2017:20 Assessment of inertia effects on transient clad-to-coolant heat transfer and coolant flow under reactivity initiated accidents

In this report, simple hydrodynamic models are used for studying the effects of coolant inertia on fuel-to-coolant heat transfer and coolant flow under reactivity initiated accidents in light water reactors. The objective is to assess if the inertia effects are important enough to warrant modification of QT-COOL, a coolant channel module for two-phase flow that has recently been implemented as an optional model in the SCANAIR fuel rod analysis program.

The results of our study suggest that inertia has a negligible impact on the growth kinetics of the continuous vapour film that forms between the fuel rod cladding tube and the surrounding subcooled liquid water, when the fuel rod is rapidly heated.

This vapour film has an insulating effect, and it has a significant impact on the fuelto-coolant heat transfer. For conditions expected under reactivity initiated accidents in light water reactors, the growth of the vapour film seems to be rate controlled by transfer of heat and mass across the liquid-vapour interface rather than by inertia of the liquid that is displaced radially by the growing film.

On the other hand, our assessment indicates that the coolant inertia is important for the axial flow kinetics in the coolant channel when the accident conditions are such that net vapour generation occurs anywhere along the fuel rod. The reason is that the vapourization involves a large local volume expansion, which in turn entails acceleration and displacement of a significant amount of water along the coolant channel.

Coolant inertia effects are not accounted for in the current version of the QT-COOL coolant channel module, but suggestions are given for how the module can be improved to overcome this limitation. The report also contains a review of relevant experimental data that are deemed suitable for future calibration and validation of the QT-COOL module.