Influence analysis of mitigative measures on evolution of SBO scenario

In this study, the full-range Station Black-out accident scenario for QINSHAN phase II 3 and 4 Unit NPP is performed and analyzed by application of MELCOR code 1.8.5. Effect of mitigative measures such as primary circuit depressurization, passive hydrogen recombination, on evolution of accident phenomena were compared in the calculation analysis. Radial and axial failure mode of fuel and cladding are simulated; Also axial and radial ablation trends of cavity induced by MCCI were quantitatively determined; For the late phase of scenario, production, distribution of hydrogen and its recombination by PAR system were evaluated and analyzed. The results indicate that timely depressurization of primary circuit by execution of pressurizer extended function can delay both core collapse and melt through of RPV for about 5 h, as well as eliminate the risk of early radioactive release through SG U-tube to the environment; Solution 3C indicates the 10 PARs have successfully recombined 667 kg of hydrogen within 24 h. As a result, maximum hydrogen concentration is as low as 3.12%, pressure inside the containment accumulates to about 0.4 MPa. (authors)