05000382/FIN-2008004-01
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Finding | |
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| Title | Intersystem loss of coolant event |
| Description | Introduction. The inspectors identified a Green noncited violation of 10 CFR Part 50, Appendix B, Criterion III (Design Control) for an inadequate "pressure locking" design calculation involving shutdown cooling Valves SI-405A and SI-405B. Plant engineers also used the calculation to support valve operability following a valve malfunction, which appeared to be caused by pressure locking. Entergy engineers had derived valve bonnet leakage rates (for pressure locking conditions) from local leak rate testing results. However, a national laboratory had already proven the Entergy theory invalid and plant engineers had taken no steps to validate the theory themselves. Finally, in response to an NRC generic letter concerning pressure locking and thermal binding of valves, the licensee's conclusions were based on incorrect facts and improper assumptions. Background: Valves SI-405A/B are opened to place shutdown cooling and lowtemperature/ over-pressure protection (Valves SI-406A/B) in service. Valves SI-405A/B are in series between Valves SI-401A/B and SI-407A/B, which are also opened for shutdown cooling operations. In the past refueling outage (RF-15, Spring 2008), plant craftsmen replaced Valves SI 405A/B hydraulic actuators with air-operated actuators. Plant engineers specified the modification to resolve performance problems with the hydraulic units. However, the behavior of the air-operated actuators was fundamentally different than that of the hydraulic model. Specifically, the hydraulic actuators provided up to the maximum amount of stem thrust starting at the beginning of the opening valve stroke, when thrust demands were greatest. In contrast, the air-operated actuators gradually increased valve stem thrust as air on the top of the actuator piston was vented off. If a load existed on the stem (typically the case, i.e. packing and unwedging), the valve would not initially move when commanded to open. It would remain closed until the actuator developed sufficient thrust to move the valve from its seat. Once off the seat, the valve disc would pop to a midposition and then continue to the open position at the normal speed. On May 14, 2008, during post-installation testing, a plant engineer observed that Valve SI-405A did not initially move on the first two attempts. As noted in CR WF3-2008-02326, approximately 2 minutes following the second attempt, the valve opened with a: . . . loud abrupt sound and shaking of the surrounding area. The valve coupling on the stem immediately moved approximately one half of the travel distance following the loud noise. Just prior to the valve operation, operators had pressurized the shutdown cooling piping system to approximately 1700 psig. The valve had pressure locked following the test. Pressure locking occurs when elevated residual pressure is trapped in a valve's bonnet and prevents the valve from opening. The NRC has issued several industry wide generic communications concerning pressure locking, most notably Generic Letter 95-07, "Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves," dated August 22, 1995. Entergy was required to respond to the generic letter, documenting their plans to address these generic concerns. Entergy engineers had concluded that pressure locking of these valves would not occur. Specific deficiencies with the Entergy generic letter response are provided later in this report. Intersystem Loss-of-Coolant Event: On September 1, plant operators shut down Waterford-3 in preparation for Hurricane Gustav. During initiation of Train B shutdown cooling operations, control room operators identified and responded to an intersystem loss-of-coolant event that lasted approximately 4 minutes. About 800 gallons of reactor coolant was lost through low-temperature over-pressure protection Relief Valve SI-406B. The relief valve unexpectedly opened following an apparent malfunction of Valve SI-405B. While placing Train B shutdown cooling in service, Valve SI-405B had failed to initially move when the control room operator repositioned the valve's control room switch to open. The indication showed full closed for about 12 minutes. The valve indication then showed midposition, which was followed by full open indication. The air-operated valve normally had a 5 to 6 minute stroke time. Licensee personnel determined that the valve was stuck in its seat until the valve actuator developed sufficient thrust to move the valve disc. Then, the valve popped to almost the full open position. Entergy engineers determined that the sudden motion of Valve SI-405B created a pressure transient in the residual heat removal piping. Since system pressure was about 350 psig at the time, system pressure combined with the pressure transient that resulted from the opening of Valve SI-406B exceeded the setpoint of Relief Valve SI-406B, which was approximately 430 psia. Once lifted, relief valves will not normally seat at their set pressure but will close at a pressure somewhat below the setpoint. Since system pressure was already relatively high, the valve did not immediately reseat. Operator action was necessary to stop the leakage by closing Valve SI-401B. This was the preferred isolation valve because of the slower closing time of Valve SI-405B. At the end of the inspection period, the inspectors were still evaluating information related to the intersystem loss-of-coolant event. The inspectors provided questions to licensee personnel concerning the valve actuator design change package. This is an URI pending further evaluation by the NRC inspectors: URI 05000382/2008004-01, intersystem loss-of-coolant event |
| Site: | Waterford  |
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| Report | IR 05000382/2008004 Section 1R15 |
| Date counted | Sep 30, 2008 (2008Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.15 |
| Inspectors (proximate) | B Henderson S Graves G Replogle R Smith W Sifre C Paulk R Azua D Overland S Makorp Elkmann G Replogle J Clark R Azua D Overland B Correl |
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Finding - Waterford - IR 05000382/2008004 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Waterford) @ 2008Q3
Self-Identified List (Waterford)
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