Sodium Boiling Experiments at Decay Power Levels(4); Summary Assessment of the Low-Flow and Low-Heat Flax Sodium Boiling Expeiments at PNC

Yamaguchi, Katsuhisa; not registered; Aoki, Tadao

The objective of the Sodium Boiling Experiments at Decay Power Levels is to examine the heat removal capability of reactor fuel subassemblies under sodium boiling condition, which is a matter of arguments in analyzing the accidents like the loss of piping integrity and the loss of shutdown heat removal system. Prior to progressing the test program, a survey study was conducted to fix the scope within which an advanced investigation should be required to analyze the event sequence following after sodium boiling at low flow. The study focused on the results of the past low-flow and low-heat-flux boiling tests performed with the SIENA Facility, with special attension to summarizing the critical (dryout) conditions of the two-phase flow heat transfer. The corresponding data for water cases were also examined. The topical results are as follows: (1)The dryout phenomenon reproducible under annular flow condition at relatively high flow is well predicted to occur by the criterion that the exit quality equals to 0.5. (2)Even if the mismatching ratio of power to flow is increased at low flow range, the slug flow pattern is sustained, repeating the void expansion and contraction synchronized with the unstable flow oscillation. In this case, the extra-superior heat removal capability is expected due to strong heat sink around the voided region. The tendencies of the dryout quality data at annular flow on several parameters are resembled to those of water data, from which one can reach the conclusion that the dryout criterion confirmed here would be reasonable for the sodium flow cases. The forthcoming experiment should be, therefore, concentrated on examining the factors influential to the flow pattern transition and on generalizing the dryout data base at annular flow having less coolable nature.