Analysis of the effect of water ponds on HTR confinement behavior under accident conditions

• Aerosol leakage to environment in a postulated VHTR under very large break accident is analysed.
• Water pool is an efficient aerosol removal system that complements filters.
• Pool scrubbing efficiency is correlated with pool geometrical features.
• Water pools are a passive system of confinement isolation once depressurization was completed.

The HTR/VHTR is one of the six system concept chosen within the Generation IV Forum (GIF). A great deal of research is presently being done worldwide on innovative features of this system among which safety is of major importance. One specific aspect which is deserving attention is the potential use of a confinement instead of a containment. To do so, it should be demonstrated that the confinement approach would be capable of reducing early and late offsite doses.This paper presents an analysis of the performance of postulated HTR/VHTR confinements. Two main configurations have been modeled, which major difference is the presence of pools in the vent pathway to the environment (wet confinement). These pools would provide a passive closure of the confinement once the depressurization is completed. However, it could change strongly the thermal-hydraulics and decontamination capability of the confinement. To illustrate the effect of water pools on accident scenarios and of the pool design on the confinement response, a Very Large Break accident has been simulated with the ASTEC v1.3 code. Results, given in terms of the fraction of radioactive material that would reach the environment, show that water pools strongly change the thermal-hydraulic evolution within the building. Moreover, water pools are efficient aerosol traps which scrubbing efficiency depends on their configuration (i.e., vent cross section and pool submergence). A correlation has been obtained for the decontamination factor (DF) in terms the pool geometrical features. Finally, by assuming moderate filter efficiency, the addition of filters downstream the water ponds would result in total source term attenuation ten times higher than in the case of a dry confinement.