IR 05000324/1981018
| ML20032B566 | |
| Person / Time | |
|---|---|
| Site: | Brunswick  |
| Issue date: | 10/05/1981 |
| From: | Jape F, Kelley W, Whitener H NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II), NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| To: | |
| Shared Package | |
| ML20032B561 | List: |
| References | |
| 50-324-81-18, 50-325-81-18, NUDOCS 8111050668 | |
| Download: ML20032B566 (6) | |
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UNITED STATES o,,
NUCLEAR REGULATORY COMMISSION o
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I REGION 11
101 MARIETTA ST., N.W., SUITE 3100 o,
A TLANTA, GEORGIA 30303
% ***** s Report Nos. 50-324/81-18 and 50-325/81-18 Licensee: Carolina Power and Light Company 411 Fayetteville Street Raleigh, NC 27602 Facility Name: Brunswick Docket Nos. 50-324 and 50-325 License Nos. DPR '1 and DPR-62 Inspection at Brunswick site near Southport, North Carolina, Rockwell International Plant in Raleigh, North Carolina, and Carolina Power and Light Company Laboratory in Raleigh, North Carolina.
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Inspector :
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Dat6 Sig'ned L' YRe g no u,L Q ~ee
. Whitener
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W. D. Kelley, Vendor (nspfction Branch, Region IV Da e Signell
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Approv d by:
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F.' Jape, Section %ction Branch Date Signed eff Engineering Inspe Engineering and Technical Inspectior. Division SUMMARY Inspection on July 21-25, 1981, at the Brunswick Plant; August 3-8, 1981, at the Rockwell International Plant; and August 12, 1981, at the Carolina Power and Light Company Laboratory.
Areas Inspected This special, announced inspection involved 66 inspector-hours on site in the investigation of main steam isolation valve failures.
Results Of the area inspected, no violations or deviations were identified.
8111050668 811008 PDR ADOCK 05000324 O
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REPORT DETAILS 1.
Persons Contacted Licensee Employees C. Dietz, Plant Manager
- R. Morgan, Plant Operations Panager
- W. Tucker, Technical Supervisor
- M. Hill, Maintenance Superintendent
- E. Bishop, Engineering Supervisor S. Grant, Principal Engineer, CP&L Harris Center Lab
- J. Boone, Project Engineer E. Cathey, Engineer G. Locklear, Senior Generation Specialist S. Bohanon, Regulatory Ccmpliance
- R. Poulk, Regulatory Compliance Other Organizations Rockwell International
- J. V. Grasso, General Plant Manager B. Milleville, Senior Technical Advisor
- R. A. Bandukwala, Manager, Quality Assurance J. P. Tucker, Product Specialist
- S. L. Adams, Supervisor, Project Engineering N. West, Field Service Representative NRC Resident Inspector
- D. Johnson, Senior Resident Inspector
- Attended exit interview July 25, 1981
- Attended exit interview August 13, 1981 2.
Exit Interview The inspection scope and findings were summarized on July 25, 1981 at the Brunswick Plant and on August 13, 1981 at the Rockwell International Plant with those persons indicated in paragraph 1 above.
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Licensee Action on Previous Inspectioc Findings Not inspected.
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Unresolved Items Unresolved items were not identified during this inspection.
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5.
Main Steam Isolation Valve Failure '
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History.
Since Brunswick Unit 2 was licensed in December 1974, there have been six maf r steam isolation valve (MSIV) disc separations. Separation has occurred at the pressure balancing valve between the stem and stem disc on three occasions and between the piston ar:d main valve disc.on three occasions. Of these MSIV disc separations, four have occurred in 1981.
A Region II inspector and a Region IV vendor inspector visited the Brunswick facility on July 21-25 and later the Rockwell International plant in Raleigh, North Carolina, on August 11-13, 1981 to review the cause of repeated valve failures and tha corrective action to prevent future failure.
At this time the only Known Rockwell MSIV disc separation other than at Brunswick occurred at Hatch 2.
The failure experienced at Hatch 2 was due to improper seating of the stem to stem disc pin. A brief summary of MSIV problems at Brunswick is listed below:
(1) July 30, 1976:
Unit 2 steamline
"D" inside isolation valve (F0220) main disc separated from the piston.
The main disc unscrewed from the piston due to improper pin installation. The nole for the lock pin in the piston and di;c had not been drilled deep enough.
(2) January 29, 1979: Unit 2 steamline "A" inside isolation valve (F022A) stem disc separated from the stem.
The stem disc
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unscrewe( from the stem after the locking pin had failed due to fatigue cracking.
It was noted during inspection of tnis valve that the main disc to piston connection was loose but not separated.
A number of :abservations indicate that torque is occurring in the valve which tends to unscrew the threaded connections as follows:
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Circumferential gall marks on top surface of the stem disc,
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elongated locking pin hole in the stem and stem disc,
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wear rings on valve body bore,
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main disc to piston locking pin deformed in a manner indicating attempted rotation in a direction which would unscrew the connection.
(3) January 1979 refueling outage:
Unit 2 steamline "0" isolation valves F0220 and F028D were disassembled due to failing the local leak rate test.
