A reactor design whose '' inherently '' safe process directly provides a '' passive '' safety component during a specific failure condition in '' all '' operational modes is typically described as relatively fail-safe to that failure condition . However most current water-cooled and - moderated reactors , when scrammed , can not remove residual production and decay heat without either process heat transfer or the active cooling system . In other words , whilst the inherently safe heat transfer process provides a passive safety component preventing excessive heat while the reactor is operating , the same inherently safe heat transfer process '' does not '' provide a passive safety component if the reactor is shut down ( SCRAMed ) . The Three Mile Island accident exposed this design deficiency : the reactor and steam generator were shut down but with loss of coolant it still suffered a partial meltdown
Three Mile Island accident
Three Mile Island accident
Passive nuclear safety