Long Term SBO With Selected Mitigative Measures: MELCOR Parametric Calculations for a 2-Loop PWR

Analyses of three long term PWR Station Black-Out (SBO) scenarios with and without mitigation are performed using the US NRC source term code MELCOR. Refilling of the secondary side of the steam generator (SG) with fire water pumps was previously identified as a potentially important accident management measure. To assess parametrically the effect of the restored availability to refill of SG secondary, SBO sequences are analyzed both without any mitigation and with mitigation at different times into the accident representing different stages of the accident progression. The scenarios studied were (i) base-line SBO without any thermally-induced Reactor Coolant System (RCS) breach, and then sequences with assumption of (ii) Surge Line failure and (iii) thermally-induced SG tube rupture (SGTR). A detailed model was used for the description of flows in the hot legs and in SGs to enable us to simulate the counter-current natural circulation which is inherent in these types of scenarios, with the cold leg plugged by water in the loop seal. These were long-term simulations, some of them up to 5 days of the transient, with the analyses of the containment response and fission product (FP) releases to environment. For the relevant cases, the molten core-concrete interactions (MCCI) are modeled in detail in the complex lower containment geometry with several distinct volumes (cavity and other rooms with concrete walls) subject to ablation by the molten corium; the challenges to containment barrier by the late overpressurization and by concrete ablation are evaluated. With the Passive Autocatalytic Recombiners, PARs, installed the hydrogen is found to be of less threat to containment integrity. For the thermally-induced SGTR the impact of the chosen mitigation strategy on potential bypass FP release to environment is also assessed for different times of the refill. The results indicate that mitigation by the SG secondary refill can be very effective for the base-line SBO sequences. For the thermally-induced SGTR it is effective only when the refill is achieved early after the SG tube failure. Mitigation by the pressurizer-loop SG refill was much less successful in the case of the Surge Line failure sequences.