An Assessment of the Effectiveness of Potential Improvements for Ice Condenser Containments

Abstract

Using Sequoyah as a representative plant, the CONTAIN severe accident analysis code has been employed to evaluate the ability of several proposed containment improvements to reduce the threat to containment integrity associated with direct containment heating (DCH) and hydrogen combustion in ice condenser plants during station blackout accidents. Calculations were performed using 4-cell, 6-cell, and 26-cell representations of the containment. Potential improvements considered included containment venting, improved igniter systems (including backup power supplies), containment inerting, subatmospheric containment, reduced ice condenser bypass, backup power supplies for the air return fans, and intentional depressurization of the reactor coolant system (RCS). The most effective improvements considered were inerting of the containment atmosphere, and the combination of depressurization of the RCS and provision of backup power for the igniter systems. Either of these approaches could substantially reduce, though perhaps not totally eliminate , the threat to containment integrity associated with DCH and hydrogen detlagrations. The other improvements considered offered at best only moderate mitigation, which would probably be insufficient to substantially alter the contribution of DCH to risk in PRAs such as those reported in NUREG-1150. With or without partial depressurization, calculations indicated that, in station blackout accidents, highly detonable gas mixtures were likely to form in the ice condenser region. Providing backup power supplies for the existing igniter system and for the air return fans substantially reduced the detonation threat, but still did not totally eliminate it. Installing additional igniters in the ice condenser and the lower plenum regions (which currently lack igniters) may be the most effective strategy, other than inerting. Sensitivity studies were performed using the NUREG-1150 PRA methodology and models to estimate the potential impact of the improvements considered upon the overall Sequoyah risk profiles for early fatality and latent cancer fatality potentials. Depressurization combined with hydrogen control offered an approximately three-fold reduction in the contribution to the mean risk potentials associated with early containment failures; other improvements considered were less effective. However, containment bypass accidents are also important risk contributors at Sequoyah, and these sequences were largely unaffected by the containment improvements considered here. Hence, reductions in total mean risk potential were about 30% to 40% at best.