05000306/LER-2014-002
| Prairie Island Nuclear Generating Plant Unit 2 | |
| Event date: | 5-19-2014 |
|---|---|
| Report date: | 7-15-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3062014002R00 - NRC Website | |
During a Unit 2 Containment closeout inspection it was discovered that the lower face on the northeast corner was intermittently leaking on 23 Fan Coil Unit (FCU)1 Heat Exchanger. A sample was taken from the leaking water. The sample results indicated hardness values consistent with river water.
The FCU is considered part of the Containment boundary. The leak caused a breach to this boundary, which could result in radioactivity traveling from Containment into the CL system, and resulting in off-site dose to the public.
Containment inoperable and entered Technical Specifications (Tech Specs) Limiting Condition for Operation (LCO) 3.6.1 Condition A, for Containment inoperable in Modes 1, 2, 3, and 4.
At the time of the Containment inoperability, Unit 2 was in Mode 3 for a Maintenance Outage. Tech Specs LCO 3.6.1 Condition A, states to "Restore Containment to OPERABLE Status" within one (1) hour.
Operations personnel isolated the Cooling Water (CL) supply to and return from 23 FCU, opened the associated Motor Valve (MV) breakers, and pressurized the space between the Cooling Water outlet motor valves to greater than 46 psig per C35 AOP4 "Cooling Water Leakage in Containment." This allowed the Operations Department to exit Tech Specs LCO 3.6.1 Condition A, at 1319 on May 19, 2014.
At approximately 1420 on May 19, 2014, Construction Pipefitters entered the Containment under Work Request (WR) 00103604 to check and apply torque to the lower northeast face header bolts.
Using PM 3154-4-23, 23 Containment FCU Cooling Coil Maintenance and Inspection, the bolting was checked to 40ft-lbs. All bolts appeared to be at equal torque in the as found condition. The bolts were then tightened to a torque value of 60ft-lbs, the maximum allowable per Appendix B in PM 3154-4-23.
At 1634 on May 19, 2014, Operations invoked Tech Specs LCO 3.0.5, this allows restoration of 23 FCU under administrative control solely to perform testing required to demonstrate operability of Containment.
At approximately 1645 on May 19, 2014, Operations personnel restored Cooling Water to 23 Containment FCU, the Preventative Maintenance Testing (PMT) on the northeast lower face flange was satisfactory. No leakage was observed. During the PMT, a second leak was identified on the 1 EIIS System Code - FCU APPROVED BY OMB: NO. 3150-0104 EXPIRES: 1/31/2017 hours. Reported lessons learned are Incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the Records and FOIA/Privacy Service Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by Internet e-mail to Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
identified on the southwest Cooling Water supply inlet flange as required by Tech Specs LCO 3.6.1 Condition A to restore Containment operability.
At 0601 on May 20, 2014, PMT was completed satisfactorily following CL leak repairs for 23 FCU northeast corner flange and southwest CL supply inlet flange. No leakage was observed.
Containment is operable.
This event is reportable under 50.73(a)(2)(v)(C), Event or Condition That Could Have Prevented Fulfillment of a Safety Function to control the release of radioactive material.
EVENT ANALYSIS
The Containment Cooling System consists of four fan-coil units located in the Containment Vessel.
These will re-circulate and cool the Containment Vessel atmosphere. The heat sink for the fan coils is provided by the containment and auxiliary building chilled water system or by the Cooling Water System. During emergency situation the heat sink for the fan coils is provided by the Cooling Water System. Additional circulating fans are provided as required to ensure a positive flow of air to the areas around the Control Rod Drive Mechanisms (CRDMS), the reactor cavity, and Reactor Primary Coolant Pump Motors.
Fan coil units inside containment are provided with water from the plant Cooling Water System when they are operating in their safeguards mode. Portions of the Cooling Water System serving the fan coil units are designed to tolerate a single active failure, designed as Class I seismic, and are missile protected. With the exception of the initial hours after an accident, Cooling Water System pressure exceeds postulated containment accident pressure. Thus, there is minimal potential for leakage of radioactive material out of the containment via the Cooling Water System. Any leakage would be detected by the Radiation Monitors and the affected FCU isolated as discussed below.
