ML20212G250

Discusses through-wall Cracks Found in Main Steam Bypass Lines at Facility in Sept 1985.Mods,admittedly Compromised Because of Physical Limitations & Cost,Appear to Have Been Done W/O Going Significantly Beyond Applicable ASME Codes

ML20212G250
Person / Time
Site:	Fermi
Issue date:	01/12/1987
From:	Lainas G Office of Nuclear Reactor Regulation
To:	Adensam E Office of Nuclear Reactor Regulation
Shared Package
ML20209J145	List: ML20209J138 ML20212G250
References
TAC-63376, NUDOCS 8701130428
Download: ML20212G250 (3)

Text

1. b o UNITED STATES NUCLEAR REGULATORY COMMISSION 8 /\ k),'J(/[,,n

1. +, u E WASHINGTON, D. C. 20555 f* N .12 9991 Docket No.: 50-341 MEMORANDUM FOR: Elinor G. Adensam, Director Project Directorate No. 3 Division of BWR Licensing FROM: Gus C. Lainas, Assistant Director Division of BWR Licensing

SUBJECT:

FERMI UNIT 2: MAIN STEAM BYPASS LINE FAILURE (TAC #63376)

Introduction In September 1985, Detroit Edison (DECO) discovered through-wall cracks in two main steam bypass lines at Fermi Unit 2. Subsequently, DECO replaced, both lines with a thicker wall pipe, redesigned some of the pipe supports, and analyzed the piping vibration. DECO has identified the root cause of the problem to be acoustically induced flutter of the pipe wall. On December 11, 1986 a meeting was held in Bethesda, Maryland for DECO to present their assess-ment of the problem and corrective actions that they have taken to resolve the issue. The Engineering Branch and Region III staff have evaluated this issue from the viewpoint of sound engineering practices rather than from assessment of meeting license requirements since the lines in question are a part of balance of plant. The assessment by the staff is to critique Deco on the basis of performance for this issue as an indicator of how they might address a similar problem in the " safety-related" part of the plant.

Both bypass lines (the West line and East line) are made of 30 inch diameter carbon steel pipe (API SL Grade B) and are reduced to 24 inch diameter at the inlet to condenser. The cracks were found at various locations on the pipe

! surface including, pipe support lug attachments, small test connections, and flow measurement orifices. DECO replaced the original 0.375 inch thick pipe with a 1.0 inch thick pipe for the 30-inch diameter section and 1.25 inch j thick pipe for the 24-inch diameter section. DECO also eliminated all pipe

weld attachments. As a part of the modification, DECO conducted a test program to obtain as-built stresses and strains at various locations of the lines and

, contracted Hopper and Associates and Stone and Webster to predict the service i life of the pipe using the test data. After resolutions of disagreements

Contact:

4. Tsao x29408 s

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Elinor G. Adensam j l

1 regarding the fatigue life of the pipe predicted by the two contractors, DECO I concluded that the west line is acceptable if cumulative operation with bypass valve position between 30% and 45% opened for a period not to exceed 100 days, i and that the east line can be operated indefinitely. l Discussion and Evaluation i

The staff believes the licensee has properly identified the root cause of this

} vibration as acoustically induced pipe wall fluttering resulting from high

{ pressure drop across the bypass dump valve seat. The dump valve opening is i about 9 inches whereas the pipe opening is 28 inches. The supersonic steam l entering the pipe is choked at the valve exit. When the steam is expanded

! into the larger pipe, the flow becomes turbulent and generates the acoustic j vibrations.

I

The modification that has been performed by the licensee has not been success-j ful in elminating the acoustically induced vibration from the piping system.

1 The arguments presented by the licensee on alternative designs and why they

! could not be implemented at the facility appear plausible. However, in the j design of the modification, it does not appear that sufficient considerations

were given to this system with its inherent vibration problem. For instance, j several supports on the system have cracked and nuts on clamps have fallen j off. We understand that fillet welds rather than full penetration welds were used in the support design. Although this practice would meet the usual code l requirements for supports of this type for power pipings, we believe that the i fillet welds, with their inherent stress raisers, were not appropriate for

this application. Further, no special care appears to have been taken by the licensee to preclude loosening of bolting in the system. As with the supports, no special considerations were taken with regard to the circumferential weldments

in the piping. Weldments were made using backing rings. This type of joint is i less desirable than an full penetration weldment made without a backing ring.

l A joint made without a backing ring would not only have been more readily

! inspectable but also would have been less likely to provide a site for potential

! fatigue crack initiation. With regard to inspection, volumetric examinations

! of the girth weld following fabrication could have provided added assurance l

that discontinuities from fabrication would not be present to serve the fatigue crack initiation sites. Also, no volumetric inservice inspection has been proposed to monitor if service-induced degradation has occurred.

CONCLUSION

, The staff concludes that the licensee has been thorough and effective in identifying l the root cause of the pipe vibration. However, the modification which is i

admittedly a compromise because of physical limitations and cost appear to have been done without going significantly beyond what the applicable ASME Codes would require. The licensee has attempted to reduce some stress raisers but has neglected quite a few others. Further, non-destructive inspections per-formed in past and those proposed by the licensee for the future are minimal.

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JAN 12 E7 Elinor G. Adensam Had this problem occurred in the " safety-related" part of the facility, the staff would have expected the licensee to have been more attentive to design, fabrication and inspection factors discussed above.

t This completes our action on this TAC.

f. ),'

Gus C. Lainas, Assistant Director Division of BWR Licensing cc: R. Bernero W. Houston G. Lainas J. Stefano E. Adensam B. D. Liaw R. Hermann J. Jacobson, R III

0. Danielson, R III J. Harrison, R III J. Tsao

Contact:

J. Tsao X-29408 ee .