Heat transfer limitation during rapid depressurization

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. These competing processes are analyzed. Results demonstrate that during initial blowdown the boiling contribution to coolant voiding is limited by vapor nucleation and transport and not necessarily by onset of critical heat flux.