Clad-to-Coolant Heat Transfer in NSRR Experiments

Abstract

The evolution of the clad temperature during a Reactivity Initiated Accident plays a key role in the accidental sequence because it strongly influences the rod mechanical resistance against failure. The present study aimed at quantifying the heat transfer in NSRR experiments. Transient boiling curves were determined by inverse conduction calculations of NSRR experiments in which the clad outer surface temperature had been measured by spot-welded thermocouples. Critical Heat Fluxes (CHFs) as high as 13 MW/m² have been obtained, highlighting a considerable increase compared to stationary pool boiling conditions. The elevated CHFs are due to the intense transient fluid vaporization at the surface induced by a fast clad heating rate. A transient boiling model has been implemented in the SCANAIR code on the basis of the physical interpretation of the boiling curves. A good agreement between computed and experimental clad temperatures is obtained for high burnup fuel tests as for fresh fuel tests.

KEYWORDS:

• LWR
• reactivity initiated accident RIA
• NSRR
• heat transfer
• transient boiling
• CHF
• film boiling
• SCANAIR