It's the other way around: If a fission reactor is insuffciently cooled (due do the cooling system getting destroyed by attack or natural causes, cf the Fukush

It's the other way around: If a fission reactor is insuffciently cooled (due do the cooling system getting destroyed by attack or natural causes, cf the Fukushima Daiichi nuclear disaster), the reaction will continue, the core can melt and breach containment. If a (plasma-based) fusion reactor gets damaged, the plasma will dissipate and the reaction will stop.