STEAM GENERATORS | COVER STORY


be removed from service, repaired or replaced. They follow the NRC’s NEI 97-06, Revision 3, Steam Generator Program Guidelines, which includes performance criteria to provide assurance that tube integrity is being maintained and guidance for monitoring and maintaining the tubes to provide assurance that performance criteria are being met between scheduled inspections. Inspection frequency is governed by the Diablo Canyon


Technical Specifications and the inspection scope and methods are evaluated as part of a degradation assessment made before refuelling outages. It assesses degradation of all components that affect steam generator tube integrity such as tubes, plugs, and tube supports. Tube sleeves are not an approved method of repair at Diablo Canyon. Plugging criteria for removing tubes from service are consistent with the technical specifications. The AMP includes foreign material exclusion guidance and plant procedural guidance includes measures to prevent the introduction of foreign material when access is provided to the primary and secondary sides of the steam generators. A secondary side foreign object search and retrieval effort is conducted when the hand hole covers are removed for maintenance to search for and remove any identified loose parts and foreign material. During each outage when tubes are inspected or


plugged, a condition monitoring and operational assessment is conducted to confirm that the structural and leakage integrity performance criteria have been satisfied.


The objective of the inspections is to ensure that the


components continue to function consistently with the design and current licensing basis of the facility. These inspections, based on the plant’s technical standards, are performance-based, and the actual scope of the inspections and the expansion of sample inspections are justified based on the results of previous inspections. If degradation or evidence of degradation is detected, then more detailed inspections or evaluations are to be performed. The programme procedures reflect these requirements


and outline the inspection programme to detect degradation of tubes, plugs, and secondary side internals, as well as providing inspection frequencies. The inspections


and monitoring are performed by qualified personnel using qualified techniques. A so-called Primary-to-Secondary Leakage Monitoring Program is a further potential indicator of a loss of steam generator tube integrity. A condition monitoring assessment is performed at


the conclusion of each inspection to determine whether inspection criteria is met. An operational assessment, which is a forward-looking evaluation, is used to determine whether the structural integrity and accident leakage performance will be acceptable during the operating interval until the next in-service inspection. Degradation of divider plate assemblies, tube-to- tubesheet welds, heads (internal surfaces), or tubesheets (primary side) may have safety implications. Therefore, all of these components and the steam generator tubes, plugs, and secondary side components are included in the asset management programme. The steam generators at Diablo Canyon – four in each unit – were replaced with Westinghouse Delta 54 models, which contain Alloy 690 thermally treated tubes. The replacement took place in 2008 for Unit 2 and 2009 for Unit 1. The tubing and secondary internals in these units are more resistant to corrosion. There have been no reported instances of cracking in thermally treated Alloy 690 tubes at any plant in the USA. All degradation indications to date are from wear (fretting) due to loose parts, tube supports or anti-vibration bars, or from manufacturing or handling anomalies. A690TT was used as the tubing material for some initial replacement steam generators and some original steam generators in the USA. An NRC information note, Experiences During Recent


Steam Generator Inspections, addressed the importance of recognising potential degradation in new areas and of assessing the significance of indications. At Diablo Canyon the assessment evaluates industry experience as well as that at the plant itself to identify active, relevant and potential tube damage mechanisms. Some of the important features of the Degradation Assessment include choosing techniques to test for degradation based on the probability of detection and sizing capability, establishing the structural limits, and establishing the flaw growth rate or a plan to establish the flaw growth rate.


Above: The Diablo Canyon steam generator aging management programme covers tubes, plugs,


divider plate assemblies, tube-to-tubesheet welds, heads, tubesheets, and secondary side components Photo credit: clayton harrison/Shutterstock.com


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