Simulated AP600 response to small-break loss-of-coolant-accident and non-loss-of-coolant-accident events: Analysis of SPES-2 integral test results

As part of the AP600 design certification program, a series of component separate effects tests and two integral systems tests of the nuclear steam supply system were performed. These tests were designed to provide data necessary to validate Westinghouse safety analysis codes for AP600 applications. In addition, the tests have provided the opportunity to investigate the thermal-hydraulic phenomena that are expected to be important in AP600 transients. One series of integral systems tests was undertaken on the SPES-2 facility in Italy, a full-height, full-pressure, 1/395th-power and -volume scale simulation of the AP600 nuclear steam supply system and passive safety features. A series of thirteen design-basis events were simulated at SPES-2 to obtain data for verification and validation of the computer models used for the safety analysis of the AP600. The modeled initiating events included a series of small-break loss-of-coolant accidents (SBLOCAs), single steam generator tube ruptures, and a main steam-line break. The results of the analyses of the SPES-2 test data, performed to investigate the performance of the safety-related systems are reported. These analyses were also designed to demonstrate, through mass and energy inventory calculations, mass and energy balances, and event timing analyses, the applicability of the SPES-2 tests for computer model verification and validation. The key thermal-hydraulic phenomena simulated in the SPES-2 tests and the performance and interactions of the passive safety-related systems that can be investigated through the SPES-2 facility are emphasized. The latter includes the impact of accumulator nitrogen and nonsafety-related system actuation on the passive safety-related system performance. It is concluded that the key thermal-hydraulic phenomena that characterize the SBLOCA and non-LOCA transients have been successfully simulated in the SPES-2 facility, and the test results can be used to validate the AP600 safety analysis computer codes. The SPES-2 tests demonstrate that the AP600 passive safety-related systems successfully combine to provide a continuous removal of core decay heat. The SPES-2 tests also showed no adverse interactions between the passive safety-related system components or with the nonsafety-related systems. In particular, it was found that the effect of noncondensable nitrogen on passive safety-related system performance was negligible.