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A quibble:

The point about meltdowns is cute, but probably wrong; the insurance against meltdown is a negative thermal coefficient of reactivity, if I understand correctly. In other words, the thermal expansion drives the fuel out into the piping as the reaction heats up, away from the compact reacting core, and also reduces the density in the core, allowing more neutrons to escape.

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All reactors have negative temperature coefficients of reactivity, including LWRs (the water moderator boils away), SFRs (the core thermally expands), and MSRs (the fuel density decreases). The key to avoiding radiation leakage out of any nuclear plant is decay heat removal [1]. If you can passively remove decay heat, your reactor is very safe. MSRs do this easily, as do other reactors with low-pressure coolants (like sodium metal-cooled reactors).

[1] https://whatisnuclear.com/decay-heat.html

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