A phenomenological analysis of melt progression in the lower head of a pressurized water reactor

• We propose a phenomenological description of melt progression into the lower head.
• We examine changes in heat loads on the vessel.
• Heat loads are more severe than emphasized by the bounding situation assumption.
• Both primary circuit and ex-vessel reflooding are necessary for in-vessel retention.
• Vessel failure conditions are examined.

The analysis of in-vessel corium cooling (IVC) and retention (IVR) involves the description of very complex and transient physical phenomena. To get round this difficulty, “bounding” situations are often emphasized for the demonstration of corium coolability, by vessel flooding and/or by reactor pit flooding. This approach however comes up against its own limitations. More realistic melt progression scenarios are required to provide plausible corium configurations and vessel failure conditions.Work to develop more realistic melt progression scenarios has been done at CEA, in collaboration with EDF. Development has concentrated on the French 1300 MWe PWR, considering both dry scenarios and the possibility of flooding of the RPC (reactor primary circuit) and/or the reactor pit.The models used for this approach have been derived from the analysis of the TMI2 accident and take benefit from the lessons derived from several programs related to pool thermal hydraulics (BALI, COPO, ACOPO, etc.), material interactions (RASPLAV, MASCA), critical heat flux (CHF) on the external surface of the vessel (KAIST, SULTAN, ULPU), etc.Important conclusions of this work are as follows:(a)After the start of corium melting and onset of melt formation in the core at low pressure (∼1 to 5 bars), it seems questionable that RPV (reactor pressure vessel) reflooding alone would be sufficient to achieve corium retention in the vessel;(b)If the vessel is not cooled externally, it may fail due to local heat-up before the whole core fuel inventory is relocated in the lower head;(c)Even if the vessel is externally cooled, transient 3D corium configurations in the lower head can induce high thermal loads on the vessel leading to a fast failure. Early in-vessel reflooding may prevent this risk. As a consequence, ex-vessel flooding should be accompanied by in-vessel reflooding;(d)In case of failure with external cooling, the breach is plausibly located on the lateral surface of the lower head. The failure is most likely local.The present work identifies effects that should be taken into account and modelled in numerical tools describing core degradation and vessel failure.