PWSCC was identified in the early 1990’s within the roll expanded region near the primary tubesheet face in alloy 600 tubes of Once Through Steam Generators. Axial tube end cracks (TECs) located behind the tube-to-tubesheet roll expansion joint are of no structural concern, but nonetheless require repair per plant Technical Specifications. Therefore, there was a need to develop a method of permitting affected tubes to remain in-service. Framatome ANP completed an extensive program that was documented in a topical report and was approved by US NRC (1999) to allow these tubes to remain inservice by accounting for primary to secondary leakage under postulated MSLB conditions. Leakage is calculated following each tube inspection, must remain below site specific limits and is based on test results that applied simulated axial loads and tubesheet bow to a tube/tubesheet mockup and measured the resultant leakage though EDM notches. Testing demonstrated that joint tightness is the key parameter which correlates with leakage. Joint tightness is quantified via “delta dilation,” which depends upon axial tube load, tubesheet deformation, and primary side pressure. Test results were used with plant specific delta dilations to develop bounding leakage estimates for various regions of the tubesheet. The bounding leak rate was assigned to each identified TEC and all were summed to determine the total leak rate. Due to its deterministic nature, and its use of delta dilation values which do not reflect the difference between actual plant axial loads and those employed in the tests, this approach produces excessively conservative results. During recent inspections, the number of TECs has continued to increase and continued initiation is expected in both the hot and cold tube end regions. This, coupled with the conservatisms discussed above, led to increases in the number of tube repair rolls required to meet leakage limits. Consequently, the authors undertook an effort to reduce the conservatisms in the leakage determination process by accounting for the differences in tube loading conditions in the testing (Poisson effect) and by eliminating the deterministic leakage calculation approach. A Monte Carlo code (LeakTEC) was developed to perform the calculations in this manner. This new approach was recently approved by the NRC for use at one of the affected plants.

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