ML20115H927
| ML20115H927 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 07/18/1996 |
| From: | Jeffery Wood CENTERIOR ENERGY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 2392, GL-95-07, GL-95-7, NUDOCS 9607230377 | |
| Download: ML20115H927 (4) | |
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l GNTERDOR W
ENERGY 6200 Ook Tree Boulevard Mod Address incependence OH PO Box 94661 216 44T3100 Devetona OH 44101466!
Docket Number 50-346 License Number NPF-3 f
Serial Number 2392 i
July 18, 1996 United States Nuclear Regulatory Commission Document Control Desk Washington, D.C.
20555
Subject:
Response to NRC Request for Additional Information to Generic Letter 95-07, " Pressure Locking and Thermal l
Binding of Safety-Related Power-Operated Gate Valves" Ladies and Gentlemen:
On August 17, 1995, the U.S. Nuclear Regulatory Commission (NRC) issued Generic Letter (GL) 95-07, " Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves," which requested licensees to provide a written response indicating whether or not they will implement the actions requested in the GL and provide a schedule for completion.
As stated in Toledo Edison's (TE) initial response to GL 95-07 (Serial Letter Number 2330), evaluations of pressure locking and j
thermal binding for safety-related power-operated gate valves had previously been performed.
TE provided a follow-up response to GL 95-07 (Serial Letter Number 2360), which included a summary of the methodology used in the evaluations and tables of the valves included in the scope of the evaluation. Modifications for some valves have been implemented to alleviate any valve reliability concerns.
The NRC's Letter dated June 10,-1996, (TE Log Number 4861) requested additional information regarding two questions.
The requested additional information is provided in the attachment' to this letter.
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Cleveland Electnc illuminoting Toledo Edison
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Should you have any questions regarding the information provided by this letter, please contact Mr. James L. Freels, Manager -
l Regulatory Affairs, at (419) 321-8466.
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Very truly yours, t
J. K. Wood Vice President - Nuclear Davis-Besse Nuclear Power Station KNH/ tam Attachment I
cc:
L. L. Gundrum, NRC Project Manger H.
J. Miller,-Regional Administrator, NRC Region III S.
Stasek, DB-1 NRC Senior Resident Inspector i
Utility Radiological Safety Board I
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d QUESTION 1:
The licensee's submittal states that valve RC11, Pressurizer PORV Block Valve, was converted from a solid wedge to a flexible wedge gate valve to address thermal binding concerns.
The NRC staff agrees that, in general, flexible wedge gate valves are less susceptible to thermal binding than solid wedge gate valves. However, flexible wedge gate valves can exhibit susceptibility to thermal binding.
Please provide justification for your view that the new disk configuration is not susceptible to thermal binding. Please provide any diagnostic test results or analysis used to support this conclusion for our review.
RESPONSE TO QUESTION 1:
During return to power following the sixth Refueling Outage in 1990, RC11, a solid wedge gate valve, failed to open during stroke testing. The failure was attributed to thermal binding.
Although RC11 did not have a safety function to open, it was determined that the problem should be addressed from a plant reliability standpoint. A Modification was implemented to replace RC11 with a flexible wedge gate valve, procured to Toledo Edison Design Specification M-212AQ.
The flexible wedge gate replacement valve was a standard design manufactured by Velan.
Flexible wedge gate valves, while not completely eliminating susceptibility to thermal binding, are inherently better than solid wedge gate valves for higher temperature applications and reduce the potential for failure. To further reduce this potential failure mechanism, a drain line is provided to prevent the accumulation of water in this line; thereby, allowing for the presence of steam in the line after RC11 is closed.
The presence of steam in this line should prevent the valve body from cooling to ambient temperature, which in turn should decrease the change in temperature differential across the valve after it is closed, reducing the potential for thermal binding.
In addition, when the new valve was installed, the control circuitry was modified to bypass the torque switch until the valve was seated.
This allowed the closing thrust to be set at a lower value, thus increasing the margin between the closing thrust and the capability to reopen the valve. This makes the valve less susceptible to thermal binding. Also, the stroke i
testing during which the original failure occurred has been successfully performed during subsequent refueling outages.
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Motor-Operated valve RC10, located in the Pressurizer spray line, was replaced with a flexible wedge valve.
This valve, while not identical to RC11, is similar in design and operating conditions.
As part of the qualification testing for RC10, the valve was tested for the effects of thermal binding by heating to over 600*F, closing using the guidance for hot cyclic testing L
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i contained in ASME B16.41, allowing to cool, and reopening.
The success of this testing provides additional evidence of the I
reduced susceptibly of this type valve to the effects of thermal bir: ding.
l QUESTION 2:
Through review of operational experience feedback, the staff is aware of instances where licensees have completed design or procedural modifications to preclude pressure locking or thermal binding, which may have had an adverse impact on plant safety, due to incomplete or incorrect evaluation of the potential effects of these modifications.
Please describe evaluations and I
training for plant personnel that have been conducted for each design or procedural modification completed to address potential pressure locking or thermal binding concerns.
l RESPONSE TO QUESTION 2:
While no plant modifications or procedure changes were made in response to Generic Letter 95-07, Toledo Edison does have in-place modification and procedure change processes that include mutli-discipline reviews.
The modification process includes design development by a cross-disciplinary team of plant personnel, final design development, and interdisciplinary design review. These interdisciplinary reviews generally include Operations and Maintenance personnel when appropriate.
These steps minimize the potential for modifications to have an adverse impact on plant safety due to incomplete or incorrect evaluation of the potential effects of the modification.
To ensure that the potential effects of modifications made specifically to address pressure locking and thermal binding were properly included in the modification process, the plant Technical Support Staff was trained on the subject. The training included causes of the phenomena, potential designs which could alleviate the potential for occurrence, and some of the potential impacts of the modifications on plant safety. These actione helped to ensure that the pressure locking and thermal binding phenomena and potential effects of modifications to address these phenomena were understood and properly considered l
in the modification process.
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