05000254/LER-2014-001
| Quad Cities Nuclear Power Station Unit 1 | |
| Event date: | 03-04-2014 |
|---|---|
| Report date: | 05-05-2014 |
| Initial Reporting | |
| ENS 49870 | 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 2542014001R00 - NRC Website | |
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PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 2957 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
Fuel Pool Radiation Monitor Spiked High Due to an Invalid Actuation and the Unit 1 and Unit 2 Reactor Building Ventilation System Isolated which Caused Secondary Containment Differential Pressure to be Momentarily Lost
A. CONDITION PRIOR TO EVENT
Unit: 1 Reactor Mode: 1 Event Date: March 4, 2014 Event Time: 1917 hours0.0222 days <br />0.533 hours <br />0.00317 weeks <br />7.294185e-4 months <br /> Mode Name: Power Operation Power Level: 100%
B. DESCRIPTION OF EVENT
On March 4, 2014, at 1917 hours0.0222 days <br />0.533 hours <br />0.00317 weeks <br />7.294185e-4 months <br />, a 901-3 H16 alarm [RA], Fuel Pool [DA] Channel 1B High Radiation, occurred. The 1B fuel pool radiation monitor [MON] had spiked high at 50 mRem/hr and then trended downward and returned to its previous steady-state value of 14 mRem/hr. The high spike caused the Unit 1 and Unit 2 Reactor Building (RB) ventilation [VA] and Control Room (CR) ventilation [VI] to isolate as designed. The Standby Gas Treatment System (SBGTS) [BH] was already in operation for a scheduled surveillance as of 1900 hours0.022 days <br />0.528 hours <br />0.00314 weeks <br />7.2295e-4 months <br /> on March 4, 2014.
Isolation of the Unit 1 and Unit 2 RB ventilation system caused the secondary containment [NG] differential pressure to be momentarily lost (pressure went positive). Secondary containment was then declared inoperable for the short positive pressure duration.
The RB ventilation system was then shut down for scheduled maintenance and the CR ventilation system was returned to its normal configuration. The SBGTS was operating to support planned RB ventilation system maintenance. Troubleshooting on the radiation monitor spike was initiated.
Operations Log recorded on March 4, 2014 at 1917 hrs that RB differential pressure went positive for a short time.
Entered QGA 300, Secondary Containment Control (No Emergency exists), and entered Technical Specification (TS) 3.6.4.1, Condition A; Secondary Containment was declared inoperable. QGA 300 was exited; RB negative differential pressure was reestablished at greater than 0.1 inches vacuum water gauge; and exited TS 3.6.4.1, Condition A.
Since both Units 1 and 2 share a common RB [NG], the loss of differential pressure that was observed to have occurred (RB pressure went positive) for approximately three (3) minutes, impacted both Units 1 and 2 secondary containments when the Unit 1B fuel pool radiation monitor spiked high.
The RB to atmosphere pressure will momentarily transition to a positive differential pressure until the RB ventilation isolation dampers close within 60 seconds. The required negative differential pressure (0.10 inches water vacuum and greater) is typically then restored within a 2 - 3 minute period (per Station Event Recorder alarm data) once the RB ventilation isolation dampers are fully closed and SBGTS is fully running.
On March 5, 2014, at 0037 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />, ENS#49870 was made to the NRC under 10 CFR 50.72(b)(3)(v)(C), to report this event as an event or condition that could have prevented the fulfillment of a safety function.
An initial investigation was conducted on the fuel pool radiation monitor. Troubleshooting determined that the detector had failed. The failed detector was replaced with a new detector, and the failed detector was then sent to the vendor for failure analysis.
Given the impact on the secondary containment, this report is submitted (for Units 1 and 2) in accordance with the requirements of 10 CFR 50.73 (a)(2)(v)(C), 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 control the release of radioactive material.
