DC-Power availability analysis with MELCOR: insights on a SBO in a 3-Loop PWR-W NPP

The Fukushima nuclear accident highlighted the fact that external events can significantly impact the restoration of AC power supply at nuclear power plants. This can result in longer recovery times than initially anticipated. As a result, it may be necessary to evaluate Extended Loss of Alternating Current Power (ELAP) conditions and determine whether existing batteries can operate for a sufficient duration to support core cooling.

In this study, an evaluation model of a generic 3-loop PWR-W has been developed with the MELCOR code following the state-of-the-art best practices. The evaluation model has been employed for studying the impact of the DC power durability on the plant progression during an eventually station blackout with no mitigation during the first 24 hours. A total of 5 cases has been analyzed assuming different battery capacity and assumptions related with the availability of the auxiliary feed water system.

As a result, it can be said that the duration of the batteries is relevant to the timing of the main events during the transient. The larger the batteries capacity the longer it takes to get the core damaged. However, since no mitigation measures have been assumed, this statement is limited by other factors such as the CWST capacity. There is a threshold where the DC power becomes irrelevant since there is no water to inject into the steam generators, leading to the inevitable core damage. In addition, although the cooling capability by means of the turbo powered AFWS extends the time available for acting with FLEX measures, it has to be accounted that an uncontrolled water injection in the secondary could lead to the failure of one of the SGs, initiating an MSLB event in the containment, converting the scenario in an asymmetric transient in the RCS, and loosing 1/3 of the core cooling capability. This MSLB is followed by a SB-LOCA produced through the PZR safety valves when the heat sump is lost, releasing the RCS inventory in the lower part of the containment, and finishing with a LB-LOCA as a consequence of the hot leg creep rupture. In those conditions, the containment pressure could surpass the design limits producing an eventually radionuclide release to the environment.

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