Filtered-vented containment systems

During the past year and a half, the potential benefits of filtered-venting containment systems as a means for mitigating the effects of severe accidents have been investigated. The studies so far have focused upon two operating reactor plants in the USA, a large-containment pressurized-water reactor and a Mark I containment boiling-water reactor. A number of design options have been formulated that could be retrofitted to these plants, including single-component once-through venting systems, multiple-component systems with vent and recirculation capabilities, and totally contained venting systems. A variety of venting strategies have also been considered, including both simple low-volume containment pressure relief strategies and more complicated, high-volume venting strategies that require anticipatory actions. The latter type of strategy is intended for accidents that produce containment-threatening pressure spikes. It was found that relatively simple filtered-vented containment systems have the potential for reducing the consequences of some acci accidents significantly. For the PWR, a single-component system could provide enough fission product retention to eliminate early fatalities and reduce latent cancer fatalities tenfold, compared to the consequences resulting from an overpressurization rupture of containment. It was estimated that a state-of-the-art system with that capability would cost about 16 million US dollars per reactor, but it was also concluded that it might be possible to develop less costly systems with comparable capabilities. The as-yet unanswered questions are whether or not a complicated venting strategy is necessary in order to circumvent pressure spikes, and whether or not the competing risks would render such a strategy undesirable. These questions are now being dealt with by conducting experiments and by performing a comprehensive probabilistic risk assessment. (author)