05000483/LER-2015-002, Manual Auxiliary Feedwater System Actuation
| Callaway Plant Unit 1 | |
| Event date: | 07-23-2015 |
|---|---|
| Report date: | 09-17-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 51253 | 10 CFR 50.72(b)(2)(xi), Notification to Government Agency or News Release, 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown |
| 4832015002R00 - NRC Website | |
1. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The main feedwater system [EIIS System SJ] supplies feedwater to the secondary side of the steam generators [EIIS Component SG] in order to remove heat from the reactor coolant system (RCS) [EIIS System AB] during normal operations. The system consists of two main feedwater pumps (MFPs) [EIIS System SJ Component P] and one startup feedwater pump, which is only used for very low-power conditions.
The MFPs are controlled by a digital control system, which is operated via touch screen controls in the main control room.
The safety-related auxiliary feedwater (AFW) system [EIIS System BA] supplies feedwater to the steam generators to remove decay heat from the reactor coolant system upon the loss of the normal feedwater supply via either manual actuation or automatic response to an Auxiliary Feedwater Actuation Signal (AFAS). The auxiliary feedwater system may also be used following a reactor shutdown to cool the reactor coolant system to below approximately 350 degrees Fahrenheit and 400 psi gauge.
2. INITIAL PLANT CONDITIONS:
On July 23, 2015 at 0115, Callaway Plant operators initiated a shutdown required by Technical Specification (TS) 3.4.13, "RCS Operational Leakage," as reported to the NRC in Event Notification 51253. At 1357 Callaway was in Mode 3 (Hot Standby) and on the way to Mode 5 (Cold Shutdown). The plant was being operated per procedure with one MFP supplying feedwater flow to the steam generators. The startup feedwater pump was unavailable due to a fire protection requirement to have its breaker [EIIS Component BKR] in the racked out position.
3. EVENT DESCRIPTION:
On July 23, 2015, during a plant shutdown, main feedwater flow was lost to all four steam generators.
A manual actuation of the auxiliary feedwater system was performed to prevent a reactor trip signal and AFAS. As described in Section 2 above, the plant was in Mode 3 at the time of the manual actuation.
Prior to the AFW manual actuation, the balance of plant operator was lowering the 'B' Main Feedwater Pump (MFP) speed manually per procedure via the pump's digital control board to regulate pump differential pressure (dP). After performing the manual speed decrease, the operator turned his attention to other main control board indications. A `B'MFP vibration annunciator returned the operator's attention to the digital feedwater control indications at 1333. He noticed that the discharge valve position and pump speed demand went to 0 percent and 0 RPM respectively and the pump was coasting down. No pump trip alarms were received.
An equipment operator was dispatched to investigate the 'B' MFP and reported a metallic banging sound. This was later confirmed to be the main feedwater pump turbine 13' low pressure steam line check valve. While the equipment operator was investigating the pump locally, the control room operator was able to control pump speed manually using both increase and decrease pushbutton commands. When the equipment operator reported to the control room that he heard an unusual noise at the pump, the decision was made to trip the pump at 1342.
The 'A' MFP had been tripped earlier during the plant down-power, per procedure. The startup feed pump is not maintained in a standby condition, so was unavailable. Therefore, following the 13' MFP trip all main feedwater sources were unavailable. With none of the normal sources of main feedwater readily available and `D' steam generator level approaching 23%, the decision was made to put the AFW system in service to establish makeup to the steam generators.
Following the procedure for a manual AFW actuation, the operators manually closed all four AFW Discharge Control Valves (ALHV0005, ALHV0007, ALHV0009, and ALHVO011) to prevent unnecessary water hammer and thermal shock in the steam generators. When the operators attempted to reopen ALHVO011, the valve did not respond. After no response was received, the operator reduced the valve demand to 0% on the main control board. An equipment operator was dispatched to the valve to manually open the valve. The operator locally opened ALHVO011. After this local action, control was restored from the main control board.
4. ASSESSMENT OF SAFETY CONSEQUENCES:
The actual and potential safety consequences of the loss of the Main Feedwater system are not significant, as Main Feedwater is not a safety-related system. By design, the safety-related Auxiliary Feedwater system performs the credited function of supplying feedwater to the steam generators in the event of a postulated accident. In addition, the deterministic Accident Analysis includes evaluation of an assumed or postulated complete loss of feedwater supply from the Main Feedwater system to the steam generators. Thus, the loss of Main Feedwater delivery leading to the manual AFW actuation was consistent with the results contained in the Accident Analysis of record.
When called upon for this event, the Auxiliary Feedwater system was able to perform its specified safety function despite the anomalous behavior of the ALHVO011 valve. (The ALHVO011 issue, which itself is not reportable, was subsequently corrected). Postulating reasonable and credible alternative conditions, including normal plant operating conditions (Mode 1) and accident conditions, would have no impact on the significance of loss of Main Feedwater as these conditions are accounted for in the Accident Analysis of record.
5. REPORTING REQUIREMENTS:
This LER is submitted pursuant to 10 CFR 50.73(a)(2)(iv)(A) to report an Auxiliary Feedwater system actuation.
Specifically, 10 CFR 50.73(a)(2)(iv) states in part, "The licensee shall report:
(A) Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section...
(B) The systems to which the requirements of paragraph (a)(2)(iv)(A) of this section apply are:
(6) PWR auxiliary or emergency feedwater system.
The auxiliary feedwater system was manually actuated in response to a decrease in water level in the steam generators following the MFP trip. This fulfills the reporting requirement of 10 CFR 50.73(a)(2)(iv)(A) due to actuation of the system specified in 10 CFR 50.73(a)(2)(iv)(B)(6).
6. CAUSE OF THE EVENT:
The manual actuation of the auxiliary feedwater system was necessitated by the manual trip of the 13' MFP. Following the event, the main feedwater digital control touchscreen from the main control room was replaced. The touchscreen removed from the plant was tested and the failure has not recurred. It is not likely that the touchscreen was the cause of this event. The most likely cause is a software malfunction. The software malfunction has only occurred one time and has yet to be re- created in the test system.
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 testing and consultation with the software design firm for the control system is required to identify a definitive root cause. Since these tests cannot be performed with the plant online, the testing is currently scheduled during the upcoming refueling outage in April 2016.
7. CORRECTIVE ACTIONS:
Since a definitive root cause analysis is delayed until the upcoming refueling outage, correction actions have not been developed. Upon further investigation, these actions will be developed and submitted in a supplemental report. A communication has been issued to all operators to notify all crews of this condition.
8. PREVIOUS SIMILAR EVENTS:
An Operating Experience (OE) review for Callaway since the main feedwater digital control system was installed in 2013 revealed no other manual actuations of the auxiliary feedwater system due to failure of MFPs or their associated digital control system.