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

A simple analytical method, which describes uncovery and heatup in the core under accident conditions, is derived, tested against experiment al data, and used for generating the scaling criteria. Void fraction a nd core uncovery levels are analytically derived integrating mass and energy equations under the assumption of quasi-steady state. The coola nt energy equation in the uncovered region is integrated to convert th e partial differential equation for the fuel temperature into an ordin ary differential equation through the assumption of the same axial dis tribution of the amount of energy loss from the fuel to the coolant as that of the decay heat generation rate. The ordinary differential equ ation for the fuel temperature, combined with the governing equation f or cladding oxidation, is analytically solved assuming a linear variat ion of fuel temperature and oxidation thickness with time over a perio d. The present analytical model is tested against the Power Bursting F acility Scoping Test (PBF-ST) and SCDAP calculation. The model produce s 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 t he fuel temperatures by the model, presumed to be mainly caused by no consideration of reflux condensation and the higher prediction of radi ation energy loss to the shroud through the treatment of one radial re gion of the bundle. The PBF-ST is examined with the scaling criteria g enerated by the present model. It is found out that the linear heat ge neration rate in the PBF should be by four times larger than that in t he prototype system and the radiation number is highly distorted in th e PBF.