Evaluation of MCCI under Various Severe Accidents in OPR1000 using MELCOR code

Abstract

During a postulated severe accident, if the molten materials of the reactor core cannot be retained inside the reactor pressure vessel (RPV), high temperature corium can be relocated into the reactor cavity beneath the RPV and then the molten corium-concrete interaction (MCCI) occurs. If the MCCI is not fully mitigated, ablation of both the concrete floor and sidewalls is expected through an exothermic chemical reaction. This could impose significant challenges on the containment integrity. In addition, during the MCCI, if a large amount of non-condensable gas is generated by concrete erosion owing to its chemical composition of concrete, containment can be pressurized and its structural integrity can be threatened. In this thesis, severe accident code MELCOR, version 1.8.6 was used for simulating the MCCI aspect for the severe accident scenarios; small break loss of coolant accident without safety injection (SBLOCA without SI), station black out (SBO), and total loss of feed water (TLOFW). Sensitivity analyses on MCCI were performed to investigate the effect of several important parameters, i.e., concrete type, debris spreading area, heat transfer model variable, and cavity flooding. For all three cases, in light of the effects of the concrete type, LCS and Generic US concretes including the high content of non-condensable gases in concrete might result in many benefits to cool and stable the molten corium by water ingression into the debris as long as the gases do not cause pressurization of the containment. On the other hand, Basaltic and SRS concretes containing the high content of SiO2 in concrete could be beneficial because it is possible to spread widely the corium so as to reduce the heat flux. In addition, in view of the ablation rate, the ablation depth was found to increase with decreasing spreading area of debris. Especially, in case of the spreading over 25% of cavity floor area, the containment failure predicted depending on the basemet melt-through. Besides, in terms of the effect of the heat transfer parameter and cavity flooding, the sole variation of the conductivity multiplier might slightly affect the heat transfer from the debris to overlying water. However, the boiling parameter by itself might not affect the heat transfer at all. Accordingly, when the default parameters in MELCOR 2.1 was used, axial ablation depth decreased somewhat due to quenching the core debris until the cavity dry-out. Therefore, it is concluded that the concrete erosion characteristics on MCCI could be influenced by a variety of the variable. In the future, based on the sensitivity analysis, the results will be compared directly with experimental results such as CCI-2, CCI-3, and CCI-6 tests to provide framework for assessing the ability of these models to adequately capture the corium cooling behavior and to predict the MCCI phenomenon.