Laboratory examinations of OTSG tubes from Crystal River Unit 3

Portions of eleven tubes were removed from the ''B'' OTSG at Florida Power Corporation's Crystal River Unit 3 (CR-3) during Refuel 8 (June 1992) and Refuel 9 (April 1994) and subjected to detailed laboratory, evaluations. Six of the seven tubes removed in 1992 contained low-voltage eddy current indications in the free-span region between the lower tubesheet secondary face (LTSF) and the first tube support plate (TSP). The four tubes pulled in 1994 contained volumetric eddy current indications at the 7th and 9th TSPs. The objectives of the laboratory examinations were (1) to physically characterize any observed tube degradation for correlation with the field eddy current data, (2) to determine the effect of such defects on the pressure-containing capability of the tubing, and (3) to establish the damage mechanism(s). The low-voltage, first-span eddy current indications were caused by small, relatively shallow, isolated patches of OD-initiated intergranular attack (IGA). The IGA was associated with a non-uniform deposit pattern, concentrated in an area 8 to 18 inches (∼20 to 46 cm) above the LTSF. The IGA was attributed to low-temperature, reduced-sulfur attack, which most likely occurred early in plant life and is no longer active. Due to their small size, the IGA patches (< 0.100 inches [2.5 nun] long) had very little influence on the burst strength of the tubing. The volumetric eddy current indications at the 7th and 9th TSPs were caused by mechanical wear from repeated contact and rubbing against the TSP lands. Two forms of wear damage were observed: (1) pit-like wear at the bottom edges of the 7th TSPs and (2) tapered wear at the top and bottom edges of the 9th TSPs at adjacent land contacts. The wear damage also resulted in very little degradation of tubing burst strength