Containment venting after severe accident: its effect on doses at exclusion boundary

The Containment Filtered Venting System (CFVS) is proposed to be used for management of severe accident scenario in Indian PHWRs. The CFVS has twin design objectives, one, to depressurize the containment for maintaining containment integrity and two, to remove the radioactivity confined in the containment during severe accident prior to release to the atmosphere. This paper presents assessment of the radioactive doses for a postulated severe accident initiated by LOCA with simultaneous failure of ECCS and moderator cooling in a 540 MWe Indian PHWR. The study compares a case of severe accident scenario without containment venting action with case when containment venting is done early (when ILRT pressure is reached) and case when containment venting is done later (when containment design pressure is reached) in the accident progression. This paper highlights effect of containment venting on doses. It is observed that final dose depends on the balance between amounts of water soluble activities (radioactive iodine) trapped in scrubber tank before venting and amount of Noble Gases released to atmosphere during containment venting. The case of venting early in accident progression can cause higher adult effective dose as compared to situation where no venting is done. This is due to release of Noble Gases which were otherwise trapped in highly pressurized containment. (author)