Restrictive Effect of Ascending Steam on Falling Water during Top Spray Emergency Core Cooling

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Abstract
Water spraying experiments were conducted to find out a flow rate of falling water overcoming ascending steam during top spray emergency cooling with an 8×8 type simulated fuel rod bundle of real size. The bundle consisted of 64 rods, each with a diameter of 12.5 mm, arranged in the form of square lattice with a pitch of 16.3 mm. In the experiments the simulated fuel rods were not heated. Instead, steam was injected into the lower plenum vessel simulating bundle-generated steam. As the results, (1) a criterion was proposed to determine the region where the restrictive effect of ascending steam on falling water appears, considering the decrease of a flow rate of ascending steam due to condensation by a spray of subcooled water, (2) the restrictive effect was independent of water head on the upper tie plate and water injection methods, and (3) an analytical model based on the pressure balance at the upper tie plate was proposed to calculate a flow rate of falling water overcoming ascending steam.

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