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| Report date | Site | Event description | |
|---|---|---|---|
| 05000483/LER-2015-004, Auxiliary Feedwater Control Valve Inoperable Due To Faulty Electronic Positioner Card | 12 October 2015 | Callaway | Between 11/18/2014 and 12/3/2014, the 'B' MDAFW train was inoperable due to an improperly functioning positioner card installed on the control valve in the AFW flow path to the 'D' steam generator, i.e., valve ALHV0005. The 'A' MDAFW train and the TDAFW train were inoperable for short durations at different times between 11/18/2014 and 12/3/2014, although neither of those redundant trains was inoperable at the same time. During the short windows of TDAFW unavailability, only a single MDAFW train was operable, resulting in a loss of safety function. On 08/11/2015, an unexpected turbine trip / reactor trip occurred due to a latent design error in the current transformer (CT) wiring for the main transformers. The reactor trip was reported to the NRC in Event Notification 51308. While responding to the reactor trip, the 'B' train motor-driven auxiliary feedwater (MDAFW) flow control valve in the auxiliary feedwater (AFW) flow path to the `A' steam generator, i.e., valve ALHV0007, could not be manipulated from the main control room. It was determined that ALHV0007 would have performed its specified safety function; however, during the extent of condition review, it was determined that ALHV0005 was inoperable from 11/18/2014 until 12/3/2014. The 72-hour Completion Time of Condition C of Technical Specification 3.7.5 was exceeded from 11/18/2014 until a new positioner card was installed on ALHV0005 on 12/3/2014. The direct cause of the ALHV0005 failure was a failure of a bridge rectifier on the valve's electronic positioner circuit card. This type of positioner card was also installed on ALHV0007. The root cause of the card failures was determined to be a vendor design deficiency. The defective positioner cards have been replaced and measures have been taken to remove defective spares from future plant use. |
| 05000483/LER-2015-003, Reactor Trip Caused by Transmission Line Fault | 1 October 2015 | Callaway | On August 11, 2015, at 01:39 Callaway plant tripped from 100% power due to an incorrect, automatic response to a transmission line fault on the Montgomery-Callaway 8 line by transformer bus differential relaying. This resulted in Reactor Protection System (RPS) and Auxiliary Feedwater System actuations. The plant response to the trip was as expected except for a problem encountered with Auxiliary Feedwater flow control valve ALHV0007 subsequent to the plant trip. This event was caused by the inadvertent inclusion of jumpers in the current transformer (CT) circuits of the main transformers that were installed as part of Main Transformer Replacement Modification 09-0044 implemented in Refuel 19. Following the event, the inadvertently placed CT jumpers were removed and the plant was successfully restarted. The preliminary root cause of the incorrect main transformer CT wiring was failure to revise drawing E-23MA02, "Generation System - Three Line Metering & Relaying Diagram," which was missing information on connections to switchyard protective relays and included jumpers that were not supposed to be installed. Post-modification testing performed by System Relay Services did not detect the improper jumpers. Corrective actions include additional design, testing and job reviews, as well as reviews of similar drawings to identify and correct missing information. |
| 05000483/LER-2015-002, Manual Auxiliary Feedwater System Actuation | 17 September 2015 | Callaway | During plant cooldown in response to conditions reported to the NRC on July 23, 2015 in Event Notification 51253, Callaway was in Mode 3 (Hot Standby) and on the way to Mode 5 (Cold Shutdown). In accordance with cooldown procedures, Callaway was operating with one Main Feedwater Pump (MFP) when the pump speed unexpectedly lowered to 0 RPM. The pump was manually tripped in response to the condition. This led to a decrease in water level in the steam generators. Operators manually activated the Auxiliary Feedwater System to maintain water level in the steam generators. This system actuation is reportable per 10CFR50.73(a)(2)(iv)(A). Fault tree analysis and subsequent testing identified the most probable cause for the loss of the 13' MFP is a software defect introduced during the software development process for the digital feedwater control system installed in 2013. However, further investigation and consultation with the software design firm for the control system is required to identify a definitive root cause. Thus, a supplemental LER will need to be submitted to update the root cause and corrective actions. |
| 05000483/LER-2015-001, Completion of a Shutdown Required by the Technical Specifications - TS 3.4.13 | 17 September 2015 | Callaway | On July 23, 2015, plant operators became aware of indications of an increase in the Reactor Coolant System (RCS) unidentified leak rate. The indications included containment radiation alarms as well as increasing containment humidity and sump levels. An RCS inventory balance indicated an unidentified leak rate of 1.2 gpm leak which is greater than the Technical Specification limit of 1 gpm for unidentified leakage. Actions were taken to determine the source of the leak. A containment entry was made, and a steam cloud was identified to be coming from the Pressurizer Spray Valve cubicle. The plant was shut down in order to comply with requirements of the Technical Specifications. It was determined that the leak was due to seat leakage through the RCS Pressurizer CVCS Auxiliary Spray Supply Drain valve BBV0400 and then through the non-safety related pipe flange immediately downstream of the valve. The valve was tightened which reduced the leakage to 60 drops per minute. The flange gasket was replaced. Additional causes and corrective actions are still being determined. |