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Information Notice No. 89-90, Supplement 1: Pressurizer Safety Valve Lift Setpoint Shift
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 April 3, 1991 Information Notice No. 89-90, SUPPLEMENT 1: PRESSURIZER SAFETY VALVE LIFT SETPOINT SHIFT Addressees: All holders of operating licenses or construction permits for nuclear power reactors. Purpose: Information Notice (IN) No. 89-90 was issued to inform licensees about possible problems resulting from operating pressurizer safety valves in environments different from that used to establish the safety valve lift setpoints. The IN primarily focused on problems encountered when valves were operated with a loop seal containing water after setting the safety valve lift setpoints using steam under the seat. This supplement to IN 89-90 alerts addressees to an additional factor that may affect the lift setpoint of all safety (relief) valves. In particular, this problem can affect large steam relief valves such as pressurizer safety valves, main steam safety valves (MSSV), and boiling water reactor (BWR) safety/relief valves. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice supplement do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: On May 28, 1989, a pressurizer safety valve failed open at the V. C. Summer Nuclear Station, requiring the operators to initiate a manual reactor trip due to the rapid depressurization of the reactor coolant system (RCS). The safety valve reseated before the pressure reached the safety injection initiation setpoint of 1850 psig. RCS pressure was then stabilized at approximately 2000 psig. On July 15, 1990, a MSSV lifted for roughly 2 minutes at approximately 1025 psia at the San Onofre Nuclear Generating Station (SONGS). The minimum setting for this valve is 1103 psia, with a nominal setting of 1114 psia. Further testing at SONGS revealed that 4 other recently refurbished MSSVs were lifting at pressures of between 20 to 30 psi below their respective minimum settings. 9103280194 . IN 89-90, Supplement 1 April 3, 1991 Page 2 of 3 Discussion: Utilities have indicated that setpoint shifts can be caused by changes in temperature of the safety (relief) valve body and bonnet. Anything that causes an increase in the temperature of the valve results in the expansion of the body and elongation of the bonnet. This relieves spring pressure and reduces the lift setpoint of the valve. ASME OM Code-1990, Appendix I (previously ASME/ANSI OM-1987 Part 1), "Requirements for Inservice Performance Testing of Nuclear Power Plant Pressure Relief Devices," discusses the importance of ambient conditions for relief valve testing. Personnel at San Onofre and at V. C. Summer Nuclear Station determined that the installation of insulation on the valve can inadvertently increase the temperatures of the valve body and bonnet. If the valve is tested and the lift setpoint is adjusted in an uninsulated condition, the addition of insulation after the valve is installed can cause a subsequent unanticipated increase in valve body temperature and a resultant decrease in the lift setpoint. At the Calvert Cliffs Nuclear Power Plant, personnel determined that the safety valve setpoint shifted from 60 to 150 psi for a 120xF change in valve body temperature. Personnel at the Millstone Nuclear Power Station, Unit 3, indicated that a significant reduction in the lift setpoint of a safety valve was caused by setting the valve at a lower temperature than the actual inservice operating temperature. At Arkansas Nuclear One, Unit 2 (ANO-2), a pressurizer safety valve was refurbished and adjusted at Wyle Laboratories utilizing valve temperature profile data provided by ANO-2 based on measured valve temperatures. After installation of the valve, and plant heatup to hot standby, personnel found that the lift setpoint for the safety valve had decreased by 25 psi, possibly due to actual valve temperatures which were higher than those previously recorded. The temperature error at ANO-2 was attributed to the use of hand-held measuring devices instead of thermocouples that could be mounted on the valve for more accurate temperature monitoring. In addition, changes in the valve's insulation may have contributed to the setpoint shift. These events show that it is critical that the valves be tested under the same ambient conditions that will exist in the plant during operation. Other factors that may possibly affect the lift setpoint of relief valves are discussed in a recent engineering report from the Office for Analysis and Evaluation of Operational Data (AEOD). This report, Engineering Evaluation Report E90-09, "Additional Factors Affecting the Lift Setpoint of Pressurizer Safety Valves," was issued in October 1990. The report documents root causes of relief valve setpoint shifts as reported by licensees in licensee event reports (LERs). The root causes discussed in the report have not all been corroborated by independent testing, so some caution should be exercised in using this information. Westinghouse and the Westinghouse Owners Group performed testing on safety valves in November and December of 1990. These tests used steam, loop seals, and nitrogen on the valve disk. The intent of this testing was to . IN 89-90, Supplement 1 April 3, 1991 Page 3 of 3 determine the magnitude of the error in lift setpoint associated with testing the valves with these various mediums. These tests may help to clarify some of the factors affecting relief valve setpoints. A further supplement to this notice will be issued, if appropriate, after the results of these tests have been issued and reviewed. Related Generic Communications: Previous information notices issued on related topics include IN 86-56, "Reliability of Main Steam Safety Valves", IN 86-92, "Pressurizer Safety Valve Reliability", and IN 88-68, "Setpoint Testing of Pressurizer Safety Valves with Filled Loop Seals Using Hydraulic Assist Devices", as well as IN 89-90. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate NRR project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts: L. Mark Padovan, AEOD (301) 492-4445 Andrew J. Kugler, NRR (301) 492-0834 Attachment: List of Recently Issued NRC Information Notices .
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