05000388/LER-2012-001
| Docket Number | |
| Event date: | 05-11-2012 |
|---|---|
| Report date: | 20-12-0500 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 47919 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3882012001R01 - NRC Website | |
CONDITION PRIOR TO THE EVENT
Unit 2 — Mode 1, 100 percent Rated Thermal Power The OV117A Control Room Floor Cooling Unit fan's discharge damper (HD-07831A) was inoperable at the start of the event and contributed to the event.
EVENT DESCRIPTION
Timeline A chronological timeline or sequence of events leading up to and immediately following the event follows:
The Control Structure (CS) Heating, Ventilation, and Air Conditioning (HVAC) system [EllS Code: VI] was operating in a normal line-up configuration prior to 0523 hours0.00605 days <br />0.145 hours <br />8.647487e-4 weeks <br />1.990015e-4 months <br /> on 05/11/2012. The 'A' chiller and train were in service and the 'B' train was in standby, or in 'AUTO' mode. Both train fans were in 'AUTO' and slaved to the associated running chiller's operating status.
On 05/11/2012 at 0523 hours0.00605 days <br />0.145 hours <br />8.647487e-4 weeks <br />1.990015e-4 months <br />, Control Room personnel noted a change in Control Room HVAC background noise and found the OV117A Control Room Floor Cooling Unit fan's discharge damper (HD-07831A) closed. Subsequently the 'A' CS HVAC system shutdown on loss of OV117A fan and OK112A CS chiller shutdown. The "Control Room Cooling Fan Failed" alarm lit up on the control panel in the Control Room. Control Room personnel utilized Alarm Response Procedure AR-029-001 and entered Off Normal Procedure ON-030-001 "Loss of Control Structure Chilled Water." The OK112B CS chiller automatically started on the OK112A CS chiller shutdown and the Division 2 CS HVAC was in service.
Limiting Condition for Operation (LCO) 3.7.3, LCO 3.7.4, Technical requirement for Operation (TRO) 3.7.9, and TRO 3.8.6 were entered for 'A' Control Room Cooling being inoperable due to failed fan discharge damper HD-07831A.
A Condition Report was generated to document the issue and request repair support.
On 05/11/2012 at 1603 hours0.0186 days <br />0.445 hours <br />0.00265 weeks <br />6.099415e-4 months <br />, during application of the clearance order for the damper repair work, the fan hand switch, HS-07831A, was placed in "STOP" to secure power to Fan OV117A. With the hand switch in the 'STOP' position, the "Control Room Cooling Fan Failed" alarm cleared, and subsequently, the OK112A CS chiller started as well as Fans 0V103A and 0V115A in the train. With 'A' train back in service as the lead system, the 'B' system shutdown returning itself to standby status. The ventilation to the Control Room from the 'A' train was not in service and the 'B' was shutdown; therefore, there was no air flow to the Control Room creating a condition that resulted in entry into ON-030- 001. This condition caused a loss of both Control Room Fans OV117A and OV117B and LCO 3.0.3 was entered.
On 05/11/2012 at 1612 hours0.0187 days <br />0.448 hours <br />0.00267 weeks <br />6.13366e-4 months <br />, Operations took action to start OK112B CS chiller in accordance with the operating procedures to restore Control Room Cooling.
On 05/11/2012 at 1615 hours0.0187 days <br />0.449 hours <br />0.00267 weeks <br />6.145075e-4 months <br />, Operations placed the OK112A CS chiller in standby.
At 1618 hours0.0187 days <br />0.449 hours <br />0.00268 weeks <br />6.15649e-4 months <br />, LCO 3.0.3 was exited.
On 05/11/2012 at 2127 hours0.0246 days <br />0.591 hours <br />0.00352 weeks <br />8.093235e-4 months <br />, an ENS notification (47919) was made to the NRC in accordance with 10 CFR 50.72(b)(3)(v) & (vi) for an event that could have prevented the fulfillment of a safety function. As such, this event is being submitted in accordance with 10CFR50.73(a)(2)(v)(D) as a loss of entire safety function.
Background Information The Chiller Circulation Pump, 0P162A(B), is the "nerve center" for the CS chiller and CS Ventilation Fan logic. The chiller and fans start from permissives initiated by the Chiller Circulation Pump logic. When CS chiller OK112A trips, a low flow signal in the "A" loop causes Circulation Pump OP162B to auto-start with its associated control hand switch in 'AUTO.' When Circulation Pump 0P162B starts, Chiller OK112B and all associated train fans start.
The Chiller Circulation Pump Control Switch (HS08622A/B) is a "break before make" (B-B-M) switch that requires installing a jumper to change positions. This deficiency prevented operators from taking action to place the 'B' chiller in the lead after the 'A' chiller tripped and is also considered an Operator Work-around. When changing the position of the B-B-M switch, the circuit is momentarily interrupted and can cause the system to trip. If this switch had not been a B-B-M switch, the Alarm Response procedure would not have contained a warning against changing position, and the standard operating practice would have been to switch the running chiller to the lead (START). If this switching had been done, application of the Clearance Order and switching fan OV117A to STOP would not have affected the running ("B") chiller, and the event would not have occurred.
Root Cause 2: Clearance Order Preparation, Review and Application Authorization was Less Than Adequate for this Emergent Condition:
The Clearance Order prepared for working on the 'A' Chiller fan OV117A (Control Room Floor Cooling A Fan) discharge damper (HD-07831A) did not adequately address impacts and effects on the system, which resulted in an unexpected system response during Clearance Order application. Specifically, the Clearance Order did not consider the impacts of leaving the 'A' Chiller Circulation Pump switch in START and moving the fan switch to STOP.
ANALYSIS/SAFETY SIGNIFICANCE
Actual Consequences:
There are no actual safety consequences as a result of this event due to its short duration.
Although the Control Room and other vital CS areas were without ventilation for less than 15 minutes, there were no appreciable changes in CS temperature. CS temperatures remained well below FSAR Sections 6.4 and 9.4.1 design limits. Restoration of the system was timely such that a habitable environment was maintained within the specified limits and the operability of equipment was not jeopardized.
Potential Consequences:
The CS ventilation systems maintain a habitable environment for personnel. These systems also assure that CS temperatures will not exceed equipment operability limits. The loss of both CS chilled water systems may prevent the CS ventilation systems from performing these functions. Consequently, Tech Specs requires entry into LCO 3.0.3 for the loss of both CS chilled water systems.
The Control Room Operators would have been challenged with a Unit 2 shutdown in addition to the work and testing associated with the Unit 1 Refueling Outage that was underway at the time. Through Control Room knowledge and training, Operations identified the issue and responded to restore CS ventilation in a timely manner. No reactivity management challenges were encountered.
An immediate corrective action was to revise alarm response procedures to include direction to place the running control structure chiller to a "start" configuration when it is decided to maintain that configuration.
Other key corrective actions include the following:
1. The Susquehanna clearance order procedure will be revised to provide better guidance for developing clearance orders for equipment that is in an atypical configuration due to system transient.
2. Chiller Circulation Pump control switches will be replaced with switches with a 'constant contact' design.
PREVIOUS SIMILAR EVENTS
The following LER's were recently submitted related to Control Structure Cooling issues:
- LER 387/2012-01-00, "Both Control Structure Chillers Inoperable" identified a condition that could have prevented the fulfillment of a safety function associated with the CS Chillers.
- LER 387/2012-002-00, "B Control Structure Chiller Inoperable Concurrent with "A" Emergency Diesel Generator Out of Service" identified a condition that could have prevented the fulfillment of a safety function associated with the CS Chillers.