Abstract
A set of reflood tests has been performed using ATLAS, which is a thermal-hydraulic integral effect test facility for the pressurized water reactors of APR1400 and OPR1000. Several important phenomena were observed during the ATLAS LBLOCA reflood tests, including core quenching, down-comer boiling, ECC bypass, and steam binding. The present paper discusses those four topics based on the LB-CL-11 test, which is a best-estimate simulation of the LBLOCA reflood phase for APR1400 using ATLAS. Both homogeneous bottom quenching and inhomogeneous top quenching were observed for a uniform radial power profile during the LB-CL-11 test. From the observation of the down-comer boiling phenomena during the LB-CL-11 test, it was found that the measured void fraction in the lower down-comer region was relatively smaller than that estimated from the RELAP5 code, which predicted an unrealistically higher void generation and magnified the downcomer boiling effect for APR1400. The direct ECC bypass was the dominant ECC bypass mechanism throughout the test even though sweep-out occurred during the earlier period. The ECC bypass fractions were between 0.2 and 0.6 during the later test period. The steam binding phenomena was observed, and its effect on the collapsed water levels of the core and down-comer was discussed.
Highlights
Some advanced pressurized water reactors such as the APR1400 [1] have adopted the new safety feature of a DVI (Direct Vessel Injection) system that supplies the ECC (Emergency Core Coolant) directly into the reactor vessel down-comer
The direct ECC bypass phenomena could limit the ECC penetration into the core through the down-comer and, degrade the core cooling capability during the late reflood phase of the LBLOCA, becoming one of the most important safety issues of an advanced pressurized water reactor, such as the APR1400, which adopts a DVI system to obtain its license from the Korean regulatory organization
This paper focuses on the analysis and understanding of the major thermal-hydraulic phenomena of core quenching, down-comer boiling, ECC bypass, and steam binding during the ATLAS reflood test program for APR1400
Summary
Some advanced pressurized water reactors such as the APR1400 [1] have adopted the new safety feature of a DVI (Direct Vessel Injection) system that supplies the ECC (Emergency Core Coolant) directly into the reactor vessel down-comer. A thermal-hydraulic integral effect test facility, ATLAS (Advanced Thermal-hydraulic Test Loop for Accident Simulation) [2], was constructed at KAERI (Korea Atomic Energy Research Institute) and has been operated to provide reliable data to help validate the safety analysis methodology for APR1400. It is based on the design features of the APR1400, an evolutionary pressurized water reactor developed by the Korean industry. The conservative condition is acquired from the EM (Evalulation Model) approach, which assumes an atmospheric containment pressure and a 120% ANS-73 decay curve, while the best-estimate condition is acquired from the BE (Best-Estimate) approach, which assumes an higher containment pressure of about 0.2 MPa and 102% ANS-79 decay curve
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