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ItsAConspiracy t1_j0z312i wrote

Reply to comment by BlueSkyToday in Nuclear fusion breakthrough: What does it mean for space exploration? by Gari_305

Fission fuel is barely radioactive before you start the reactor. It's just natural uranium, with a modestly higher percentage of U235.

It's the broken-apart atoms you get after fissioning uranium that are the really dangerous stuff. And to a lesser extent, heavier atoms that absorbed neutrons without splitting. So, just don't start the reactor until you're well away from Earth.

That would be way safer than what NASA has actually done multiple times, which is launch deep space missions powered by plutonium-238. That doesn't even need to be fissioned, its radioactivity is what powers the mission.

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BlueSkyToday t1_j106ge8 wrote

Yes, spent fuel rods are a lot more dangerous but the cost of mass to orbit is very high. Every plan that I've heard of calls minimizing the size of the reactor and the fuel load. So we're looking at highly enriched uranium or other possible designs. These are more of a problem than what we normally use in fission reactors.

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ItsAConspiracy t1_j10bji0 wrote

The half-life of U235 is 700 million years. The longer the half-life, the less radioactive something is, so even weapons-grade uranium at over 90% U235 is not particularly dangerous. You wouldn't want to eat it or inhale a large concentration of it, but you wouldn't want to do that with solid rocket fuel either.

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BlueSkyToday t1_j1p3ws0 wrote

In a world where people are deeply upset about the environmental health effects of fragments of shells made out depleted uranium, I don't think that scattering highly enriched uranium is going to fly.

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ItsAConspiracy t1_j1pzpeu wrote

And yet, plutonium-238 has flown multiple times, and that's way more radioactive.

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