Experimental Study of Heat Transfer Characteristics During Reflooding Process Under Partially Exposed Conditions in Narrow Rectangular Channels

Abstract In the event of a small and medium-sized breach accident in the primary loop of a nuclear reactor, the pressure relief and discharge causes a loss of coolant flow in the core, exposing the top of the fuel elements to overheated steam while the bottom remains submerged in residual cooling water. At this point, the fuel elements experience significant axial temperature differences, making the boiling two-phase flow behavior during the reflooding process of the fuel elements more complex compared to complete exposure. This experiment conducted reflooding tests with varying initial water levels for localized exposure, and the results showed that the minimum wall film boiling temperature in the completely exposed state gradually decreased downstream along the axial direction, while it increased in the case of localized exposure. The precursor cooling duration experienced during the reflooding process was longer for complete exposure than for localized exposure, and at the same wall superheat, the heat transfer coefficient for localized exposure was larger. The average heat transfer coefficient during the reflooding process of localized exposure was higher than for complete exposure, and as the degree of exposure decreased, the average heat transfer coefficient during the precursor cooling phase increased.