ML20058F050
| ML20058F050 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 11/19/1993 |
| From: | Marcus G Office of Nuclear Reactor Regulation |
| To: | Rhodef D FURMANITE, INC. |
| References | |
| NUDOCS 9312070299 | |
| Download: ML20058F050 (5) | |
Text
7 fo - 334 Ncntenber 19, 1993 Dr. Dusty Rhodef Quality Assurance Manager Furmanite Inc.
2645 International Parkway Virginia Beach, VA 23452 l
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Dear Dr. Rhodef:
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SUBJECT:
DRAFT INFORMATION NOTICE REGARDING MILLSTONE UNIT 2 REPEATED APPLICATIONS OF LEAK SEALANT i
The U.S. Nuclear Regulatory Commission is planning to issue the enclosed information notice (IN) discussing the repeated applications of leak sealant at the Millstone Nuclear Station, Unit 2.
Please review the draft IN to ensure that the technical information is correct and return any comments you may have as soon as possible.
Your cooperation is appreciated.
If no comments are received by close of business of November 22, 1993, we will assume the technical information in the notice is correct.
If you have any questions regarding this issue, please phone Peter Wen of my staff at (301) 504-2832.
Gail H. Marcus, Chief Generic Communications Branch Division of Operating Reactor Support Office of Nuclear Reactor Regulation
Enclosure:
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4, DRAFT UNITED STATES i
NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C.
20555 November xx, 1993 NRC INFORMATION NOTICE 93-XX:
UNISOLATABLE REACTOR COOLANT SYSTEM LEAK FOLLOWING REPEATED APPLICATIONS OF LEAK SEALANT Addressees All holders of operating licenses or construction permits for nuclear power reactors.
PurDose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to an online leak-sealing process which substantially degraded the integrity of the reactor coolant pressure boundary.
It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems.
However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required.
Description of Circumstantes On May 24, 1993, at the Millstone Nuclear Power Station, Unit 2, Northeast Utilities (the licensee) identified a leak in a body-to-bonnet gasket on valve 2-CH-442.
This valve is a Velan model M08-3543B-13MS, 2-inch gate valve which is used for manually isolating the letdown portion of the chemical and volume control system from the reactor coolant system (RCS) for maintenance and local leak rate testing of containment isolation valves.
The valve is upstream of the automatic letdown isolation valves, and is unisolatable from the RCS.
Between June 4, and August 5,1993, leak sealant was injected approximately 30 times in an attempt to stop the leakage from the body-to-bonnet joint. The first three sealant injections were performed by Leak Repairs, Inc., and the remainder were performed by the licensee and Furmanite, Inc.
The injections had marginal results; after the initial sealing, the valve would start to leak again after a period of time, i
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D IN 93-XX November xx, 1993
.e Page 2 of 4 On the initial leak-sell injection, technicians attempted to install a body-to-bonnet peripheral clamp to provide a boundary for the leak-sealant compound. The clamp could not be instal'ed because the bonnet had been installed 90 degrees away from its proper orientation preventing the clamp from fitting properly. On-line attempts to seal the leak included mechanical peening to enclose the body-to-bonnet joint to prevent injection material from extruding out of the gasket area. A second body-to-bonnet clamp was scheduled to be installed on August 5,1993. The hybrid clamp consisted of two seal-welded bars located on opposite sides of the body-to-bonnet joint, and two strongback bars located on the adjacent sides.
The hybrid clamp was supposed to form an effective sealant boundary to retain the injection material so that subsequent injections would not be required.
On August 5, 1993, leak sealant was injected to obtain the dry valve surfaces required for welding the clamp to the valve body. During the injection, technicians observed that the leakage increased dramatically, from slight seepage to a 3.05-meter [10-foot) steam jet. The technicians also reported that the valve bonnet appeared to lift ano that one body-to-bonnet stud moved.
In response to the increased leakage, the director of Hillstone, Unit 2, ordered a controlled normal shutdown. During the shutdown, the maximum RCS leakage was 16.28 liters [4.3 gallons] per minute.
