RESIDUAL STRESSES IN OTSG TUBE EXPANSION TRANSITIONS

A section of once through steam generator (OTSG) tubing, approximately 33 cm. long and encompassing the roll expansion region in the upper tubesheet (hot leg), was removed from the Davis Besse Nuclear Power Station in April 1996 for laboratory failure analysis. Rotating pancake coil (RPC) inspection of this region had determined that an axial defect was present in the expansion transition approximately 2.5 cm below the primary face of the hot leg tubesheet. Examination of the pulled tube section identified the defect as an ID-initiated primary water stress corrosion crack (PWSCC) approximately 78% throughwall and 2.3 mm long. Additional axial cracks with less throughwall extent were also observed within the roll transition. During fabrication, OTSG's were subjected to a fullvessel stress relief at 593° - 621°C for ~12 to 15 hours. This practice was shown in early testing to reduce the residual stresses caused by roller expansion. Operating experience for OTSG tubing has demonstrated that it is more resistant to stress corrosion cracking than low temperature mill annealed Alloy 600 tubing used in early recirculating-type steam generators. Thus, the appearance of PWSCC in this tube was not expected. A review of manufacturing records for the Davis Besse OTSG's revealed that this specific tube end had been repaired following stress relief and hydrotest operations. It was not clear from the records, however, if the tube had been rerolled as part of the repair procedure. Based on this uncertainty, a project was initiated to measure the residual stress distribution and cold work in the Davis Besse roll transition and in assorted rolled tube mockups, using X-ray diffraction techniques and finite element analysis. The objective was to determine if the Davis Besse tube had been rerolled following stress relief and thus more likely to experience PWSCC. Results confirm a definite effect of rerolling the tube following stress relief on both cold work and residual stresses. It was further concluded that the Davis-Besse tube data have a high degree of correlation with the reroll data.