Penetration through operational leaks in the containment steel shell

1 Penetration through operational leaks in the containment steel shell 

In safety analyses it is frequently assumed that the rate of penetration of fission products through the usually very narrow slits would be identical to the leak rate that would be measured using air as a penetrating medium. To be sure, this assumption is true for the fission product noble gases for airborne aerosols, however, narrow slits can act as a trap, so that under otherwise identical conditions aerosol leak rates will usually be lower by some orders of magnitude than the air leak rate. Early observations in this field showed that the presence of steam in a heated vessel atmosphere results in a drastic decrease of the leak rate to values that are well below those measured with dry air at ambient temperature (Witherspoon, 1970). Likewise, quick plugging of natural leak paths was observed when higher steam contents were introduced into the containment atmosphere under such conditions, even artificially made leaks in the vessel effected an appreciable retention of both airborne iodine and particles (Witherspoon and Postma, 1971). 

Similarly, elemental iodine h as well as HI can be expected to be retained during passage through narrow slits. With steel materials, the mechanisms discussed in Section 7.3.3.4.7. will be effective, and revolatilization of deposited iodine can be assumed to be small at the comparatively low temperature of the containment 

These results mean that penetration of fission products (except the fission product noble gases) through operational leaks in the containment shell in the course of a severe reactor accident will be very small. The largest fraction of the penetrating fission products will be plated out in the adjacent rooms and compartments with the condensing steam, while the still airborne fraction will be transported to the filters of the annulus air extraction system. Fission product iodine plated out into the water pools formed in the annuli will be subjected to the reactions to be discussed in Section 7.3.4.3. 

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