emergency situations. That is simply not where you want a reactor, period.

I don't understand this knee-jerk pro-fossil fuel arguments, using your words. Fossil fuels, especially those in backup generators, are very harmful if run long term. We are currently in a pandemic where one of the primary causes of death is the inability to take up oxygen, and in this context we want to add air pollution?

I'm not sure I understand how you're concerned about a portable nuclear generator failure or attack releasing nuclear materials, but you suggest that it would be somehow safer (?) to strap it on top of hundreds of tons of explosive rocket fuel and launch it into the sky?

Cost/benefit analysis isn't about deciding what's "safer" in an absolute sense. It's about how much you get for how much risk you're willing to accept. The risk of one meltdown is real, but not impossible to tolerate in all circumstances.

Is it worth keep a bunch of these reactors around to deploying to urban Mogadishu or wherever to supply an invading military force with power? I argue not. The risk is high, and electricity is cheaply available already.

Is it worth launching one reactor into space if it enables an entirely new capability in solar system exploration? Maybe yes. It's just one launch, you can do it in remote areas. The people directly involved can be made aware of the risks (and are taking significant risks already). I don't have a full whitepaper to hand you on this but my intuition says that this might be OK, yeah.

So the question is what would you rather be hit by a missile? A nuclear reactor or a diesel engine?

The emissions of a diesel engine are irrelevant here because that's a specific use-case that is going to be negligible compared to emissions by cars and industry in general (or even by the army).