05000254/LER-2015-009
| Quad Cities Nuclear Power Station Unit 1 | |
| Event date: | 07-27-2015 |
|---|---|
| Report date: | 09-24-2015 |
| Initial Reporting | |
| ENS 51272 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident, 10 CFR 50.72(b)(3)(xiii), Loss of Emergency Preparedness |
| 2542015009R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 2957 Megawatts Thermal Rated Core Power Energy Industry Identification System (El IS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
The failure of a fan discharge damper to close during toxic gas testing caused the Control Room Emergency Ventilation (CREV) System to be declared inoperable, which impaired the ability to pressurize the Main Control Room (MCR).
A. CONDITION PRIOR TO EVENT
Unit: 1 / 2 Event Date: July 27, 2015 Event Time: 1730 hours0.02 days <br />0.481 hours <br />0.00286 weeks <br />6.58265e-4 months <br /> Reactor Mode: 1 / 1 Mode Name: Power Operation / Power Operation Power Level: 100% / 100%
B. DESCRIPTION OF EVENT
On July 27, 2015, at 1730 hours0.02 days <br />0.481 hours <br />0.00286 weeks <br />6.58265e-4 months <br />, the discharge damper [DMP] for the running Air Filtration Unit (AFU) [AHU] booster fan [FAN] failed to close when the toxic gas analyzer received a high ammonia signal. The 'B' AFU Booster Fan discharge damper stuck open in mid-position during closure. In this condition, the CREV system [VI] cannot be guaranteed to achieve required design flow rate. At that time, Instrument Maintenance was performing a calibration and functional test on the ammonia analyzer. Upon introducing the trip input signal, the running AFU booster fan shut off and the discharge damper became stuck in a partially open position while traveling to the closed position.
The CREV System was declared inoperable at 1730 hours0.02 days <br />0.481 hours <br />0.00286 weeks <br />6.58265e-4 months <br /> due to uncertainty in achieving rated airflow through a throttled damper and the recirculation that could occur if the standby AFU booster fan was operated. During troubleshooting, the damper was reopened and closed identifying evidence of mechanical binding during the closing stroke. As a result, Technical Specification (TS) 3.7.4, Condition A, was entered.
On July 28, 2015, at 0008 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />, ENS #51272 was made to the NRC under 10 CFR 50.72(b)(3)(v)(D) to report this event as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
An investigation of this event was performed which determined that a deficient damper design did not allow for adequate clearances between damper fasteners and the damper sealing area when the damper is in the open position. This resulted in a misalignment condition that caused the damper shaft to bend. Previous internal damper inspections did not detect the design flaw since they were performed while the damper was in its normally-closed position.
Given the impact on the CREV System, which is common to both Units, this report is submitted (for Units 1 and 2) in accordance with the requirements of 10 CFR 50.73 (a)(2)(v)(D), which requires the reporting of any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
C. CAUSE OF EVENT
The cause of the stuck damper was a design flaw that resulted in mechanical binding when the damper is stroked closed from the full open position to shut. The bolts that connect the damper blade to the shaft are located such that the outer bolt hex heads come into contact with the metal seating surface of the damper assembly. The resulting misalignment caused the damper shaft to bend, increasing internal friction, and caused the damper to become stuck.
D. SAFETY ANALYSIS
System Design Habitability systems are provided to ensure that control room operators are able to remain in the MCR [NA] and operate the plant safely under normal conditions and to maintain the plant in a safe condition under accident conditions. The worst-case design basis accident (DBA) for habitability considerations is postulated as a loss of coolant accident (LOCA) with main steam [SB] isolation valve [ISV] leakage at Technical Specification limits.
Per Updated Final Safety Analysis Report (UFSAR) Section 6.4.2, the Control Room HVAC systems are capable of maintaining the control room atmosphere suitable for occupancy throughout the duration of a Design Basis Accident (DBA). The HVAC systems are capable of both automatic and manual transfer from the normal operating mode to the isolation mode. Transfer of the Control Room HVAC systems to the emergency (pressurization) mode of operation is not a fully automatic operation, since some Control Room HVAC system components must be manually started to operate the Control Room HVAC systems in the emergency (pressurization) mode. The manual actions required when placing the Control Room HVAC system into the pressurization mode following an accident include: (1) starting the refrigeration compressor unit; and (2) starting one AFU booster fan.
Per UFSAR Section 6.4.4.1, the Control Room HVAC system provides radiation protection by pressurizing the control room emergency zone with filtered air, isolating the normal outdoor air intakes, and isolating the areas not included in the control room emergency zone. This zone isolation with filtered pressurization air provides radiation protection by minimizing the infiltration of unfiltered air into the control room emergency zone. A positive pressure of 1/8 in. H2O is maintained by supplying 1800-2200 scfm of outdoor air through the AFU with an iodide removal efficiency of 99%.
