Acceptance criteria have been developed for indications found during inspection of reactor vessel in upper head penetrations. These criteria were originally developed for inside surface flaws, as part of an industry program coordinated by NUMARC (now NEI) in 1992. These criteria were not inserted into Section XI at the time, because inspections were not required for these regions. In developing the enclosed acceptance criteria, the approach used by the industry group was similar to that used in other portions of Section XI, in that an industry consensus was reached using input from the operating utility technical staff, each of the three PWR vendors, and representatives of the NRC staff. The criteria developed are applicable to all PWR plant designs. The discovery of leaks at Oconee, ANO-1, and several other plants, have led to the imposition of inspection requirements for head penetration regions, and therefore the need to develop criteria for indications in all portions of the tubes. This would include indications on the inside diameter of the tube, as well as on the outside diameter of the tube below the attachment weld, and flaws in the attachment weld itself. The criteria presented herein are limits on flaw sizes which are acceptable. The criteria are to be applied to inspection results. It should be noted that determination of the period of future service during which the criteria are satisfied is plant-specific and dependent on flaw geometry and loading conditions. It has been previously demonstrated by each of the owners groups that the penetrations are very tolerant of flaws. It was concluded that complete fracture of the penetration would not occur unless very large through-wall flaws were present; therefore, protection against leakage during service is the priority. The approach used here is more conservative than that used in Section XI applications where the acceptable flaw size is calculated by putting a margin on the critical flaw size. In this case, the critical flaw size is far too large to allow a practical application of this approach, so protection against leakage is the key element used to define the acceptance criteria. Also, the use of flaw acceptance standards tables is not allowed for this region, for penetrations which are susceptible to stress corrosion cracking. The acceptance criteria apply to all flaw types regardless of orientation and shape. The same approach is used by Section XI, where flaws are characterized according to established rules and their future predicted size is then compared with the acceptance criteria.
Asymptotic Characteristic of Exact Distribution of Fracture Strength Based on Gamma Distribution as Initial Flaw Size Distribution
