05000370/LER-2011-003
| Mcguire Nuclear Station, Unit 2 | |
| Event date: | 12-20-2011 |
|---|---|
| Report date: | 2-16-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3702011003R00 - NRC Website | |
BACKGROUND
The following information is provided to assist readers in understanding the event described in this LER.
Applicable Energy Industry Identification [EllS] system and component codes are enclosed within brackets.
McGuire unique system and component identifiers are contained within parentheses.
The McGuire containment building has a secondary containment which is a concrete structure that surrounds the steel primary containment vessel. Between the steel containment vessel and the reactor building inner concrete wall is an annulus that collects any containment leakage that may occur following a loss of coolant accident (LOCA) or rod ejection accident.
The Annulus Ventilation System (AVS) [VD] establishes a negative pressure in the annulus between the concrete reactor building and the steel containment vessel. The AVS consists of two separate and redundant trains. Each train includes a moisture eliminator [SEP], two heaters [HTR], a pre-filter [FLT], upstream and downstream high efficiency particulate air (HEPA) filter banks[FLT], an activated charcoal adsorber [ADS] between the HEPA filter banks, and a fan [FAN]. The HEPA filters and charcoal adsorbers control the release of radioactive contaminants to the environment. The heaters are designed to reduce the relative humidity of the incoming airstream.
Per Technical Specification (TS) 3.6.10 Condition A, with one AVS train inoperable, the inoperable train must be restored to operable status within seven days. If the AVS train cannot be restored to operable status within seven days, the unit must be brought to at least Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and to Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
Per TS 3.6.10 Condition B, with one or more AVS heaters inoperable, the heater must be restored to operable status within 7 days. Alternatively, a report must be initiated within 7 days in accordance with TS Reporting Requirement 5.6.6, which details the reason for the heater's inoperability and the corrective action(s) required to return the heater to operable status. This Report shall be submitted within the following 30 days.
Surveillance Requirement (SR) 3.6.10.1 operates each AVS train for at least 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> with the heaters operating.
This SR ensures operability of the AVS trains and controls and eliminates moisture on the HEPA filter banks and charcoal adsorbers. This SR is performed monthly.
SR 3.6.10.2 performs the AVS filter testing in accordance with the Ventilation Filter Testing Program (VFTP). The VFTP also specifies a heater dissipation test. This SR is performed on an 18 month frequency.
EVENT DESCRIPTION
On Saturday, October 29, 2011, ten minutes after starting the 2B AVS Filter Train Operability Test to satisfy SR 3.6.10.1, the indicating lights for 2B AVS heaters on the HVAC panel went dark. The Operability test operates the respective AVS fan for ten hours with the heaters energized. After the test was stopped, the light bulbs were inspected and it was found that all four bulbs had blown. The bulbs were replaced but the indicating lights failed to turn back on. An Operator was dispatched and reported that the "Heater On" and "Heater Power On" lights on the local panel were also off. It was then discovered that the incoming power breaker for the heaters had tripped.
A Work Request was written to investigate the cause of the breaker trip. The AVS filter train has two matching 21.5 Kilowatt (kW) delta connected heaters which preheat the air before it reaches the charcoal adsorbers.
Maintenance technicians took resistance readings on the heaters and found that one had values of 47.3, 95.2 and 48.2 ohms; while the other had values of 31.6, 32.1 and 32.2 ohms. The technicians suspected that the heating element with the unbalanced resistance readings may have partially burnt out. They checked the circuit for grounds and did not find any.
On Sunday, October 30, after a verbal discussion between Operations and the Maintenance technicians, it was decided to close in the heater power breaker and re-run the 2B AVS Train Operability test while the technicians took current readings. Current values were determined to be 33.9, 43.4 and 33.7 amps which appeared normal and below the trip setpoint of the heater breaker. The Operability test continued and was completed successfully.
Since the heater power breaker did not trip during the ten hour test and the heater "ON" indicating lights remained lit, the work request was closed and the 2B AVS train and heater were declared operable.
On December 6, 2011, Engineering initiated a cause evaluation of the October 29 2B AVS breaker trip after a Maintenance Rule evaluation determined the breaker trip was a Maintenance Preventable Functional Failure.
While researching the troubleshooting actions of October 29 and 30, Engineering determined that based on the data collected by the Maintenance technicians, one leg of one of the two delta connected heaters was probably open as suspected by the technicians. Engineering realized that the Work Request had been closed without repair or replacement of the heater, thus the heater was still degraded and its operability was in question.
On December 6, Engineering contacted the on-duty Operations Shift Manager who conservatively declared the 2B AVS heater inoperable, and a new Work Request was written to re-test and repair the heater.
On December 7, the degraded heater was re-tested by applying the heater dissipation test per the VFTP. It was first believed that the degraded heater barely passed the dissipation test.
On December 9, the degraded heater was replaced and retested by applying the AVS heater dissipation test.
After successful completion of the test, the 2B AVS Train and heater were declared operable on December 10, 2011.
On December 20, Engineering concluded that the true, total kW for the degraded heater did not meet the minimum required value from the VFTP and was thus past inoperable from October 29, 2011 until December 10, 2011. The formula used to calculate total kW from the heater dissipation test procedure would not apply to a heater with one degraded/failed element.