UFSAR Section 6.4.4.2.3 provides that the Control Room HVAC System provides toxic gas protection to the control room emergency zone in case of either an onsite or offsite toxic chemical accident. The system provides this protection by either manual isolation through operator action or automatic isolation through the use of a toxic gas analyzer. A monitor is provided for ammonia since the control room concentrations for this chemical reaches the toxicity limits faster than the operator can manually isolate the system after detection of odor.
Safety Impact Per TS Bases 3.7.4, the CREV System is considered operable when a booster fan is operable, High Efficiency Particulate Air (HEPA) filter and charcoal adsorbers are not excessively restricting flow and are capable of performing their filtration functions, the heater, ductwork, valves, and dampers are operable, and air circulation through the filter train can be maintained.
There are two redundant AFU booster fans in the CREV System. Immediately downstream of each AFU booster fan is a discharge damper that opens when its AFU booster fan is running and closes when its AFU booster fan is tripped or manually secured. The discharge damper for the non-running AFU booster fan remains closed to prevent recirculation and to maintain 1800-2200 scfm of filtered air supply to the CREV system. If an AFU booster fan discharge damper becomes stuck in a throttled position, rated airflow cannot be assured and the CREV system is considered inoperable per TS 3.7.4, Condition A.
The primary purpose of the discharge damper is to prevent reverse airflow, or recirculation, through the non-running fan. Although the CREV System was momentarily inoperable per TS 3.7.4, Condition A, there was no DBA condition in progress requiring operation of the CREV system. Furthermore, the CREV function was restored when the affected AFU booster fan and damper were taken out of service with its discharge damper secured closed, thus ensuring that recirculation could not occur.
Per UFSAR Section 6.4.4.1, Radiological Protection, the control room emergency zone will be filtered with outdoor air no later than 40 minutes following a LOCA. Procedural requirements are in place for Operations to manually start the AFU associated with the Control Room HVAC System within 40 minutes of a LOCA. Operators are also trained on the procedures that require the manual start of this system.
Troubleshooting that occurred immediately following the event verified that the damper did not experience any mechanical binding when opening, which allows for its associated AFU booster fan to be available in the event of a DBA. However, since mechanical binding of the AFU booster fan damper occurred during closing, this event is a functional failure of the CREV system because the manual AFU start procedures do not require Operators to check the system to identify booster fan damper position failures, and a failure of the non-running fan damper to close could allow recirculation and impede the required 1800-2200 scfm of filtered air supply to the CREV system.
Risk Insights The plant Probabilistic Risk Assessment (PRA) model gives no credit to the Control Room Envelope (CRE) and does not include it in the model; hence, the CREV AFU Booster Fan damper failing to close did not contribute to an increase in risk.
In conclusion, the overall safety significance and impact on risk of this event were minimal.
E. CORRECTIVE ACTIONS
Immediate:
1. The toxic gas test was secured and the stuck damper was cycled open and then closed.
Follow-up:
1. Extent of condition evaluation was performed for other dampers in the CREV System.
2. Design enhancements were made to the replacement damper to ensure adequate mechanical clearances and freedom of movement.
3. One additional CREV System damper will be modified.
4. The CREV system PM will be revised to manually stroke the dampers being inspected.
F. PREVIOUS OCCURRENCES
The station events database, LERs, and INPO Consolidated Event System (ICES) were reviewed for similar events at the Quad Cities Nuclear Power Station. This event was attributed to a design flaw that resulted in mechanical binding when the damper is stroked. Based on the cause of this event and the associated corrective actions, the event listed below, although similar in topic, is not considered a significant station experience that would have directly contributed to preventing this event.
- Station Issue Report (IR) 2384445, "Internal Resistance On TB Vent Isolation Damper 2-5772-38A," (9/22/2014) — Internal resistance on a Turbine Building (TB) Ventilation isolation damper caused the discharge damper for TB supply fan 2A to stick in a throttled position. This event is similar in that it involved a mechanical failure that required replacement of the damper. However, the failure mode differed in that the damper design in this event is a louver-type, compared to the butterfly-type damper being evaluated in this LER. Hence, this event is not directly applicable to the event of this LER.
G. COMPONENT FAILURE DATA
Failed Equipment: Damper Component Manufacturer: Pacific Air Component Model Number: SL100R Component Part Number: N/A This event has been reported to ICES as Failure Report No. 317745.