Assessment of ex-vessel steam pressure spikes in BWR Mark II containments

This paper addresses the pressure response of a Mark II containment from rapid steam formation during postulated severe accidents. For example, the flow of hot corium from a breached reactor vessel into water can lead to rapid steam generation. The resulting maximum pressure depends on the rate and duration of steam formation, the containment volume, and the relief capacity of venting flow paths. The rate of corium discharge largely determines its progression in the containment and the rate of steam formation when contact with water occurs. If the corium enters a dry region, there is no immediate steam pressure spike. If corium spreads on a floor, arrival at vertical downcomers can provide a flow path to pool water below. The objective of this paper is to provide first order analyses which help display how various parameters affect both the corium progression and magnitude of steam pressure spikes from postulated severe accidents. State-of-the-art corium discharge computations are used to estimate the pressure response of a typical Mark II containment. Results of the analysis indicate that steam pressure spikes are not large enough to cause containment failure