05000395/LER-2010-001

LER-2010-001, Reactor Building Cooling Units Reduced Air Flow Rate Below Technical Specification Limits

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3952010001R00 - NRC Website

PLANT IDENTIFICATION

Westinghouse - Pressurized Water Reactor

EQUIPMENT IDENTIFICATION

XAA0001A, Reactor Building Cooling Unit 1A XAA0001B, Reactor Building Cooling Unit 1B XAA0002A, Reactor Building Cooling Unit 2A XAA0002B, Reactor Building Cooling Unit 2B

IDENTIFICATION OF EVENT

During Refuel 18, Surveillance Test Procedure STP-453.001 was performed in accordance with Technical Specification (TS) 4.6.3.b. to verify operability of the Reactor Building Cooling Unit (RBCU) HEPA (High Efficiency Particulate Air) filter banks. This TS requires the testing to be conducted per the requirements of the Ventilation Filter Testing Program (VFTP) which is described in the TS administrative section 6.8.4.1. The VFTP requires that each HEPA filter bank be capable of passing an air flow rate of 60,270 +1- 10% ACFM (Actual Cubic Feet per Minute). During testing of RBCU XAA0002A, the calculated air flow rate was found to be low outside of the

• required air flow rate range. In order to increase the air flow rate air filters were replaced. However, during the investigation of this event, it was determined that a non-conservative filtration area factor was used in the calculation of the air flow rate. Since STP-453.001 is a common surveillance test for the RBCUs, the non-conservative filtration area factor had previously been used to calculate the air flow rate for all four units. Based on a historical data review for determining past operability, the use of this non-conservative filtration area factor resulted in RBCUs XAA0001A and XAA0002A with air flow rates below the required Technical Specification range between Refuel 16 and 18. Also, RBCUs XAA0001B and XAA0002B had reduced air flow rates below the Technical Specification range between Refuel 16 and 17. The evaluation for past operability determined that this had been a violation of the Limiting Condition for Operation for TS 3.6.2.3 and 3.6.3. This issue was corrected prior to the end of Refuel 18.

EVENT DATE

April 27, 2010 Condition Report CR-09-05126 Condition Report CR-10-01783

REPORT DATE

June 28, 2010

CONDITIONS PRIOR TO EVENT

Mode 1, 100% Power

DESCRIPTION OF EVENT

Surveillance testing is performed every 18 months on the RBCUs to demonstrate the required air flow rate and HEPA filter bank performance. To determine air flow rate through each RBCU the air velocity is measured through each filter element and then summed to give a total velocity through the unit. The total air flow rate through the unit is calculated using the summed velocity and a filtration area factor for each filter element and then corrected to actual conditions. The calculation used a filtration area factor of 4 square feet based on the nominal size of a filter element which is 2 feet by 2 feet. However, this factor did not account for the flange area around each element which does not pass air flow. Thus, the actual area for air flow passage is 3.52 square feet which is the filter area factor that should have been used in the calculation.

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CAUSE OF EVENT

The cause of this event was due to the use of a non-conservative filtration area factor in the calculation of total air flow rate through a RBCU. The surveillance test for determining RBCU air flow rate was based on a procedure which was developed by a contractor who performed the original air flow test for the RBCUs. The contractor used a filtration area factor of 4 square feet which was based on the nominal size, 2 feet by 2 feet, of a filter element. However, this factor did not account for the flange area around each element which does not pass air flow. Thus, the actual area for air flow passage is 3.52 square feet which is the filtration area factor that should have been used in the calculation. The reduced area results in a smaller total air flow rate through the RBCU.

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ANALYSIS OF EVENT

In an emergency, the RBCUs, in conjunction with the Reactor Building Spray System, provide cooling of the reactor building atmosphere. The HEPA filters in the RBCUs also provide cleanup of the reactor building atmosphere following an accident to limit radiological dose. Since the flow rates of the RBCUs have been accounted for in the reactor building analyses for a Loss of Coolant Accident (LOCA) and Main Steamline Break (MSLB) and off-site and control room dose analyses for a LOCA, engineering analyses were performed to determine the affect of the reduced flow rates on these accident analyses.

To assess the impact on the reactor building analyses, heat removal performance was evaluated using the reduced air flow rate and actual plant cooling water temperature. This evaluation determined that the revised heat removal performance had no impact on peak reactor builiding pressure or temperature following a LOCA. Small increases in reactor building pressure and temperature occurred in the long term. However, these small increases remained within existing margins. The revised heat removal performance was also determined to be less limiting than that used in the MSLB which resulted in no impact on the reactor building pressure or temperature for this accident. Based on these determinations, the increases in reactor building post-accident pressure and temperature are not considered safety significant.

The results of the engineering analysis for off-site and control room dose following a LOCA determined that the reduced air flow rate had a negligible impact on whole body gamma and beta skin dose and only a very slight increase in thyroid dose. The LOCA dose results remainded within 10 CFR 100 and General Design Criteria (GDC) 19 limits. The calculated increases were well below what would be considered "more than minimal increase in the consequences of an accident previously evaluated" under 10 CFR 50.59. Based on these determinations, the increases in off-site and control room dose are not considered safety significant.

The engineering analyses support the conclusion that the RBCUs remained capable of performing their design functions with the reduced air flow rates.

CORRECTIVE ACTIONS

Surveillance Test Procedure was-revised-to incorporate the correct filtration area factor into the calculation. for RBCU-flow-rate.-- ---

PRIOR OCCURRENCES

There have been no prior occurrences of this event.

�NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER