Abstract
During the reflood phase of a large-break loss-of-coolant accident (LBLOCA) in a pressurized-water reactor (PWR), the fuel rods can be ballooned or rearranged owing to an increase in the temperature and internal pressure of the fuel rods. In this study, an experimental study was performed to understand the thermal behavior and effect of the ballooned region on the coolability using a 2 × 2 rod bundle test facility. The electrically heated rod bundle was used and the ballooning shape of the rods was simulated by superimposing hollow sleeves, which have a 90% blockage ratio. Forced reflood tests were performed to examine the transient two-phase heat transfer behavior for different reflood rates and rod powers. The droplet behaviors were also investigated by measuring the velocity and size of droplets near the blockage region. The results showed that the heat transfer was enhanced in the downstream of the blockage region, owing to the reduced flow area of the subchannel, intensification of turbulence, and deposition of the droplet.
Highlights
The effect of the ballooned fuel rods on the coolability has been an important issue since the 1980s
The main experimental programs can be summarized as the FEBA [1], SEFLEX [2], THETIS [3, 4], ACHILLES [5], CEGB [6], and FLECHT-SEASET programs [7]
The nonblockage and blockage tests were performed to investigate the effect of the ballooned fuel rod on the coolability in a 2 × 2 rod bundle test facility
Summary
The effect of the ballooned fuel rods on the coolability has been an important issue since the 1980s. The previous studies conducted forced or gravity reflood tests for various test conditions to examine the thermal behavior of the blockage region and to determine an upper limit of the blockage coolability with respect to the blockage geometry and configurations. They concluded that the coolability at the blockage region greatly depends on the blockage characteristics (blockage ratio, maximum blockage length, blockage shape, and blockage configuration) and the coolant conditions (flow rate, system pressure, and inlet temperature). The first group test is intended to understand the heat transfer phenomena and to examine the effect of the blockage characteristics on the coolability in a modeled 2 × 2 subchannel. The results were carefully discussed based on the temperature profiles of the heater rods and the droplet behavior
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