Is there a minimum viable size for a fusion reactor? If it scaled down ennough, they could just launch prototypes into space, and see if they explode.

I'm honestly not sure if you're joking but in case you're not, the "minimum viable size" is hardly the largest issue with what you're proposing and it sounds like you're not getting what the key issues are.

You're talking about taking a technology that's so finicky we've barely gotten it to work after almost 100 years and rocketing it into space? We're no where near good enough at this to get a test that would work after the extreme violence of an escape velocity launch.

Further fusion reactors aren't like fission. "exploding" really isn't a problem . Keeping the reaction going in an efficient manner is.

IF exploding was a problem, space is probably the worst place for it? Putting it way underground would be vastly easier and a hell of a lot safer because you won't have material possible falling back to earth/hitting satellites in orbit.

That was not a real question, it was a subconscious plea for elon musk to take over and make it work.

"Shooting it into space" is a reference to how SpaceX disrupted the rocket industry through a "fail fast" mentality, aggressive goals, and sheer force of will.

This take is even more baffling to me than the original question.

Let me break my logic down:

Assumptions:

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1. The average commenter here (me!) knows a little about nuclear fusion, and a lot about online tech news.

2. There are no new ideas.

Background:

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This big science project is overbudget, making incremental progress, and the timeline is stretching to forever.

That is incredibly similar to the state of rockets 10 years ago - publically funded, making little progress.

Leap of logic:

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Somewhere in his subconscious, OP pattern-matched to the only big successful science project in the news recently - SpaceX.

But the subconscious no good at talky talky, and OP didn't trace the train of thought, so what came out was "Shoot them into space".

Please don't "move fast and break things" with nuclear fusion :-)

What is the worst that can happen?

Seriously, I think I might know (a fire) but I do not.

Is there any possibility these can go ban! and spread nasty stuff around the neighbourhood?

I doubt they can go thermonuclear bang! but I am not a physicist.

What is the worst than can happen?

Well, neutron bombardment from fusion means you get some radioactive isotopes. It's not as bad as spent fuel but still something to avoid.

But that doesn't really matter. This is expensive technology you don't want to blow up. Yes, more expensive than rockets.

It's true that working on fusion is high on the list of better uses for $44B than buying an overpriced social media platform.

I'm no expert on this, but a minimum viable size exists and it's much larger than what you could just launch into space. I watched a video on this years ago, I don't recall the exact relation to size but if I'm not remembering this completely wrong there's a minimum size you need for a fusion reactor to "ignite", ITER is huge for a reason.

ITER is huge because it uses weak magnets.

Fusion reactors are pretty non-explodey. Really all they can do is spring a leak, then fill with air and extinguish the plasma. Maybe if you quench the magnets hard enough you might get something dramatic like leaking a gram of tritium.

Yeah there is, at least for a tokamak. The smaller the device, the stronger the magnets need to be, otherwise you can get instabilities that will prevent the plasma from being contained. This is why commonwealth fusion is building their own superconducting magnets, for example.