This issue addresses the possibility of a causal relationship: a main steam or feedwater line break in an unisolable portion of the secondary system is postulated to cause a number of SG tubes to leak or rupture. Moreover, a SGTR is assumed to occur spontaneously in just one tube. However, these accident initiators are generally assumed to occur independently unless there is severe core damage.

Main steam line break and steam generator tube rupture (SGTR) are both included as design basis accidents in Chapter 15 of most FSARs and the SRP, 11 and are addressed as accident initiators in most plant-specific PRAs. Such leakages, concurrent with containment bypass, might cause offsite radiation doses in excess of 10 CFR Part 100. The resulting SG secondary side blowdown could further increase tube leakage due to resonance vibration within the affected SG tube bundle. (2) Significant SG tube leakage could lead to secondary system breaches from a variety of causes.

(1) Operating experience and design information suggested that the potential existed for a line breach to significantly increase SG leakage, because resonant vibration of SG tubes from a secondary side blowdown could cause increased tube leakage. The issue raised the following two potentially risk-significant events that are not fully addressed as design basis accidents in FSARs, industry analyses, the SRP, 11 or staff reviews: The issue is related to Issue163, 'Multiple Steam Generator Tube Leakage.' The related technical issues include the ability to correctly predict SG secondary side thermal-hydraulic behavior, physical loadings, component response, resonance vibrations within the tube bundles, eddy current testing, iodine spiking, operator response, and risk. The concern is that an unisolable secondary system opening outside containment coupled with multiple steam generator tube leaks or ruptures could result in releases in excess of 10 CFR Part 100. This issue was identified when it was believed 1799 that the validity of steam generator (SG) tube leak and rupture analyses could be affected by resonance vibrations in steam generator tubes during steam line break depressurization.

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