Discussion When valve 2-CH-442 was disassembled, it was discovered that one stud was broken.
Since the body of valve 2-CH-442 is an unisolatable RCS pressure boundary, the broken stud changed the character of the incident from a routine gasket-related failure to a significant structural failure. ABB-CE metallurgical study indicates that the stud broke in response to loads applied as part of the online leak-sealing process. The most likely potential sources of loads were evaluated.
These likely load sources are:
(1) drilling, tapping, and injection port installation, (2) peening, and (3) injecting.
Inspection revealed that two of the four studs had drill holes in them from the injection port and clamp installation drilling processes.
The broken stud and one other stud each had 0.49-centimeter [0.19-inch] diameter holes that penetrated 0.25 centimeter [0.10 inch] and 0.28 centimeter [0.11 inch] deep, respectively.
The holes did not appear to contribute to the stud failure.
The licensee had intended to limit the injection port locations to low stress zones.
However, the injection ports had actually been drilled in restricted high-stress zones located near the studs.
Later analysis determined that the drilling caused very little stress on the studs.
The leak-sealant injection procedure used at Millstone permits peening between the body and bonnet to prevent sealant extrusion but does not allow peening along the entire perimeter of the valve.
Inspection of valve 2-CH-442
4 A E >N 93-xx.
November xx, 1993
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revealed a significant amount of metal moved by the peening process.
Peening i
essentially obscured the body-to-bonnet interface, leaving a groove-like l
indentation along the split line, and metal was peened so that it was in contact with all four bonnet studs.
In addition, chisel-like marks were s
evident around the leak-sealant injection ports where hand peening was performed. The extent of peening could have developed significant stud loads.
j Because the injection ports were incorrectly located next to the body-to-bonnet studs instead of between them, the load of each injection would be borne by the adjacent stud rather than being distributed over two studs. The-licensee had made no provision to limit the amount of leak sealant injected i
into the valve. As a result, approximately 2.38 liters [0.63 gallon] of leak sealant was injected into the valve. On August 5, 1993, alone, technicians injected 32.8 cubic centimeters [2 cubic inches] of leak sealant instead of the 8.19 cubic centimeters [0.5 cubic inch] specified in the procedure used.
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.The repeated attempts to seal the valve at Millstone Unit 2 indicated that an i
acequate engineering evaluation was not performed.
The.cause of the leak was j
not adequately evaluated to determine if online leak sealing was appropriate.
i The evaluations dia not adequately consider the effects of the sealing process and the borated water on the fasteners. Also, the evaluation did not l
adequately consider the amount or effect of sealant entering the system after repeated injections.
Further, the evaluation did not adequately consider the operational and safety consequences of structural failure of the component or the fasteners during and after the leak-seal attempts. Additionally, the management and quality assurance oversight were not adequate to identify the numerous failures to follow procedures and failures to adhere to engineering
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documents.
Because events such as the one discussed above have the potential to cause a loss-of-coolant accident, it is important that the safety evaluations in support of the use of leak sealants on the primary system components are properly performed in accordance with 10 CFR 50.59 and 10 CFR 50 Appendix B, as applicable. This is especially true when ASME Code Class 1 pressure l
boundary components are involved.
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At this time, the licensee is developing significant new information about the 4
effects of excessive peening. Ongoing laboratory experiments are showing that l
i when body-to-bonnet joints are peened to the point that the edges contact, the l
adjacent bolt can be loaded to failure. This is apparently due to the wedging action of the deformed metal.
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IN 93-XX ovember xx, 1993 i
Page 4 of 4 This information notice requires no specific action or written response.
If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts:
Eric J. Benner, NRR (301) 504-1171 Geoffrey P. Hornseth, NRR (301) 504-2756 i
Charles G. Hammer, NRR i
(301) 504-2791 1
Attachment:
List of Recently Issued NRC Information Notices l
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