Experimental analysis with RANNS code on boiling heat transfer from fuel rod surface to coolant water under reactivity-initiated accident conditions

Udagawa, Yutaka  ; Sugiyama, Tomoyuki ; Suzuki, Motoe; Amaya, Masaki  

In order to promote a better understanding of the temperature evolution of fuel rod under reactivity initiated accident (RIA) conditions, we have investigated the effects of coolant subcooling, flow velocity, pressure, and cladding pre-irradiation on the heat transfer from fuel rod surface to coolant water during RIA boiling transient, based on a computational analysis with the RANNS code on the transient data from RIA-simulating experiments in the NSRR. The analysis showed that the film boiling heat transfer coefficients during RIA boiling transient increase with coolant subcooling, flow velocity, and pressure as predicted by the model for stable film boiling. The estimated boiling heat transfer coefficients were significantly larger than those predicted by semi-empirical correlations for stable film boiling. The analysis also suggested that the heat transfers during both transition and film boiling phases are strongly enhanced by pre-irradiation of the cladding.