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Comment by dahfizz | Hacker News Reader

Surely with a little imagination you can conceive of some risks that might be associated with having a small sun in your backyard.

For one, fusion reactions generate neutrons that activate all the structural material of the reactor, creating radioactive waste. The waste is shorter lived than fission waste but is still a risk.

A more obvious risk is the pure destructive power of that much energy. High pressure, 100 million degree plasma is full of risks. It's nice that fusion reactors can't "run away" like fission does, but you still have to deal with material 6x hotter than the sun escaping confinement.

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When operational a fusion reactor has less then a couple grams of fuel in the vacuum vessel.

Okay, let's do some back of the napkin calculation.

The specific heat of Hydrogen gas is roughly 20 J/gC at high temperatures[1]. It would be much higher for a plasma but I couldn't easily find a number.

At 100M degrees C, a single gram of hydrogen would have over 2 Gigajoules of thermal energy. 2 grams of hydrogen would have the same energy as a literal ton of TNT.

It's not untenable, but its not really something that should be brushed off as "its only a couple grams of fuel" or "it can't have a runaway reaction".

[1] https://www.engineeringtoolbox.com/hydrogen-d_976.html

Having energy does not demonstrate danger to life and limb. 2 tons of fat have more energy than 2 tons of TnT. Obviously it does not release energy in the same way.

That plasma is at less than atmospheric pressure, and is contained in a massive steel vessel. Even if it expands and melts through the vessel, the atmospheric pressure will collapse it. I do not see a plausible scenario where anyone is in jured.

Funnily enough the much bigger danger is magnet quenches. Thousands of amps boiling off a couple dozen tons of liquid whatever is going to be a hell of a danger, but it's still "violent industrial accident" and not "fireworks factory" - and definitely nothing which people think of when you say "nuclear".

My comment wasn't very focused, so I'll clarify: GP seemed to imply that there is some hidden risk about fusion power that nobody is aware of or talking about, as if such a risk is the greatest factor currently bogging down fusion research. I wanted them to expand on that thought, as it's definitely not a perspective I'm familiar with.

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