The Cooling Water Supply lines to the fan coil units are provided with a remote manual motor operated gate valve outside containment. Return lines are provided with a remote manual motor operated gate valve inside containment and a remote manual motor operated globe valve outside containment.
In the event of accident, the cooling water supply and return isolation valves position to full open to satisfy their safeguards function. In the event of a fan coil unit or associated piping rupture the containment remote manual motor operated isolation valves would be closed to prevent the entry of non-borated water into containment. Pressure against the closed isolation valves is maintained by APPROVED BY OMB: NO. 3150-0104 EXPIRES: 1/31/2017 hours. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the Records and FOIA'Privacy Service Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
equalizing lines. The water supply for this "seal" is provided by the cooling water system pumps (3 motor driven and 2 diesel driven) which take suction from the Mississippi River.
The Containment Air Cooling System is sized such that any three fan coil units will provide adequate heat removal capacity from the Reactor Containment during normal and full-power operation, to maintain interior air temperatures below the maximum temperature allowable at any component, and to obtain temperatures below 104 degrees Fahrenheit (F) in accessible areas during hot standby operation. The fan coil units are also utilized for emergency cooling under post-accident conditions.
SAFETY SIGNIFICANCE
There were potential nuclear, radiological, and industrial safety impacts due to the loss of Containment integrity which could result in off-site dose to the public during a Design Basis Accident.
There was regulatory impact in that the inoperable Containment Fan Coil Unit was a loss of Containment cooling and loss of safety function that required a 10 CFR 50.72 report.
CAUSE
The Root Cause evaluation determined that the maintenance procedures and work plans for installation of Containment FCU Head Face Flange gaskets and inlet/outlet piping spacers/gaskets do not meet station planning standards for critical component maintenance and repair to prevent leakage through all ranges of operation.
The contributing causes were inadequate design of Fan Coil Unit inlet and outlet flange spacers allows for misalignment during installation and that the station has not tracked and identified a sufficient torque value within the design range to ensure a leak tight joint at the Containment FCU header face flange.
CORRECTIVE ACTION
1. Corrective Action to Prevent Recurrence (CAPR), Revise Containment FCU Maintenance and Inspection procedures to specify a 45ft-lbs torque value for the Containment FCU Head Face Flanges vice providing an allowable range.
2. (CAPR), Revise Containment FCU Maintenance and Inspection procedures to direct workers to take measurements of inlet/outlet flange spacers during installation to ensure spacers are centered in the flange.
APPROVED BY OMB: NO. 3150-0104 EXPIRES: 1/31/2017 hours. Reported lessons teamed are incorporated Into the licensing process and fed back to industry. Send comments regarding burden estimate to the Records and FOIA/Privacy Service Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the Information collection.
3. Condition Evaluation (CE), Evaluate the design of the CL inlet and outlet flange spacers to identify a spacer solution that eliminates the possibility of misalignment or a design that eliminates the pipe flange spring/pull.
PREVIOUS SIMILAR EVENTS
On May 20, 2012, with Unit 2 in Mode 5 a leak was identified on 23 Containment FCU, CL flange gasket. Per the Work Order, the gasket removed for the FCU to CL flange that was leaking on the 23 FCU was inspected and found to be in good condition. The CL pipe flanges were also inspected during the leak repairs. The sealing surfaces were experiencing heavy pitting. The Cooling Water piping at this joint is constructed of carbon steel. Carbon steel is susceptible to corrosion and can cause pitting on unprotected surfaces. The most likely cause of the leak on the 23 FCU was pitting of the pipe flange mating surfaces.
On October 24, 2012, with Unit 1 in Mode 4 a gasket leak was identified on the #1 face of the 12 Containment FCU. The most likely cause of the leak on the 12 FCU was exceeding the service life of the header box gasket.