Three mile island 16.422 · March28th,1979 Main feedwater pump failure, caused reactor to shut down Relief valve opened to reduce pressure but became stuck in the open position No indication to controllers Valve failure led to a loss of reactant coolant water No instrument showed the coolant level in the reactor Operators thought relief valve closed water level too high High stress Overrode emergency relief pump16.422 Three Mile Island • March 28th, 1979 • Main feedwater pump failure, caused reactor to shut down • Relief valve opened to reduce pressure but became stuck in the open position – No indication to controllers – Valve failure led to a loss of reactant coolant water • No instrument showed the coolant level in the reactor • Operators thought relief valve closed & water level too high – High stress – Overrode emergency relief pump