SCALING STUDY OF IN-CORE BOIL-OFF AND HEATING PROCESS

Abstract

A simple analytical method, which describes uncovery and heatup in the core under accident conditions, is derived, tested against experimental data, and used for generating the scaling criteria. Void fraction and core uncovery levels are analytically derived integrating mass and energy equations under the assumption of quasi-steady state. The coolant energy equation in the uncovered region is integrated to convert the partial differential equation for the fuel temperature into an ordinary differential equation through the assumption of the same axial distribution of the amount of energy loss from the fuel to the coolant as that of the decay heat generation rate. The ordinary differential equation for the fuel temperature, combined with the governing equation for cladding oxidation, is analytically solved assuming a linear variation of fuel temperature and oxidation thickness with time over a period. The present analytical model is tested against the Power Bursting Facility Scoping Test (PBF-ST) and SCDAP calculation. The model produces the estimation of inlet flow rates and its results which are in good agreement with the measured levels. There is an overprediction of the fuel temperatures and an underprediction of the rate of increase of the fuel temperatures by the model, presumed to be mainly caused by no consideration of reflux condensation and the higher prediction of radiation energy loss to the shroud through the treatment of one radial region of the bundle. The PBF-ST is examined with the scaling criteria generated by the present model. It is found out that the linear heat generation rate in the PBF should be by four times larger than that in the prototype system and the radiation number is highly distorted in the PBF.