The objectives of the work described in this report are to prepare a comprehensive list of fault causes and to estimate the relative contribution made by each to the probability of severance in reactor primary piping. Severance is regarded as the sudden failure of a pipe without prior detectable leakage, with either circumferential or axial opening of substantial area. A fault cause is an event in the course of design, fabrication or operation of the piping system which ultimately proves to be the cause of some fault contributing to piping severance These faults are principally an increased actual stress, decreased critical stress for severance due to unfavorable fracture properties of the material, increased crack frequency or growth rate, and decreased crack detection capability.
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