Influence of ATF Coatings on the Critical Heat Flux in a Vertical Rod Bundle during Flow Boiling of Water at Low Pressure

Abstract

This paper issued the experimental testing of ATF cladding materials as rod bundles in loss of coolant accidents of pressurized water reactors from the onset to the departure of nucleate boiling under low pressure conditions. A CrN-coated rod bundle is compared with an untreated Zircaloy-4 rod bundle as a promising candidate from ATF material research in terms of corrosion/ hydrogen generation rates. To clarify the influence of ATF-grade coatings, the heat flux required to reach the boiling crisis is used as a benchmark. With halved and doubled reference conditions (20 K subcooled mass flux of 200 kgs^-1m^-2 at 1400 mbar), the influence of the CrN coating on the steady state temperature and maximum heat transfer is shown in consecutive heating steps until the boiling crisis is reached. The experimental setup consists of a directly electrically heated and magnetically compensated 5-rod bundle within a vertical 43.7 x 43.7 mm flow channel with controlled boundary conditions and a temperature excursion-triggered rapid shutdown of the transformer, which enables multiple non-destructive CHF experiments with identical cladding tubes. The outer and inner diameter of each rod is 9.5 and 8.36 mm with a layer of 8 μm CrN and a heated length of 300 mm. The sight glasses in the flow channel with high-speed cameras and transformer-synchronized ring buffer enable the observation of the heated surface at the time of the CHF. The investigated material influence of heated surfaces can be used for the further development of mechanistic CHF prediction and CFD models.