05000315/FIN-2010006-02
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| Description | The inspectors identified an unresolved item (URI) concerning a non-conservative analysis. Specifically, the design calculation performed to determine the pressurizer PORV lift setting while in the LTOP mode of operation failed to consider the instrument uncertainty associated with the pressure instrumentation that actuates the PORV to open when required.
In order to limit RCS pressure at low temperatures, LTOP is established so the integrity of the RCS pressure boundary would not be compromised by violating the pressure and temperature (P-T) limits of 10 CFR Part 50, Appendix G, Fracture Toughness Requirements. The reactor vessel is the limiting RCS pressure boundary component for demonstrating such protection, and is less tough at low temperatures than at normal operating temperature. The potential for vessel overpressurization is most acute when the RCS is water solid, occurring only while shutdown, because a pressure fluctuation can occur more quickly than an operator can react to relieve the condition. Exceeding the RCS P-T limits by a significant amount could cause brittle cracking of the reactor vessel. While in the LTOP mode of operation, vessel protection for overpressure is provided by the PORVs, and, in some cases, in combination with RHR system relief valves. Based on the current Appendix G methodology, the P-T limit was determined to be 621 psig. The LTOP PORV setpoint of 435 psig was determined by Westinghouse in 1989 based on the 10 CFR Part 50, Appendix G limit curves, and LTOP setpoint methodology at the time. Although there were several updates to the P-T limits since 1989, Westinghouse determined there was sufficient margin between the maximum allowable LTOP setpoint and the current PORV setpoint of 435 psig, such that no setpoint change was warranted. Information Notice 93-58, Nonconservatism in Low-Temperature Overpressure Protection for Pressurized-Water Reactors, documented LTOP setpoint errors identified by Westinghouse. These errors resulted in a pressure increase of 88.7 psi to the LTOP setpoint for D. C. Cook. The inspectors reviewed ECP-12-N1-05, LTOP Setpoint Calculation, which documented the LTOP setpoint basis and reconciled the errors mentioned in the IN. This calculation had previously determined the contribution from overshoot (78 psi), the pressure the reactor vessel would be subjected to above the setpoint based on the relief rate of the PORVs size and stroke time. Considering the adjustments for the errors and the contribution from overshoot, the margin to the Appendix G limits was about 19.3 psig (621 psig - 435 psig - 88.7 psig - 78 psig). However, the inspectors determined that the licensee did not account for instrument uncertainty in the PORV lift setting of 435 psig. The IN recommended method to compensate for the pressure increase was to demonstrate that the available margin in the LTOP calculation, taking into account instrumentation uncertainty, was sufficient to offset the plant-specific pressure differences. The Westinghouse letter, AEP-93-208, dated March 10, 1993, contained a similar recommended action to offset the pressure increase by using the instrument uncertainty used in the development of the P-T curves. However, the D. C. Cook P-T curves did not include instrument uncertainty in their development such that it was not available to offset the plant-specific pressure differences. Based on the licensees interpretation of the Westinghouse letter, they did not believe instrument uncertainty needed to be accounted for in the setpoint calculation, especially since they believed it was not within their license basis. In Calculation 1-2-UNC-211 Calc 2, RCS Wide Range Pressure, the licensee had determined there was about 75 psi pressure uncertainty associated with the PORV lift setting, which had not been incorporated in the basis for the current setpoint of 435 psig. After further evaluation during this inspection, the licensee concluded the pressure uncertainty associated with the PORV lift setting was about 68.6 psi. The inspectors reviewed the calculation, and noted there was no seismic contribution to instrument uncertainty in the calculation. Upon review of NUREG-0933, Resolution of Generic Safety Issues, Issue 70, PORV and Block Valve Reliability (Rev. 3), the inspectors determined that the seismic contribution to instrument uncertainty needed to be addressed. If instrument uncertainty, without the seismic consideration, was included in the calculation, the results would be a negative margin of 49.3 psi (19.3 psi 68.6 psi) to the Appendix G limit of 621 psig. The inspectors were concerned that the existing PORV setpoint would not limit an overpressure condition such that Appendix G limits could be exceeded. In summary, based on the above, the available margin to the Appendix G limit was as follows: _ 435 psig, the existing PORV setpoint; _ + 88.7 psi error determined in the response to Westinghouse/IN 93-58; _ + 68.6 psi instrument uncertainty (not including the seismic contribution); _ +78 psi maximum overshoot applied to the LTOP setpoint (12-N1-05, Rev. 13); and _ = 670.3 psig which exceeded the Appendix G limit of 621 psig. In response to inspectors concerns, the licensees initiated AR 2010-10197 to further evaluate the margin issue. Westinghouse performed an evaluation that was documented in letter, AEP-10-127, dated September 24, 2010. This evaluation identified approximately 50 psi of additional margin based on changes in P-T limits calculation methodology or removal of excessive calculational conservatisms (e.g., use of ASME Boiler and Pressure Vessel Code Case N-640, Alternative Reference Fracture Toughness for Development of P-T Limit Curves,Section XI, Division 1). Applying the 50 psi margin available via Code Case N-640, the Appendix G limit would increase to 671 psig (621 psig + 50 psi). The Westinghouse letter also identified an analytical margin of approximately 35 psi based on the margin between the current PORV setpoint and the maximum allowable setpoint and conservatisms in the LOFTRAN computer code analyses. Therefore, the overall margin would be approximately 35.7 psi (- 49.3 psi + 50 psi + 35 psi). The licensee also concluded that any seismic contribution to instrument uncertainty would be small and would not exceed the margin determined by Westinghouse. The licensee concluded that based on the above information from Westinghouse, the PORV setpoint was acceptable and the PORVs were operable but non-conforming. With the apparent 35 psi of margin, there was reasonable assurance that the existing LTOP setpoint of 435 psig will provide adequate overpressure protection, even when considering the seismic contribution to instrument uncertainty. The inspectors concluded that the above analysis was acceptable from an operability standpoint. Although there was no immediate technical concern based on the licensees analysis, the licensee disagreed with the NRCs position on whether the inclusion of instrument uncertainty in the LTOP setpoint was within their license basis. The licensee did provide the inspectors with correspondence during the upgrade to Improved Technical Specifications where they informed the NRC that instrument uncertainty was not included when the LTOP setpoint was established. However, since the setpoint was not being revised, it was not clear whether Office of Nuclear Reactor Regulations (NRR) reviewed and agreed with the licensees position on instrument uncertainty. Since the inspectors were unable to verify whether the use of instrument uncertainty was part of the license basis and the perceived difference between the recommended actions in the IN and the Westinghouse letter, this issue is considered an unresolved item (URI 000315/2010006-02; 000316/2010006-02) pending further discussions with NRR to resolve these two concerns. |
| Site: | Cook  |
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| Report | IR 05000315/2010006 Section 1R21 |
| Date counted | Dec 31, 2010 (2010Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | A Dunlop M Marshfield J Lennartz R Walton T Go P Laflamme T Brileya Dunlopr Langstaff G Hausman B Sherbin O Mazzoni L Jones J Corujo-Sandin N Melly |
| INPO aspect | |
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Finding - Cook - IR 05000315/2010006 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Cook) @ 2010Q4
Self-Identified List (Cook)
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