Engineering now suspects that one leg of the delta connected heater element faulted on October 29, ten minutes after starting the 2B AVS Filter Train Operability Test. This fault caused some of the indicating lights to burn out and tripped open the breaker. The fault was self clearing such that when the event was over, no short remained.
One leg of the heater element was an open circuit the other two legs retained their designed resistance. When Operations returned the heater to service, the delta connected heater provided approximately two-thirds of the rated kW. This caused the unit to pull less current than designed, thus the breaker did not trip during the subsequent ten hour Operability Test.
REPORTABILITY DETERMINATION
Per TS 3.6.10 Condition B, with one or more AVS heaters inoperable, the heater must be restored to operable status within 7 days. Alternatively, a report must be initiated within 7 days in accordance with TS Reporting Requirement 5.6.6. This Report shall be submitted to the NRC within the following 30 days.
Since it was determined the 2B AVS heater inoperability existed since the breaker trip on October 29, 2011, the Completion Times for Condition B were exceeded. As such, this condition represents an "Operation or Condition Prohibited by Technical Specifications" reportable under 10 CFR 50.73(a)(2)(i)(B).
CAUSAL FACTORS
1. The AVS Filter Train Operability Test Procedure used to satisfy SR 3.6.10.1 was incorrect in stating that procedure performance would demonstrate operability of the AVS heaters.
The purpose statement of the procedure states, "To demonstrate operability of 2B AVS fan, HEPA filters and pre- heaters." In addition, the acceptance criteria states, "Flow through AVS filter train for at least 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> with pre- heaters energized. (Pre-heater operability met by checking pre-heaters energized at end of 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />). " The actual determination of AVS heater operability is tied to the kW output of the heaters which is defined in the VFTP, Section 5.5.11e, as 43+/- 6.4 kW (heater dissipation test). The VFTP tests are required by SR 3.6.10.2.
The SR 3.6.10.1 Operability Test Procedure does not determine the kW rating of the heaters, nor does it reference the heater dissipation test procedure.
2. Insufficient troubleshooting was applied to the heater breaker trip event on October 29, 2011.
The purpose for writing the Work Request was to investigate the cause of the breaker trip. Work performed under a Work Request on safety related components is narrow in scope and non-intrusive. In this scenario, the most that could be accomplished is to perform a visual inspection and/or take readings to determine possible causes.
To validate the assumption of the possible degraded heater would have required acknowledgement by the Controlling Group (Operations) in agreement with Maintenance for more intrusive work/testing which would require a Work Order to be generated. The Work Order would then require additional inputs and planning to permit Maintenance to perform additional troubleshooting and/or work. Once it was decided to perform the AVS Train Operability Test again, no additional trouble shooting was performed.
CORRECTIVE ACTIONS
Immediate:
1. The 2B AVS heater was replaced and tested satisfactorily.
Subsequent:
1. All AVS Filter Train Operability Test Procedures used to satisfy SR 3.6.10.1were revised to clarify that performance of this procedure does not demonstrate operability of the AVS heaters and to provide a reference to the heater dissipation test procedures.
2. A Ventilation System Heater Failure Report per TS 5.6.6 was developed and submitted to the NRC on January 24, 2012.
Planned:
1. Clearly delineate the Operations Procedure requirements for reviewing work request or corrective work order completion comments when restoring inoperable systems and components to operable status related to failures involving Technical Specifications and/or Surveillance Requirements.
SAFETY ANALYSIS
The McGuire TS Bases 3.6.10 states that the AVS heaters are not required or credited for the operability of the AVS Filter Trains.
The AVS heaters were originally designed to limit the incoming airstream to 70% relative humidity in order to ensure the efficiency of the charcoal adsorber. The quantity of water retained by charcoal is dependent on temperature, and less water is retained as the temperature rises. The water retained by the charcoal decreases its efficiency in adsorbing other contaminants. The AVS filter efficiencies are inputs into the LOCA and Rod Ejection dose analyses.
The operability requirements of the AVS heaters were separated from the operability requirements for AVS Filter Trains by License Amendment and NRC Safety Evaluation dated July 15, 1991. The AVS heaters are not required to maintain the airstream relative humidity to 70% or less due to the more stringent charcoal adsorber testing requirements now in place (methyl iodide penetration test per ASTM D3803-1989). Charcoal adsorber efficiency testing is performed at 30°C and 95% relative humidity as required by TS Program 5.5.11, the Ventilation Filter Testing Program (VFTP).
NRC R012M 366A U.S. NUCLEAR REGULATORY COMMISSION (io-2010) With the addition of the heater specific TS Action in 1991, McGuire is committed to maintaining the AVS heaters in an operable status as an added conservatism to the dose analysis by maintaining the relative humidity of the airstream to well below 95%.
The McGuire accident analysis shows that the site boundary radiation and Control Room doses are within the 10 CFR 50.67 limits during a DBA LOCA under the AVS airstream conditions of 95 % relative humidity. As such, during this period of heater reduced capacity and inoperability, the 2B AVS Filter Train was always capable of performing its safety function.
ADDITIONAL INFORMATION
To determine if this event was recurring, a search of the McGuire Problem Identification Process (PIP) database was conducted for a time period covering five years prior to the date of this event. Each corresponding cause code, "Technical inaccuracies" and "Incorrect assumptions used to make decision," brought back five similar events. A thorough review of these ten events concluded that none were similar enough to call this event recurring.