05000454/LER-2014-001
| Byron Station, Unit 1 | |
| Event date: | 01-23-2014 |
|---|---|
| Report date: | 03-24-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4542014001R00 - NRC Website | |
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A. Plant Condition Prior to Event Event Date/Time: January 23, 2014 / 0539 hours0.00624 days <br />0.15 hours <br />8.912037e-4 weeks <br />2.050895e-4 months <br /> CST Unit 1 - Mode 1 - Power Operations, Reactor Power 100 percent Unit 2 - Mode 1 - Power Operations, Reactor Power 100 percent Reactor Coolant System [AB]: Normal operating temperature and pressure.
B. Description of Event
On January 23, 2014, Byron Unit 1 and Unit 2 were in Mode 1 with reactor power at 100 percent. Operators were restoring the OB SX Makeup Pump to an available on line status following a scheduled work window.
In preparation to execute an auto-start of the OB SX Makeup Pump, the shift began lowering SX tower basin level per Byron operating procedure BOP SX-12, "Makeup to an Essential Service Water Mechanical Draft Cooling Tower Basin." During the lowering of the SX basin level, an auto-start of the OA SX Makeup Pump occurred due to a sensed low level in the OA SX tower basin.
The minimum allowed Technical Specification (TS) level for SX Cooling Tower basin water level is 60 percent per Byron TS LCO 3.7.9, "Ultimate Heat Sink (UHS)." The auto-start level setpoint for each pump is 62 percent. The Byron SX Cooling Tower contains separate basins for the OA and OB SX trains which are hydraulically connected at 64 percent level and above and are physically separated below 64 percent level.
The SX basin level was being lowered on both basins, as they are cross-tied to the 64 percent level. Basin level was being lowered in automatic mode on the Manual/Auto (M/A) station for both OA and OB basin level controllers by reducing the demand signal on the M/A station. This practice causes the makeup to the basins to remain closed until the setpoint on the M/A station is reached. The demand setpoint on the M/A station was set to 71 percent in accordance with BOP SX-12, which is well above the expected auto-start setpoint of 62 percent.
As the controller was responding in automatic mode to the setting at 71 percent basin level, it overshot and reached 68 percent, at which time the OA SX Makeup Pump auto started. Additionally, there were adverse environmental conditions outside including high winds that were causing wave action in both basins, which likely contributed to the auto-start of the OA SX Makeup Pump. This level is approximately 6 percent above the designed auto start level of 62 percent.
C. Cause of Event
The cause of the event was that the operating procedure directs the controller to be set at a level that leaves little margin to the SX Makeup Pump automatic start level switch setpoint, especially when adverse environmental conditions such as high winds and wave action exist.
D. Safety Significance
The OA SX Makeup Pump was Operable per Byron TS 3.7.9, "Ultimate Heat Sink (UHS)." The pump was demonstrated to start on low basin level and was within the frequency requirements of the TS Surveillance Requirement. In this event the pump started early, which is conservative relative to the design setpoint.
Therefore, the safety significance of this issue is considered low.
E. Corrective Actions
Corrective actions include revising the operating procedures for SX tower basin level manipulations to set the controller at a more conservative target level relative to the SX Makeup Pump trip setpoint. Additionally, notes and cautions will be inserted to better alert the operator of potential SX Makeup pump auto-starts during SX tower basin level manipulations.
F. Previous Occurrences
There have been no previous occurrences of this nature in the previous three years.