05000251/LER-2015-001, Automatic Auxiliary Feedwater System Actuation during a Planned Reactor Trip
| Turkey Point Unit 4 | |
| Event date: | 11/30/2014 |
|---|---|
| Report date: | 1/29/2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2512015001R00 - NRC Website | |
On November 30, 2014, at approximately 1354 the Unit 4 reactor was manually tripped as a pre-planned evolution to facilitate the repair of an unidentified steam leak in the High Pressure (HP) Turbine. While Unit 4 was in Mode 3, at approximately 1358 hours, the Auxiliary Feedwater System initiated when the 4C Steam Generator (SG) level reached the low-low SG level setpoint setting. The AFW system was restored to standby alignment at approximately 1454 hours. The causal analysis determined that: 1) The appropriate operating margin to prevent AFW actuation was not established prior to the reactor trip for the planned shutdown, and 2) The just-in-time training did not prepare crews to reduce the probability of having an unnecessary AFW actuation on a planned reactor trip. Corrective actions include: 1) Change the applicable operating procedures to establish available margin to avoid unnecessary AFW actuation during a planned reactor trip, and 2) Develop simulator scenarios that more closely model the plant response during a planned shutdown and train Operators to reduce the probability of an AFW actuation during a planned reactor trip.
DESCRIPTION OF THE EVENT
On November 30, 2014, Turkey Point Unit 4 reactor [AC, RCT] was in Mode 1, with reactor power reduced to 23% to facilitate the discovery and repair of an unidentified steam leak on the Unit 4 High Pressure (HP) Turbine. Unable to determine the exact location of the HP turbine steam leak, at approximately 1354 hours, the Unit 4 reactor was manually tripped in accordance with normal operating procedure guidance, as a pre-planned evolution. The planned Unit 4 reactor shutdown was conducted in accordance with Operating Procedure 4-G0P-103, Power Operation to Hot Standby.
Following the reactor trip, as expected, Operators entered Emergency Operating Procedure 4-EOP-E-0, "Reactor Trip or Safety Injection," and transitioned to 4-E0P-ES-0.1, "Reactor Trip Response," to stabilize As part of the reactor trip response, 4-E0P-ES-0.1, Step 2b, Operators were verifying that the Main Feedwater Control Valves [SJ,FCV] had closed, when they observed dual indication on the 4A main manually close the Main Feedwater Control Valves, by performing the step in the "Response Not Obtained" (RNO) column. This unexpected manual action, delayed the expeditious performance of subsequent steps in the procedure to establish feedwater flow using the Main Feedwater Bypass Valves, as directed in 4-E0P-ES-0.1 Step 2f.
At approximately 1358 hours, following the reactor trip, Unit 4 was in Mode 3 when the level in the 4C Steam Generator (SG) [SB,SG] level decreased to the low-low level setpoint, 16% Narrow Range (NR), causing an Auxiliary Feedwater (AFW) System [BA] actuation.
At approximately 1407, the Main Steam Isolation Valves [SB,ISV] were procedurally closed in response to the cool down. AFW was restored to standby alignment at approximately 1454 hours. The 4A Main Feedwater Pump was secured at 1526 hours, with the 4A Standby Feedwater Pump supplying feed to the SGs and decay heat removed via the atmospheric dump valves.
The NRC Operations Center was notified by Event Notification 50645 in accordance with 10 CFR 50.72(b)(3)(iv)(A) for valid actuation of the AFW system.
This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as "...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." The AFW System was automatically actuated during the event and is included in the systems listed in paragraph 10 CFR 50.73(a)(2)(iv)(B).
CAUSE OF THE EVENT
The causal analysis determined the following:
- The appropriate operating margin to prevent AFW actuation was not established prior to the reactor trip for the planned shutdown.
- The just-in-time training did not prepare crews to reduce the probability of having an unnecessary AFW actuation on a planned reactor trip.
ANALYSIS OF THE EVENT
On November 28, 2014, an unidentified steam leak was discovered on the south end of the Unit 4 HP turbine. Due to an excessive amount of steam, personnel could not gain access to identify the source of the steam leak. A load reduction was necessary to allow personnel to enter the area but visibility and accessibility was limited. On November 30, the Unit 4 was manually tripped from 23% power.
Following the trip, the 4C SG level decreased to 16% NR, the low-low level setpoint, and AFW actuated.
Providing feedwater through the main feedwater bypass valves to the generators would have been sufficient to maintain reactor coolant temperature stable after the trip, since Unit 4 was at the beginning of cycle (BOC) plant conditions with low decay heat load.
On a planned shutdown, reducing steam demand prior to tripping the reactor and the turbine reduces the SG level shrink, and therefore reduces the probability of an AFW actuation following the reactor trip. The operating procedure, 4-GOP-103, permitted Operators to trip the reactor manually when reactor power decreases to approximately 15 to 25% power. However, prior to that step, the procedure includes a note to caution the Operators that a " Manual trip below 20% power reduces the probability of unnecessary For this event, the reactor was tripped from 23% power, in order to minimize the risk of having a secondary transient due to the unidentified HP turbine steam leak, which increased the probability of an AFW actuation. This operating margin reduction, along with the delay in the expeditious performance of the resulted in not establishing feedwater flow to the generators using the Feedwater Bypass Valves promptly, reaching the SG low-low level setpoint setting of 16% NR, and thus actuating AFW.
ANALYSIS OF SAFETY SIGNIFICANCE
AFW initiated on a low-low SG level signal and added cooler water to the generators. As expected, maintaining the reactor in a safe condition. As such, the safety significance of this event is very low.
CORRECTIVE ACTIONS
Corrective actions are in accordance with condition report AR 2009853 and include:
1. Change the applicable operating procedures to establish available margin to avoid unnecessary AFW actuation during a planned reactor trip.
2. Develop simulator scenarios that more closely model the plant response during a planned shutdown and train Operators to reduce the probability of an unnecessary AFW actuation during a planned reactor trip.
ADDITIONAL INFORMATION
ENS Codes are shown in the format [IEEE system identifier, component function identifier, second component function identifier (if appropriate)].
FAILED COMPONENTS IDENTIFIED: None PREVIOUS SIMILAR EVENTS: None