How do fusion reactors differ fundamentally from fission reactors regarding the risk of a runaway chain reaction?

Answer

There is no risk of a runaway chain reaction meltdown because the reaction stops instantly if conditions fail.

Fusion possesses an inherent safety characteristic: the reaction ceases immediately if the plasma conditions (temperature, density, confinement) are lost, meaning a runaway meltdown is impossible.

How do fusion reactors differ fundamentally from fission reactors regarding the risk of a runaway chain reaction?
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