In October 2001, during a bobbin eddy current inspection (ECT) of steam generator tubing at AmerGen's Three Mile Island (TMI), Unit 1, located in Pennsylvania, wear indications were discovered in the B generator on four tubes at the secondary face of the upper tubesheet. The four tubes surrounded a tube that previously was plugged.

Since the wear indications were significant, the ECT analysts surmised that the cause might be a severed tube. Once the plug was removed from the upper tubesheet, a visual inspection confirmed that the tube had indeed severed at the upper tubesheet secondary face.

To understand the cause, AmerGen decided to remove tube samples (tube harvest) so that a destructive examination could be done for failure analysis. Framatome ANP received a call on Friday, October 19, 2001 asking for help. Since this was a first-of-a-kind (FOAK) effort, a team was quickly formed and a preliminary plan was mapped out to accomplish the tube harvest.

The plan involved removing the SG secondary manway cover and elliptical wrapper cover to gain access and allow removal of tube sections from the secondary side. Cutting the sections would take place from the primary side but retrieval and accountability of the pieces would be from the secondary side.

The team developed tooling, mockups, video and other support items in 4 days. The site team went through training in Lynchburg and the final execution plan was completed in 3 days. One week after the initial request, the team deployed to site to begin equipment staging and setup, and site training.

Special remotely operated tool designed especially for the TMI tube harvest.

Close-up of the clamp on the special tool to hold the tube in place during the operation.

Tube section removal work began on October 28th. The team encountered stuck studs on the secondary manways, and the studs on the elliptical wrapper cover had never been removed since plant startup. The team also discovered tube sections that had swelled considerably larger than design. Furthermore, the non-harvested section of the tube had to be left in a condition suitable for future operation. The sections removed were ECT tested as a baseline for the laboratory destructive exams including metallurgical evaluation and pressure testing. The entire process, even with the stuck studs and swelled tubing, was completed in two days.

The process removed all of the planned tube sections except one; the only section not removed was a 4" (10 cm) long section of the tube within the upper tubesheet region. This section was roll expanded in place to capture it and prevent it from becoming a loose part. Based on subsequent testing and analysis of the other sections removed, it turned out that removal of this 4" section was not necessary. Testing and analysis revealed the cause to be attributed to a combination of three factors; 1) Tube expansion from water within the tube, 2) OD intergranular attack (IGA) and 3) Flow-induced vibration from high cross flow. Identification of the cause of this failure was key to AmerGen's plan for mitigation that allowed for the plant's return to power operation.

Rich Freeman, steam generator program engineer at TMI, commented, "Framatome did a very good job of preparing and executing the removal of these tubing sections from the secondary side of our generator. Removal of these sections was important and helped us to discern the cause of our tubing degradation."