Development of MELCOR thermal hydraulic model of AP1000 and its verification for a DECL break

Abstract The AP1000 is currently an advanced pressurized water reactor with emergency core cooling systems (ECCS) completely dependent on natural circulation. It is under construction in the USA and has been commissioned in China and is being considered by several countries to use this advanced technology for a power source. With this objective in mind, a representative MELCOR model (for the AP1000) has been developed to better understand the thermal hydraulics involved and to assess the performance of the safety systems for a design basis accident such as large break LOCA (LBLOCA) with availability of the safety systems. The results from these calculations were compared to those obtained using WCOBRA-TRAC, developed by Westinghouse Electric Company, which are reported in the AP1000 European Design Control Document. MELCOR was selected for the design assessment activities specifically because of its capabilities to model severe accident scenarios and features such as containment behaviour. It is one of the system thermal hydraulic codes, which are recommended for severe accident analysis in light water reactors. The aim of the current study is to assess the thermal hydraulic models and correlations in MELCOR against a specific code developed for a design basis accident such as a LBLOCA. This exercise will validate the input model for a LBLOCA and then use the input model for station blackout studies and the impact on the containment. A double ended cold leg (DELC) break in the loop containing the Core Makeup Tanks (CMT) is considered for this study. The calculated break flow, peak cladding temperature, accumulator, and CMT injection flow rates are compared to those reported. These calculated results are found to be satisfactory according to ECCS acceptance criteria for the temperature and hydrogen generation. This study has been further extended to analyse the containment behaviour during the accident. The calculations for the transient were extended to observe the behaviour of the Automatic Depressurization System (ADS) and IRWST injection capabilities which can effectively avoid core uncovery for a longer duration of the time. These systems can only work when the reactor system is depressurized. MELCOR calculated a PCT value (occurs just after the break) as 1242 K while the total hydrogen produced was calculated to be 2.216 kg which is only 0.61% of the total potential hydrogen production from oxidation.