C. CAUSE OF EVENT
The cause of the 1B fuel pool radiation monitor spike was a detector failure. The failed detector was sent to the vendor for failure analysis. During testing the vendor identified a double pulsing in the GM-tube [RT]. Multi-pulsing would cause the output of the sensor/converter [CNV] to increase, even when the actual background gamma field had not changed, because the sensor/converter integrates all pulses into the output signal regardless of its origin. If sufficient multiple pulses occur in a short period of time, a spike in the sensor/converter output would then be produced even though there was no actual change in the gamma field intensity. As a result, the vendor determined that the detector failure was caused by a manufacturing defect that caused double pulsing in the GM-Tube.
When the 1B fuel pool radiation monitor spiked due to the detector failure, the Unit 1 and Unit 2 RB ventilation system isolated which caused the secondary containment differential pressure to be momentarily lost (pressure went positive). However, the RB to atmosphere pressure will momentarily transition to a positive differential pressure until the RB ventilation isolation dampers close within 60 seconds. The required negative differential pressure (0.10 inches water vacuum and greater) is typically then restored within a 2 - 3 minute period (per Station Event Recorder alarm data) once the RB ventilation isolation dampers are fully closed and SBGTS is fully running.
D. SAFETY ANALYSIS
System Design The function of the secondary containment is to contain, dilute, and hold up fission products that may leak from primary containment following a Design Basis Accident (DBA). In conjunction with operation of the Standby Gas Treatment System (SBGTS) and closure of certain valves [V] whose lines penetrate the secondary containment, the secondary containment is designed to reduce the activity level of the fission products prior to release to the environment, and to isolate and contain fission products that are released during certain operations that take place inside primary containment, when primary containment is not required to be operable, or that take place outside primary containment.
Updated Final Safety Analysis Report (UFSAR) Section 6.2.3.1 provides that the safety objective of the secondary containment system, in conjunction with other engineered safeguards and nuclear safety systems, is to limit the release of radioactive materials so that offsite doses resulting from a postulated DBA will remain below 10 CFR 100 guideline values.
The SBGTS is designed to maintain the RB (RB is common to both Units 1 and 2) at a negative pressure and to filter the exhaust of radioactive matter from RB spaces to the environment (by particulate filtration and halogen adsorption) in the unlikely event of a DBA, including the Loss of Coolant Accident (LOCA) and the refueling accident. It is also instrumental in maintaining the integrity of secondary containment during a primary to secondary containment instrument line break. Two parallel trains are provided, each of which is capable of producing greater than 0.25 inches water negative pressure required in the RB while processing 4000 cubic ft /min of exhaust air.
Safety Impact Since both Units 1 and 2 share a common RB (secondary containment), when the fuel pool radiation monitor spiked high due to an invalid actuation, the Unit 1 and Unit 2 RB ventilation system isolated which caused secondary containment differential pressure to be momentarily lost.
TS 3.6.4.1, Action A.1, requires restoration of secondary containment to operable status within four hours. This four hour Completion Time provides a period of time to correct the problem that is commensurate with the importance of maintaining secondary containment during Modes 1, 2, and 3, since the probability of an accident occurring during this short period where secondary containment is inoperable is minimal.
The primary purpose of the secondary containment is to minimize the ground level release of airborne radioactive materials and to provide a controlled, elevated release of the building atmosphere under accident conditions. An engineering analysis was performed to demonstrate that during the time that secondary containment differential pressure increased to positive for approximately three (3) minutes, there would be a negligible effect on the resulting dose calculations. Secondary containment would have sufficiently contained radioactive materials during a LOCA such that all current dose limits would remain to be met. Secondary containment would have been able to perform its safety function. Therefore, the dose consequence from postulated releases from the reactor building during this short duration would be bounded by the existing design basis LOCA dose analysis. The safety significance of this event was minimal.
The engineering analysis that was performed demonstrated this event did not constitute a Safety System Functional Failure (SSFF). (Reference NEI 99-02, Revision 7, Regulatory Assessment Performance Indicator Guideline, Section 2.2, Mitigating Systems Cornerstone, Safety System Functional Failures, Clarifying Notes, Engineering analyses.) As such, this event will not be reported in the NRC Performance Indicator (PI) for safety system functional failures since an engineering analysis was performed which determined that the system was capable of performing its safety function during this event when the secondary containment differential pressure increased to positive for approximately three (3) minutes.
