05000530/LER-2010-001
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 10-07-2010 |
|---|---|
| Report date: | 12-6-2010 |
| 5302010001R00 - NRC Website | |
All times are Mountain Standard Time and approximate unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This LER is being submitted pursuant to 10 CFR 50.73 (a)(2)(i)(B) as a condition prohibited by Technical Specification (TS) 3.3.11, "Remote Shutdown System," which requires the remote shutdown controls and instrumentation to be Operable in Modes 1, 2, and 3. Specifically, on October 7, 2010, Palo Verde Nuclear Generating Station (PVNGS) discovered that the pressurizer auxiliary spray (Pzr Aux Spray) valve, 3JCHBHVO203, would not operate from the remote shutdown panel (RSP) while the local/remote refueling outage (May 2009).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The Pressurizer (Pzr) (EllS:AB) provides a point in the reactor coolant system (RCS) (EllS:AB) where liquid and vapor are maintained in equilibrium under saturated conditions for RCS pressure control. Pzr heaters are used to raise RCS pressure and Pzr spray is used to lower RCS pressure. Pzr spray is provided via both a main and auxiliary (Pzr Aux Spray) flow path. Main Pzr spray flow is provided through a main spray valve which uses pressure differential across the core as the driving force for spray flow. Pzr Aux Spray flow is provided through two auxiliary spray valves, 3JCHAHVO205 (A Train) and 3JCHBHVO203 (B Train), and uses charging pumps in the chemical and volume control system (CVCS) (El IS:CA) to provide the driving force for spray flow. These Pzr Aux Spray valves can be operated from the main control room (CR). Pzr Aux Spray valve 3JCHBHVO203 can be remotely operated at the "B" train RSP when the disconnect switch is in the local position.
The RSP provides the CR operator with sufficient instrumentation and controls to place and maintain the unit in a safe shutdown condition from a location other than the control room. A safe shutdown condition is defined as Mode 3. Disconnect switches are provided in the control circuits for specified "B" train components in order to prevent the inadvertent operation of components during a CR fire and provide for operation from the RSP.
The disconnect switches have two positions: local/remote and local. With a disconnect switch in local/remote position, operation of the component is enabled both from the CR and from the local controls on the RSP. With the disconnect switch in the local position, operation of the component is possible only at the local controls on the RSP.
3. INITIAL PLANT CONDITIONS:
On October 7, 2010, Palo Verde Unit 3 was in Mode 6 (Refueling). There were no structures, systems, or components inoperable at the time of the event that contributed to the event.
4. EVENT DESCRIPTION:
On October 7, 2010, at 1745, Unit 3 Operations personnel were performing surveillance test (ST) 40ST-9ZZ20, "Remote Shutdown Disconnect Switch and Control Circuit Operability," Appendix Q for Pzr Aux Spray valve 3JCHBHVO203. When the valve was tested from the local position, the valve failed to respond. Troubleshooting revealed that a wire in the control circuit was missing in the associated relay cabinet.
An investigation determined that in May, 2009, during the Unit 3 refueling outage, valve 3JCHBHVO203, a Valcor valve, was replaced with a Target Rock valve. This valve modification had previously been installed in Unit 1 and Unit 2 for both the "A" and "B" train Pzr Aux Spray valves. Prior to this modification in Unit 3, a parallel circuit path existed that resulted in the ability to operate the valve from the RSP with or without the missing control circuit wire. After.the modification, this parallel path was eliminated.
The investigation revealed inconsistencies between the as-built condition, and control wiring diagrams (CWD) and supplier documents (SDOC). The CWDs and SDOCs illustrated a wire in the auxiliary relay cabinet for valve 3JCHBHVO203 that was not physically installed. Since the CWDs and SDOCs identified a wire installed, the work order (WO) installing the modification did not contain instructions for the installation of the required wire. The investigation concluded the wire most likely had never been installed.
Additionally, the subsequent design validation test (DVT) focused on the operation of the valve from the CR circuit (local/remote) and did not test the operation of the valve from the RSP circuit (local). The WO that installed the modification and the DVT that tested the modification did not identify that the wire was not installed per the drawing.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
This condition did not adversely affect plant safety or the health and safety of the public.
The condition did not result in any challenges to the fission product barriers or result in any releases of radioactive materials. Therefore, there were no actual adverse safety consequences or implications as a result of this condition.
The conditions under which the Pzr Aux Spray valve may be operated from the RSP with the disconnect switch in the local position are during a CR fire event. For CR fire events, hot standby (Mode 3) is considered a safe and stable end state in the risk analysis model.
For CR fire postulated events, pressure control of the Pzr is maintained by cycling the Pzr heaters from the RSP in accordance with abnormal operating procedure, 40A0-9ZZ19, "Control Room Fire." The use of the Pzr Aux Spray valve is a contingency action if the Pzr heaters cannot be de-energized. Therefore, Pzr Aux Spray valves are not required for the CR fire event and there is no core damage risk associated with the failure of a Pzr Aux Spray valve to operate from the RSP.
The condition would not have prevented the fulfillment of the safety function; and, the condition did not result in a safety system functional failure as defined by 10 CFR 50.73 (a)(2)(v).
6. CAUSE OF THE EVENT:
The cause of the Pzr Aux Spray valve failure to operate from the local position at the RSP was a missing wire in the control circuit in the associated auxiliary relay cabinet. The investigation revealed inconsistencies between the as-built condition, and control wiring diagrams (CWD) and supplier documents (SDOC).
Additionally, an inadequate DVT failed to identify that the valve would not operate from the RSP. The DVT focused on the operation of the valve from the CR circuit (local/remote) and did not test the operation of the valve from the RSP circuit (local).
7. CORRECTIVE ACTIONS:
The missing wire was installed as an immediate corrective action, restoring the design configuration of the valve. The valve was tested to verify operation from the RSP, with the local/remote disconnect switch in the local position, with satisfactory results.
Additionally, action will be taken to enhance procedure 81DP-OEE10, "Design Change Process," to more clearly define the requirements and organizational responsibilities for DVT.
Since the CWDs and SDOCs already reflected the wire in the drawings at the time of discovery, no changes were required to those drawings.
8. PREVIOUS SIMILAR EVENTS:
Previously, PVNGS reported one event where post maintenance testing was considered to be inadequate (LER 50-529/2007-004-00). The corrective actions were specific to conditions identified in LER 50-529/2007-004-00, and would not have prevented this event.
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