Failure Modes of the Primary System of PWR

This chapter investigates the failure modes of the primary circuit system and the respective times to failure for core melt-down under high pressure and low pressure conditions (LPC). It focuses on two questions, Firstly, is there a potential for failure modes leading to depressurization of the primary circuit system prior to failure of the vessel and secondly what is the time to failure of the lower head under LPC. The chapter also discusses the behaviour of steam generator tubes, to surgeline and main coolant piping, to the upper head and flange connections of the reactor pressure vessel, and to the lower head. Steam generator tubes can be heated up by hot gas, if pump seals fail or a small break exists in a cold leg. The steam generator rests on a flexible support system which allows for thermal movement.