05000364/LER-2012-001
| Joseph M. Farley Nuclear Plant, Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3642012001R01 - NRC Website | |
FACILITY NAME DOCKET NUMBER
FACILITY NAME DOCKET NUMBER
Westinghouse -- Pressurized Water Reactor
Description of Event
As a result of the 2011 Component Design Basis Inspection (CDBI), Plant Farley received a Licensee Identified Violation (LIV 364/2011010-005) for the actual bore size of the Unit 1 Motor Driven Auxiliary Feed Water (MDAFW) system discharge flow orifice (F02861A) [OR] deviating from the nominal bore size specified in the design drawing by 0.03125 inches and exceeding the assumed hydraulic analysis maximum bore size by 0.0156 inches. To address this condition, the Unit 1 MDAFW and Turbine Driven Auxiliary Feed Water (TDAFW) flow orifices were replaced during the spring 2012 Unit 1 refueling outage to reduce the bore size to within tolerance and provide additional margin in the AFW flow analyses. Inspection of the replaced orifice plates revealed they were in good condition, of adequate thickness, and had experienced insignificant erosion.
In order to similarly restore the Unit 2 Auxiliary Feed Water (AFW) flow orifices to design specifications and provide additional margin in the Unit 2 AFW flow analysis, the Unit 2 MDAFW and TDAFW flow orifice plates were replaced during the 2013 spring U2 refueling outage. On July 27, 2012, as part of planning for the Unit 2 MDAFW orifice replacements, walk-downs of two of the three Unit 2 MDAFW flow orifice plates were conducted. These inspections identified that although the engraved bore size information matched the design drawings, the flow orifice thickness was 1/8 inch instead of 1/2 inch as design drawings specified. A subsequent evaluation of the orifice plate thickness determined that the Unit 2 MDAFW flow orifices were degraded but operable.
MDAFW orifice plate thicknesses, an inspection of the Unit 2 TDAFW flow orifice plates was conducted. On September 12, 2012 at 0614 while Unit 2 was in Mode 1 at approximately 100% power, an external inspection of the Unit 2 TDAFW flow orifice plates determined the plate thickness to be 1/8 inch versus 1/2 inch as specified in design drawings. Based on analysis, the 1/8 inch thick TDAFW orifice plates could not be assured of maintaining proper geometry and integrity when subjected to the pressure drop across the plate under maximum design TDAFW flow conditions, and therefore the operability of the Unit 2 TDAFW system might not be supported. As a result of this analysis, the Unit 2 TDAFW Pump was declared inoperable at 0614 on September 12, 2012 and Technical Specification 3.7.5 Condition B was entered.
Implementation of Temporary Modification SNC429958 to limit TDAFW orifice flow, thereby reducing the pressure drop to an acceptable level, by restricting the open movement of TDAFW flow control valves was completed at 1534 on September 12, 2012. This action allowed restoration of the TDAFW Pump to OPERABLE status within the time limit of Technical Specification 3.7.5 condition B. Unit 2 remained in Mode 1 at 100% power for the duration from the time of discovery of the non-conformance (0614 on September 12, 2012) until the restoration of operability (1534 on September 12, 2012). There were no other inoperable structures, systems, or components during this time period that contributed to the event.
Since the 1/8 inch orifice plates have been in place for the life of the plant, this represents a condition prohibited by Technical Specifications and is reportable under 10CFR50.73(a)(2)(i)(B).
This condition is additionally reportable as an unanalyzed condition that degraded plant safety under 10CFR50.73(a)(2)(ii)(B) and as a condition that could have prevented the fulfillment of a safety function under 10CFR50.73(a)(2)(v)(D).
During the spring 2013 refueling outage, all Unit 2 TDAFW 1/8 inch flow orifice plates were replaced with orifice plates of design thickness. The removed orifice plates were inspected and were all found to be intact, of proper bore size, and in good condition with the exception of noticeable deformation (bulging) of the plates. These findings were not inconsistent with the previous deterministic and risk-based assessments that were performed. No further inspections or testing will be conducted.
Cause of Event
The inadequate orifice plate thickness is considered a latent construction issue. Two design changes were issued during original plant construction that required installation of adequately sized flow orifices. For undetermined reasons, the replacement of the 1/8 inch thick orifice plates as required by the design changes did not occur. Therefore, the direct cause of the event was the failure to correctly implement these two design changes during plant construction.
Research of plant construction documentation was unable to identify reasons for the failure to implement the design changes.
Safety Assessment An Engineering analysis of design basis events with an assumed failure of the installed 1/8 inch thick TDAFW flow orifice plates has determined that during a main steam line break or a main feed water line break event with successful injection from both MDAFW pumps, there would be adequate AFW flow to the intact steam generators. It can be noted however that under these conditions, the TDAFW Pump would operate in a run-out condition, and if one of the two MDAFW Pumps were out of service or failed during either of these two events, then minimum required flow would not be achieved. This analysis of potential impact on TDAFW pump flow does not consider the ability of the control room operator to modulate TDAFW pump speed and TDAFW flow control valve position to avoid or mitigate runout if necessary. No other design basis accidents were impacted by the non-conforming Unit 2 TDAFW orifice plates.
Additionally, SNC Risk Informed Engineering (RIE) conducted a risk evaluation of the impact at Farley Unit 2 of secondary side break (i.e. main steam line break and main feedwater line break) events. The evaluation assumed failure of the TDAFW pump due to run-out for these MDAFW pumps. The current Farley Unit 2 average risk model (Revision 9, Version 2) that considers only hazards from internal events and internal flooding was used. Based on the results of this risk evaluation, it was concluded that the potential increase in risk resulting from the smaller plate thickness is considered "very small" in accordance with the acceptance early release frequency (LERF) (i.e., less than 1E-06 and 1 E-07 per reactor year, respectively).
The non-conformance to design specifications of the Unit 2 TDAFW orifice plates with respect to plate thickness had no actual adverse effect on the safety and health of the public since there has been no occurrence of accident conditions involving a main steam line break or a main feed water line break.
During the spring 2013 refueling outage, all Unit 2 TDAFW 1/8 inch flow orifice plates were replaced with orifice plates of design thickness. The removed orifice plates were inspected and were all found to be intact, of proper bore size, and in good condition with the exception of noticeable deformation (bulging) of the plates. These findings were not inconsistent with the previous deterministic and risk-based assessments that were performed. As a result, no further changes to the safety assessment are necessary.
Corrective Action To restore the Unit 2 TDAFW Pump to operable status Temporary Modification SNC429958 was implemented within the time limit of Technical Specification 3.7.5 condition B to limit the maximum TDAFW orifice through-flow and resultant pressure differential across the orifice plate to an acceptable value.
To restore the Unit 2 TDAFW flow orifices to within design specifications, the orifice plates were replaced with plates meeting design thickness criterion during the Unit 2 spring 2013 refueling outage. The Unit 2 MDAFW flow orifice plates (whose non-conformance to design specifications did not make the motor driven sub-system inoperable) were likewise replaced with plates meeting design thickness criterion during the Unit 2 spring 2013 refueling outage.
Industry Operating Experience (OE) on the event has been issued.
Additional Information
No similar events were discovered.
N RC FORM 366A )1O-2OO)