Failure sequence suppression

Fig. 50. The suppression of the formation of failure sequences by blocking of remaining truncated ones 60...

Fig. 51. The use of 3-nitrophthalic anhydride as a blocking agent in the suppression of failure sequences...

During reaction of these blocking agents an acidic function (3-nitro-benzoyl-2-carbox-ylate or benzoyl-2-sulfonate) from the intramolecular anhydride bond is Uberated in its salt form. All blocked defective sequences therefore additionally are marked by this acidic moiety. They can be separated from the end product by ion exchange chromatography, since the acidity of the marker function is much more pronounced than any peptide car-boxylate on terminal or side [168] positions (Fig. 51). Though acetylation likewise suppresses the formation of failure sequences, it effects no facilitated separability of synthetic peptide by-products. 

Accident-sequence development was discussed in Section 2.6.4. The major differences in this step for external events as contrasted with traditional internal events are the addition of external event-caused failures to the fault trees and the increased likelihood of multiple failures of safety systems due to correlations between component responses and between component capacities. There are additional considerations when determining core damage frequencies associated with fires. These considerations include the availability and effectiveness of automatic and manual fire suppression, and the locations of vital equipment with respect to potential fires. Coincident failures of fire protection systems and other systems are also considered. Only a small fraction of the fires that could occur in a nuclear power plant would be expected to lead to core damage. 

An important asset of the Mark III design is construction of the outer containment around the drywell, effectively providing a double layer of protection. If containment failure were to occur, in many cases the outer containment would fail first, leaving the drywell and suppression pool intact. Any subsequent fission product releases would still be scrubbed as they passed through the suppression pool, greatly reducing the source term. Thus, the only accidents (other than bypass sequences) likely to produce large source terms must involve failure of the outer containment plus either loss of the suppression pool or failure of the drywell. 

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