Effect of Core Spray Cooling in Transient State after Loss of Coolant Accident

Abstract

To examine the conditions brought about upon initiation of core spraying, a 36-rod assembly was heated to different temperatures and then spray cooled. The runs were made at atmospheric pressure using two kinds of assemblies, both made up of heater rods of 15 mm outer sheath diameter, arayed into 6×6 lattice accommodated in a 119×119 mm2 channel, but with heated lengths of 1,500 and 2,500 mm. From these experiments following results were obtained: 1. The surface of the exposed fuel rod after loss of coolant accident can be considered close to adiabatic state.2. The cooling water streams down the surface of the heater rods in the form of water film. If the wall surface temperature is quite high, water film is blown off the heated wall surface at a certain point along the rod (wet front), where the temperature is about 150°C. This wet front progresses slowly downward along the rod.3. The speed of downward progress of the wet front was measured, and an empirical equation obtained.4. After commencement of spraying, the heat transfer coefficient of the portion of rod not yet reached by the wet front is approximately equal to that after attainment of steady state in dry-out condition.