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2. Inserting and welding the new sleeves to repair the circumferential cracks in the 16,000 steam generator tubes in the Maine Yankee steam generator creates residual stresses in the sleeve-tube transition area which then must be heat treated to repair stress damage.
3. Both the sleeve insertion and the heat treatment to repair damage caused by the sleeve insertion create additional "far field post-weld stress in the parent tube structure" (U. S. Nuclear Regulatory Commission, 1995c).
4. Hard rolling the sleeve edges after insertion is an additional source of residual stress damage. To minimize this damage, hard rolling the sleeve edge is done after heat treatment of the damaged weld, but remains a source of both post-weld transition area stress and far field stress damage.
5. Extensive sludge deposits have been documented by the NRC and MYAPC throughout the steam generator. The largest deposits are at the base of the steam tubes at the junction of the tube sheet. Other sludge deposits are associated with the nine horizontal ("eggcrate" and drill plate) supports. The new sleeves (12, 20 and 30 inch) are located at the intersection of the steam tubes and the tube sheet, well below areas of of sludge deposits and stress which have accumulated at these horizontals. The areas of sludge deposits and stess located at these horizontals are not subject to repair in the sleeving process. Not all parent tubing subject to circumferential cracking or other corrosion will be sleeved.
6. MYAPC locked support mock-up tests indicate that sludge deposits combine with copper scaling on the steam tubes to create a locking effect at the horizontal "eggcrate" supports which results in both severe bowing and tube displacement off the vertical during the post-weld heat treatment process. (See NRC Figure 2 & Figure 3.) The MYAPC report indicates that tube bowing and lateral displacement (deformations) occur early in the stress relief process.
7. Shorter 12-inch and 20-inch sleeves are frequently used in areas with the least sludge deposits to lower the frequency of post-weld induced deformations. The sludge deposits, tube scaling, and other corrosive products result in the nonuniform distribution of pressure and temperature which cannot be quantified in pre-test mock-up trials. "Sleeve joint life will be maximized if the joints are made above the sludge pile"(Nuclear Regulatory Commisssion, 1995b).
8. Extensive copper scaling tube deposits project above the sludge deposits, sometimes overlapping into the weld transition areas and complicating the distribution of operational, weld, and post-weld heat treatment stress damage. (See NRC Figure 2.)
9. Additional mechanisms causing microstructural degradation of the older parent tubes are not fully understood and cannot be empirically reproduced in mock-up tests. "Recent experience at operating plants has emphasized the sensitivity of the Alloy 600 parent steam generator tube material to stress corrosion cracking when unfavorable residual stresses are introduced by processes such as sleeving" (MYAPC, 1995).
10. The formation of both sludge deposits and copper scaling tube deposits result from corrosion processes which are also not fully comprehended. Use of the alloy 690 in the new sleeves, which has twice as much chromium as the parent tube alloy 600, doubles corrosion resistance of the new sleeve. The older less corrosion resistant parent tube continues to be the weakest link in a steam generator being subject to all the degradation processes noted in Section 1, as well as the residual stresses from the sleeving process.
11. Welding during the sleeve insertion results in copper scale tube flaking, further increasing sludge deposits and complicating residual stress distribution.
12. Maine Yankee and Westinghouse steam corrosion tests "indicate a properly stress relieved weld has four to five times the expected life of a comparable weld joint" (Nuclear Regulatory Commission, 1995c). All 16,000 welds therefore must be carefully and successfully heat treated to complete a safe sleeving.
13. Ultra sonic testing to analyze tube sleeve welds results in extensive "attenuated backwall signals" due to the copper scaling tube deposits and other corrosive products outside the tube, making it impossible to verify the success of the welds. "Effectiveness of post-weld heat treatments in relieving residual stresses created by the welding process is unknown" (Nuclear Regulatory Commission, 1995b).
14. The MYAPC report concludes that "tube OD (outside diameter) deposits can attenuate backwall signals in structurally acceptable welds." Since successful welds can therefore not be verified with the ultra sonic tests, there is no way to locate defective welds.
15. Defective welds can only be verified by a tube-by-tube preoperational safety analysis. Such a detailed safety check is too time consuming and thus extremely expensive for the licensee, Maine Yankee, in that it would significantly delay the reopening of the plant. The verification of the weld efficacy will only be made as the steam generator is returned to service, or after the first cycle of operation through the use of ECT (eddy current testing).
16. Only full operation of the steam generator will verify
the extent of weld deficiencies, far field stress damage, and other structural
degradations, including bowing and lateral deformations, as well as additional
operational corrosion damage. The confirmation of such damage will only
come in the form of leaks or breaks in the steam generator tubes, some
types of which could lead to a serious nuclear accident.
The sleeving process, which includes the plugging of some steam tubes that cannot be successfully welded, results in a reduction in the reactor coolant flow rate. Future plant operation may approach, equal, or exceed the 9 percent reduction in maximum power which is the cutoff point for the safe operation of the Maine Yankee plant. This reduced reactor coolant flow, combined with possible weld deficiencies, sludge deposits, and microstructural degradation, provide the basis for Maine Yankee's recent controversial application for a reduced power license. The unprecedented restart of such a degraded and decrepit facility is a dangerous experiment which poses a clear and present danger to the public safety in the State of Maine.
The return of this degraded equipment to service in lieu
of replacement with a new steam generator greatly increases the chance
of a substantial nuclear accident and constitutes an act of criminal negligence
by the NRC, the supervisory agency for this obsolete and dangerous nuclear