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Maine Yankee Atomic Power Company
Steam Generator Sleeving Project 1995

A Summary of Safety Concerns

Re: Maine Yankee-NRC Meeting of September 14, 1995 Steam Generator Sleeving Update

"Laboratory testing of sleeving joints (between alloy 600 and 690) consistently produces cracking in the weld-heat affected zone of the parent tube." (Nuclear Regulatory Commission, 1995b)

1. Circumferential crack growth in the Maine Yankee steam generator tubes results primarily from primary water stress corrosion cracking (PWSCC) and cannot be predicted from simulated testing. Other processes contributing to the damage in the tubes include "outside diameter stress corrosion cracking (ODSCC), intergranular attack (IGA), pitting, denting, and vibration induced wear" (U. S. Nuclear Regulatory Commission, 1995c).

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.

Safety Issue Summary: The NRC and MYAPC locked tube test program clearly document a grossly degraded steam generator with extensive steam tube circumferential cracking due to primary water stress corrosion cracking (PWSCC) and widespread accumulations of sludge deposits (corrosion products) throughout the steam generator. The steam generator suffers additional system wide welding- induced stress damage during the sleeving process, which must be successfully repaired with a post-weld heat treatment process which in itself either reinforces pre-existing far field stress damage in the parent tube or extends the damage further down the parent tube. The successful operation of the repaired steam generator is further undermined by steam tube deformations (bowing and vertical displacement) which occur in the repair process due to extensive preexisting corrosion-derived sludge and copper scale tube deposits. These deformations cannot be repaired. The sludge deposits and scaling are located throughout the steam generator, with a particularly large accumulation of corrosion products (See NRC figure 2) at the tube sheet intersection and smaller deposits athe the horizontal supports. These widespread sludge deposits combined with tube scaling render tests to verify weld efficy useless.

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 energy dinosaur.

H.G. Brack
Director of Center for Biological Monitoring
Hulls Cove, Maine

Back to A Summary of Safety Concerns or Maine Yankee Atomic Power Company

I wish to express my thanks to Uldis Vanags, State Nuclear Safety Advisor, for providing the September 14, 1995 report on the Maine Yankee/NRC Meeting Steam Generator Sleeving Update as well as copies of other correspondence pertaining to the safety issues of the sleeving process.
RADNET editors note: 11/21/96: the bibliographic citations in the Summary of Safety Concerns have recently been changed to reflect the fact that the undated, unreferenced reports sent by the State Nuclear Safety Advisor included an MYAPC sponsored report on the Tube Sleeving Locked Support Testing Program. The only evidence that this was an MYAPC rather than an NRC report is a MYAPC cover letter which was not originally included in the material received from the State Nuclear Safety Advisor. Diagram references in the Summary of Safety Concerns are actually MYAPC and not NRC diagrams and are included in the locked support report. The second citation is also an unreferenced report; a more exact citation will be added if RADNET can ever locate the correct references for this material. The lack of referenced reports (author, title, date, etc.) pertaining to controversial subjects such as the safety of the tube sleeving project at MYAPC point out the fact that some of the most interesting information pertaining to NRC licensees is not always available through standard NRC sources (the public document room and NRC websites), and in fact, as proprietary information, may not be available to the general public at all. The MYAPC Locked Tube Program Report is an example of licensee generated information which the licensee would prefer to not be available to the general public.