Heat Transfer Limitation During Rapid Depressurization

Abstract

During depressurization events, such as large break LOCA, there is a large increase in the rate of vapour bubble nucleation on heated surfaces of fuel elements arising from the transient increase in wall superheat of the fuel sheaths. Continued nucleate boiling and associated high rates of vapour generation are dependent upon the balance between liquid resupply and vapour nucleation and transport away from heated surfaces. This paper presents a model of these competing processes and demonstrates that, during initial blowdown, coolant voiding due to boiling is limited by vapor transport as opposed to the assumption of onset of critical heat flux.