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In valve F0220 the threaded correction between the sten and stem disc was loose indicating a loss of torque preloading. The cross sectional area of the locking pin had been reduced in the range of 30 to 50 percent at the stem / stem disc interface. In valve F0280 a similar condition was found to that in F0220. The stem to stem disc locking pin was deformed. The main disc to piston assembly connection was tight in both valves..
(4) January 15, 1981:
Unit 2 steamline
"C" outside isolation valve (F028C) main disc separated from the piston.
The main disc unscrewed from the piston.
In this case it appeared that the disc to piston lock nin had not been installed.
(5) March 30, 1981:
Unit 1 steamline
"C" insid
. solation valve (F022C) stem disc separated from the stem. ihe locking pin was not rLovered but damage to the pin hole indicates that the pin had been installed.
(6) July 2, 1981: Unit 2 steamline "C" inside isolation valve (F022C)
stem disc separated from_ stem.
The locking pin was present but was rounded on the end rather than bevelled as a new pin would be.
Also the locking pin hole in the stem which should be h-inch deep was only about 1/8-inch deep.
(7) July 17, 1981:
Unit 2 steamline
"D" inside isolation valve (F0220) main disc separated from the piston.
The locking pin broke out a section of the main disc hub.
Inadequate thread engagement appears to be the failure mechanism.
(8) July 21,1981: Spare parts manufactured by Rockwell and stored on site were inspected on site. The threads on three of four stems and two of two stem disc were found to be out of drawing specifi-cation limits in a nonconservative direction.
This matter was reviewed at the Rockwell plant and the results are reported in IE Report No. 99900058/81-02.
A Rockweil International Technical Report dated August 7, 1981, indicates that for items (5) and (7) above, the separation of the threaded joints occurred without unscrewing.
In item (6) it is not clear whether separation occurred from unscrewing or pulling straight ou.
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Failure Mechanisms Rockwell engineers stated that the threaded connections in the valve are designed to be self-lockirg connections under the applied torque preloading. Pinning is an aati-rotational device only. The pin is not-intended to sustain valve operational forces.
Examination of valve components and review of maintenance records indicates that several factors may contribute to eientual failure such as: (1) loss of-preloading, (2) lack of thread engagement, (3) inadequate pinning, and (4) flow induced vibration.
The engineering evaluation of the cause of failure based on available data postulates that a loss of preloading on the threaded connections is occarring. The preloading is obtained by applying torque to the v eaded connection in order to pull the stem disc tightl1 against a 11 shoulder on the stem. This preloading may be lost momentarily during valve operation or as a result of flow induced vibration. Loss of the preload will result in movement between the threaded pieces and eventual looseness of the threaded joint due to thread wear. Thread wear will ultimately lead to axial disc separation due to lots of thread engagement Also, if the pin engagement is inadequate or if pin failure occurs, vibration induceds rotation of the threaded parts will cause disc separation. As indicated in paragraph 5.a., these conditions and failure modes have been observed.
Conditions which support this failure analysis are as follows:
(1) The presence of vibrational forces from turbulent steam flow is postulated due to the close connection uf pipe elbows to the inside isolation valves.
Five of the six disc separations have occurred on inside isolation valves. In the one disc separation on an outside isolation valve failure was due to unthreading. It appears that a pin was never installed to prevent rotation.
(2) Pin Engagement:
As indicated in paragraph 5.a., poor pin engagement such as misalighment of pin and hole, lack of depth in hole, and failure to install a pin has been identified with valve failures.
Although the pin is not intended to be a force sustaining device, it is clear from pin wear and deformation that it does retard rotation of valve parts.
(3) Thread Engagement:
Lack of thread engagement due to undersized threads would accelerate failure from thread wear because of the presence of less thread mateiial. Undersized threads on installed valve compr.nents have not been confirme _,
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Based on-the above analysis the licensee, in~ conjunction:with vendor
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. engineers, has developed an immediate corrective action plan for repair-of the failed valves. The plan includes verification of proper. thread engagement by dimensional checks of threaded parts, and improvement of-
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pin engagement by increasing the number,_ length and diameter of pins and increasing the depth of pin holes.
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Ensure proper thread engag' ment by dimensional checks of threaded parts.
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Ensure proper pin engagement by increasing the number ~, length and diameter of pins and increasir g the depth of pin holes..
Permanent corrective action to prevent recurrence of the failures is being pursued with the valve manufacturer,
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Safety Evaluation Prior to returning the plant- (Unit 2) to. operation, the Plant Nuclear Safety Committee performed a safety review and concluded that the plant could be safely returned to operation. This review. included considera-tion of the following:
(1) The transient resulting from the rapid closure of one MSIV is bounded by the FSAR analysis.
(2) CP&L and Rockwell engineers conclude that for either stem disc or main disc separation the main disc will seat.
(3) Seating of the main disc causes a loss of steam flow which will be evident to the reactor operator who will take action to close the second isolation valve and isolate the affected steamline.
(4) Although increased leakage may occur through the stem-disc seat of the failed valve, closure of the second valve 'will isolate the affected line.
The NRC concluded that the licensee's evaluation was adequate to permit continued plant operation for an interim time until the permanent corrective action can be determined and implemented. However, should an additional MSIV failure occur, the NRC will re-evaluate the corrective action and continued reactor operation based on the conditions at that time.
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