I know. Heck, it's also why I think Battletech would work in the Fallout setting. 3 reactor hits and your mechs reactor explodes.
By RAW Reactors simple shut down when they take three engine hits. The entire "fusion plants go big boom" comes from the novels where one specific author was overly fond of having it happen.IIRC reactor explosions are kinda rare in battletech, they 'do' happen but most of the time the reactor just shuts off and fizzles out.
There is some pretty cool reactor explosions in the Mechwarrior 3 and Mechwarrior 5 (the OG mechwarrior 5, not the shitty one we got) trailers.
There are OPTIONAL rules in BattleTech for having engines go critical, either via having to much heat buildup or an extraordinary number of engine crits (as in, taking five engine crits in a single round, which if you know anything about BattleTech and its statistic curve is... difficult) in a round. But BattleTech has optional rules for just about everything imaginable so...