05000338/LER-2007-002
| Document Numbersequential Revmonth Day Year Year Month Day Year | |
| Event date: | 09-26-2007 |
|---|---|
| Report date: | 11-26-2007 |
| 3382007002R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT and Unit 2, the main FW (EIIS-SJ) flow venturis installed on Unit 1 were removed from the piping system during the September 2007 refueling outage and sent to a vendor lab for calibration. The calibration resulted in flow discharge coefficients for the FW venturis that were higher than the installed flow discharge coefficients and led to a reduction in generated power output. The average flow coefficient for the FW venturis measured at the vendor lab was 0.758% higher than the average of the coefficients previously installed.
Adjustments to the new FW venturi flow coefficients resulted in a decrease in operating power by 0.758% or about 7.4 MWe. The change in FW flow coefficients is bounded by the analysis of record for the calorimetric uncertainty and no accident analysis limits for Unit 1 were exceeded.
The previous FW flow coefficients were installed in 1999. Prior to that time, both North Anna Units 1 and 2 reactor power were measured using a steam flow calorimetric calculation. Trending data had shown that the steam flow venturis were subject to fouling, resulting in reduced power output. In 1999, based on data collected using ultrasonic flow measurement equipment, new flow coefficients were installed on both units. Calorimetric instrument uncertainty calculations documented that the flow measurement uncertainty was 0.923% for the flow testing conducted to support the 1999 transition to a FW flow calorimetric. Therefore, calorimetric power measurement remained within the required 2% accuracy value in our licensing basis for compliance with 10CFR50, Appendix K.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS Operation of Unit 1 has remained within the safety analysis limits and licensing basis requirements (10CFR50, Appendix K.) Operability of equipment is not impacted for Unit 1.
Design changes installed revised FW and steam flow discharge coefficients associated with a recalibration of the FW flow venturis. In addition, a design change installed Rosemount differential pressure (dp) transmitters as replacement for the Foxboro dp transmitters.
Operability of equipment is not impacted for Unit 2. As stated above in Section 1.0, the existing Unit 2 FW flow coefficients were installed in 1999 based on data collected using ultrasonic flow measurement equipment. Calorimetric instrument uncertainty calculations documented that the flow measurement uncertainty was 0.923% for the flow testing conducted to support the 1999 transition from a steam flow to feedwater flow calorimetric.
Therefore, Unit 2 calorimetric power measurement remains within the required accuracy value in our licensing basis for compliance with 10CFR50, Appendix K. As noted in Section 4.0, recalibration of the existing flow venturis and replacement of the dp transmitters is planned during the 2008 Unit 2 refueling outage.
�NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER 3.0 CAUSE The apparent cause of the different flow discharge coefficients was the use of flow calibrations with larger uncertainties. The flow coefficients for the FW venturis at North Anna Unit 1 were adjusted in 1999 via design change using two diverse external ultrasonic flow measurement devices. For the calorimetric, a 0.758% change in FW flow coefficients was bounded by the calculated uncertainty value of +1- 0.923% and remained within the required 2% accuracy value in the licensing basis for compliance with 10CFR50, Appendix K. The uncertainty of flow coefficients determined during the September 2007 refueling outage at the vendor's lab is 0.25%.
4.0 ADDITIONAL CORRECTIVE ACTIONS Recalibration of North Anna Unit 2 FW flow venturis will be performed during the fall 2008 refueling outage via design change.
Replacement of FW flow transmitters (Foxboro dp with Rosemount dp) at North Anna Unit 2 will be performed during the fall 2008 refueling outage via design change.
NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER