Comparison of integral thermal-hydraulic behaviors of a DVI line break SBLOCA with an equivalent cold leg break

• Effects of break location between the cold leg and the DVI nozzle were investigated.
• The DVI line break case was found to be more limiting than the cold leg break.
• The cold leg break case resulted in more complex behavior than the DVI line break.
• The system code predictions for the water levels were not so satisfactory.

An APR1400 (Advanced Power Reactor, 1400 MWe) adopts a DVI (Direct Vessel Injection) method for ECC (Emergency Core Cooling) water delivery rather than a conventional CLI (Cold Leg Injection) method as an advanced safety feature. Several break scenarios of one DVI nozzle are taken into account in the safety analysis as one of the spectra of small break LOCAs. An 8.5-in. double-ended break of one DVI nozzle was simulated with ATLAS, and a counterpart test for the DVI break was performed at the cold leg with an equivalent break size for comparison. It was found from the counterpart tests and the analysis that the DVI break is more limiting than the corresponding cold leg break. This comparison will contribute to enhancing the comprehensive understanding of the thermal hydraulic behavior during transients. A constructed integral effect database can also be used to validate the existing conservative safety analysis methodology and to develop a best-estimate safety analysis methodology for small-break LOCAs. A post-test calculation was performed with a best-estimate safety analysis code, MARS-KS, to examine its prediction capability and to identify any code deficiencies for thermal hydraulic phenomena occurring during the transient.