Risk Insights The plant Probabilistic Risk Assessment (PRA) model gives no credit to the secondary containment and does not include it in the model, hence the as-found conditions did not contribute to an increase in risk. In addition, the physical integrity of the secondary containment structure was never compromised and the primary containment function was never lost.
Although a secondary containment loss of function (loss of differential pressure) occurred momentarily when the fuel pool radiation monitor spiked high due to an invalid actuation and caused the Unit 1 and Unit 2 RB ventilation system to isolate, there was no DBA condition in progress, and secondary containment function was restored within three (3) minutes when operation of the SBGTS restored the required differential pressure to the RB (secondary containment).
In conclusion, the overall safety significance and impact on risk of this event were minimal.
E. CORRECTIVE ACTIONS
Immediate:
1. An initial investigation was conducted. Troubleshooting determined the detector had failed.
2. Replaced the failed 1B fuel pool radiation monitor detector (sensor/converter) with a new detector to restore function.
Follow-up:
1. The failed detector was sent to the vendor for failure analysis.
2. Revised all associated procurement catalog numbers to require testing and issuance of a certificate to accompany the shipment that the GM-Tube testing has been completed satisfactorily.
F. PREVIOUS OCCURRENCES
The station events database, LERs, and INPO Consolidated Event System ICES (EPIX) were reviewed for similar events at Quad Cities Nuclear Power Station. This event was a fuel pool radiation monitor spiked high due to an invalid actuation and the Unit 1 and Unit 2 RB ventilation system isolated which caused secondary containment differential pressure to be momentarily lost. Based on the conditions of this event, causes, and associated corrective actions, the events listed below, although similar in topic, are not considered significant station experiences that would have directly contributed to preventing this event.
- Station Issue Report (IR) 1391595, 1-1705-16B Fuel Pool Rad Monitor Spiked High (7/22/12), and IR 1393190, 1- 1705-16B Fuel Pool Rad Monitor Spike High (7/26/12) - Were due to two separate spurious upscale "spiking" events on the 1B fuel pool radiation monitor. RB and CR ventilation isolated, and SBGTS auto started as designed. In each event, the RB differential pressure was verified to have been maintained negative through the transient. The apparent cause of each event is failure of the detector due to a manufacturing defect. During manufacturing, the routing of the high voltage (HV) lead caused a voltage breakdown through its insulating jacket.
These detector failures involved a sensor that had only been in operation for a few weeks when the failure occurred. These previous events, although similar to this LER event, where a fuel pool radiation monitor spiked high and caused RB and CR vents to isolate, however, did not involve RB differential pressure becoming positive, hence these past radiation monitor failure events are not directly applicable to the event of this current LER.
- Station Issue Report (IR) 1444822, Received Unexpected 912-5 C-1 & C-4 U1 & U-2 RB LO DP Alarms (11/27/12) — Due to signal air and main air line equipment air leaks on the RB ventilation pneumatic control system, the air leaks caused the dampers on the RB ventilation to operate erratically. The RB ventilation did not shutdown or isolate during this event, but a momentary positive RB to atmosphere differential pressure occurred.
The alarm condition cleared within 14 seconds with no operator action. This previous event, although similar to this LER event, where an equipment failure caused RB and CR vents to isolate and RB differential pressure to become positive, is not directly applicable to the event of this current LER, since the event was not caused by a failed fuel pool radiation monitor.
- LERs - A review of LERs at Quad Cities Nuclear Power Station over the past 10 years did not identify any similar events associated with this type of failure.
G. COMPONENT FAILURE DATA
Failed Equipment: Radiation Monitor Component Manufacturer: General Electric Component Model Number: 194X927G016 Component Part Number: 194X927G016 (Range 1 to 106 mR/hr) This event has been reported to ICES as Failure Report No. 310606.