05000237/LER-2009-002
| Dresden Nuclear Power Station | |
| Event date: | 03-15-2009 |
|---|---|
| Report date: | 03-31-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 2372009002R01 - NRC Website | |
Dresden Nuclear Power Station (DNPS) Unit 2 is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditions Prior to Event:
Unit: 02� Event Date: 3-15-2009 �Reactor Mode: 1� Mode Name: Power Operation Power Level: 100 percent Reactor Coolant System Pressure: 1000 psig
B. Description of Event:
On March 15, 2009, at approximately 2025 hours0.0234 days <br />0.563 hours <br />0.00335 weeks <br />7.705125e-4 months <br /> (CDT), with Unit 2 at approximately 100 percent power, DNPS Operations personnel discovered during a maintenance activity that motor operated valve (MOV) 2-2301-6, Unit 2 High Pressure Coolant Injection System [BJ] Suction Valve, would not close. The valve is a normally open valve and allows the flow of cooling water from the Condensate Storage Tanks (CST) to the suction of the High Pressure Coolant Injection System (HPCI) pumps.
The valve is required to close during the transfer of HPCI pump suction from the CSTs to the Torus to prevent high Torus water level. High Torus water level could result in exceeding Torus structural design values during a postulated accident.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(C) and (D), "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to: (C) control the release of radioactive material and (D) mitigate the consequences of an accident." The Torus is a structure that is credited to control the release of radioactive material and mitigate the consequences of an accident.
C. Cause of Event:
MOV 2-2301-6 is a 16", Crane Class 150 cast steel wedge gate valve with flanged ends. The valve is installed in a vertical section pipe with the valve stem and disc oriented horizontally. It is a normally open valve and allows the flow of cooling water from the CSTs to the suction of the HPCI pumps.
Upon receipt of a HPCI initiation signal, MOV 2-2301-6 is automatically signaled to open, unless both Unit 2 MOVs 2-2301-35 and 2-2301-36 (i.e., HPCI pump suction valves from the Torus) are open.
MOV 2-2301-6 remains open during HPCI initiation and will automatically close if both the 2-2301-35 and 2-2301-36 valves are open, changing the HPCI pump supply from the CSTs to the Torus.
Additionally, MOV 2-2301-6 will close on high water level in the Torus to prevent exceeding Torus structural design values during a postulated accident or CST low level to prevent the CSTs from draining to a level below a pre-determined set point.
control room, the valve was partially closed by manual action and again failed to close when given another signal from the control room to close. The valve was manually closed. The valve was last successfully closed electrically on January 13, 2009.
� Subsequent troubleshooting and diagnostic testing were performed to determine the failure mode.
The electrical circuit was inspected up to and including the valve actuator. There were no issues identified with the electrical control power circuit. During the diagnostic testing, the valve did go fully closed when operated electrically. Subsequent testing showed reduced thrust and torque during each of the next couple of strokes. The diagnostic test identified an internal valve binding force during the close stroke for about the first 40% of travel. The open stroke was observed to be as expected with no evidence of binding. The diagnostic testing found no issues with the Limitorque actuator.
During the MOV diagnostic testing evolution several actions were taken to ensure the valve would achieve the full close stroke: (1) The process of numerous strokes appears to have lowered the magnitude of the binding forces from the initial value of greater than 4800 lbs to approximately 3000 lbs. (2) The stem was cleaned and lubricated to improve the efficiency of the actuator torque to stem thrust conversion. The as-found stem Coefficient of Friction (COF) was measured at 0.2 and after the lubrication activity it was improved to 0.06 COF. (3) The actuator output torque switch setting was increased to ensure sufficient margin between the actuator output and the binding forces. The valve was stroked a total of 12 times.
The valve was declared operable and capable to perform its design function after it was verified that all diagnostic test parameters for torque and thrust were within prescribed specifications. Monthly diagnostic testing was performed on MOV 2-2301-6 to verify its continued operability until internal valve inspections were performed in the Unit 2 refuel outage (D2R21) which was completed in November 2009.
The Root Cause of this event was that when the valve was in the full open position, the center of gravity of the solid gate valve disc extending past the end of the in-body valve guides, which caused the disc to tip and bind with the in-body guides. Industry operating experience found similar events that were caused by either friction between disc and guide or disc alignment issues known as "tipping", however their root cause was attributed to excessive in-body guide wear which was not evident during D2R21 internal valve inspections.
A contributing cause to this event is increased stem friction. The higher than normal thrust associated with the binding caused a loss of stem lubricant through extrusion, which increased the friction at the stem and stem nut interface. Additional monthly diagnostic testing monitored the stem friction and the condition of the lubricant until the valve was repaired.
A detailed extent of condition review found 13 motor operated valves with the disc/stem oriented horizontally with some internal binding identified during the most recent diagnostic testing. In most cases, the binding load was very small. Each of these valves has been evaluated and determined to have significant margin or have been tested within the past six years, so there is minimal risk of a similar failure. The preventative maintenance activities for these valves will be revised as needed to perform diagnostic testing on an increased frequency.
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/ POINT A- F z Figure 1. This is a typical example of valve disc tipping.
D.S Safety Analysis:
The safety significance of the event is minimal. An evaluation was performed to determine the effect of this event on the operability of the HPCI and Torus during the time that it is hypothesized that MOV 2-2301-6 would not electrically close (i.e., January 13, 2009 to March 15, 2009). The evaluation concluded that HPCI would have operated long enough to fulfill its safety function. A review of the actual Torus water levels during this time frame identified that the Torus would not have exceed its structural design values during a postulated accident. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
_
E. Corrective Actions:
MOV 2-2301-6 was returned to operable status.
Monthly diagnostic testing was performed on MOV 2-2301-6 to verify its operability until internal valve inspections were performed in the Unit 2 refuel outage which was completed in November 2009.
Dresden MOV diagnostic testing for MOV Program valves with horizontal disc and stem orientation that exhibit internal binding was revised to ensure the testing frequency does not exceed six years.
Dresden MOV stem lubrication for MOV Program valves with horizontal disc and stern orientation that exhibit internal binding was revised to ensure the lubrication frequency does not exceed two years.
F. Previous Occurrences:
A review of DNPS Licensee Event Reports (LERs) for the last three years did not identify any LERs associated with failure of valve closure due to internal binding.
G. Component Failure Data:
16" Crane Class 150 cast steel wedge gate valve with flanged ends