Information Notice No. 86-12: Target Rock Two-Stage SRV Setpoint Drift
SSINS No: 6835 IN 86-12 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, DC 20555 February 25, 1986 Information Notice No. NO 86-12: TARGET ROCK TWO-STAGE SRV SETPOINT DRIFT Addressees: All nuclear power reactor facilities holding an operating license (OL) or a construction permit (CP) Purpose: This notice is provided to inform recipients of the continuing problem of setpoint drift occurring in the two-stage Target Rock safety/relief valve (SRV) which was originally described in Information Notice (IN) 82-41 and follow-up INs 83-39 and 83-82 It is expected that recipients will review the information for applicability to their facilities and consider actions, if appropriate, to preclude similar problems from occurring at their facilities However, suggestions contained in this notice do not constitute NRC requirements; therefore, no specific action or written response is required Background: The two-stage Target Rock SRV was designed to solve the problems of spurious opening and failure to reseat that had been experienced with the three-stage valve The two-stage valve exhibited some setpoint drift during testing and became a major concern in July of 1982, when 11 of 11 of the Hatch 1 valves failed to open at pressures that significantly exceeded their set pressures following a scram and isolation (Three valves subsequently opened at 1180 psig and further response to the transient was normal) The owners' group that formed after this event, General Electric Company (GE), and the Target Rock Company investigated the causes of setpoint drift and developed recommended solutions The principal causes were determined to be galling in the labyrinth seal area and corrosion-induced seat-to-disc bonding The recommended solutions were: (1) an enhanced maintenance program to ensure that adequate clearances were maintained in the labyrinth seal area and (2) a replacement disc of a material whose oxide film would be less likely to bond to the oxide film of the seat The enhanced maintenance program has been in effect for at least one fuel cycle in most plants Since the issuance of IN 83-82 in December 1983, testing results for a typical plant showed that a small number of valves were within the technical specification tolerance band of +/-l percent, the majority were within 5 percent of the setpoint, and a single valve was stuck 8602200400 IN 86-12 February 25, 1986 Page 2 of 3 As the data base grew, it became apparent that setpoint drift had not gone away The Director, Division of Licensing, Nuclear Reactor Regulation (NRR) sent a letter to the owners' group in March 1985 recognizing the benefits of enhanced maintenance, but concluding that maintenance alone was inadequate to solve the problem The next step taken by the owners' group was the selection of a replacement material for the disc which would have less tendency to develop a strong oxide bond with the Stellite alloy seat A precipitation hardenable stainless steel disc material was selected to replace the Stellite alloy currently used in the disc Description of Circumstances: Following the 1982 Hatch event, the tests performed at Wyle had consisted of a steam pressure test to determine the setpoint, followed by diagnostic tests to determine the cause of the setpoint drift on those valves which did not lift within 3 percent (initially 5 percent) of the setpoint To determine the amount to which the disc is stuck, the test of the Brunswick valves, and the current test procedure, calls for determining if the disc is stuck before the steam pop This is done by backing the stem off the seat and pressurizing below the seat with nitrogen Because the disc is unrestrained, it is expected to lift within 5 psi-nitrogen pressure The nitrogen pressure indicates the extent of seat-to-disc bonding while the labyrinth seal problem is indicated by the characteristically large drop in steam pop pressure on repeated tests On January 7, 1986, Carolina Power and Light (CP&L) reported that 6 of-the 11 Brunswick Unit 2 Target Rock two-stage SRVs being tested at Wyle Laboratories had failed to lift at a nitrogen pressure of 200 psi, 4 others lifted outside the tolerance band, and 1 lifted satisfactorily The nitrogen tests on those valves were terminated at that time and CP&L directed that 4 of the stuck valves be opened using the solenoid actuator to demonstrate operability in the automatic depressurization system (ADS) and manual modes The valves operated properly A fifth valve, having a setpoint of 1105 psig, was steam popped, lifting at 1219 psig-steam The sixth valve was disassembled before further testing Brunswick Unit 2 had completed 2 cycles with the enhanced valve internals maintenance program, but the disc replacement was yet to be done These test results were significantly different from other licensees' results reported to the NRC recently NRC's concern was heightened because of the number of valves affected and the extent to which the discs appeared to be stuck The 5 stuck valves as well as the remaining 5 were successfully steam popped the required number of times before being disassembled and refurbished This demonstrated that the clearances in the labyrinth seal area were satisfactory Four of the stuck valves and 2 of the others are to have discs of the new material retrofitted On January 9, 1986, the internals of the valve which was steam popped at 1219 psig were examined by an NRC inspector, who reported no differences in appearance of this disc from previously observed discs The disc area exposed to steam appeared to be covered with a light gray film There was a reasonably narrow, bright seating band with no evidence of other problems The labyrinth seal showed very light wear CP&L personnel could not identify any significant IN 86-12 February 25, 1986 Page 3 of 3 conditions that may have contributed to the severity of the sticking such as the primary water chemistry transient that Hatch had experienced in 1982 The licensee noted that the three valves on the C-steamline exhibited the least amount of drift and that they were located in the same drywell quadrant The main steam line isolation valves in this line had been closed for about a six-week period, just prior to the end of the fuel cycle and there was a minor primary water chemistry transient during that time The licensee, also mentioned drywell temperature problems and steam leaks from the high-pressure coolant injection (HPCI) system as possible contributors Results of recent testing of the 11 Hatch Unit 1 valves identified two valves outside the tolerance band for the nitrogen test that determines disc-to-seat sticking Two valves lifted outside the tolerance band, 1 was leaking too much to test on nitrogen, and the remaining valves opened at less than 6 percent over setpoint The tests of the Brunswick and Hatch valves followed the same procedures and the results for both reactors are detailed in Attachment 1 for comparison Discs made of precipitation hardenable stainless steel will be installed in 50 percent of the SRVs in each plant as they become available Hatch 1 and Brunswick 2 have just had the new discs installed Experience data will begin to be received in 12 to 18 months The NRC investigation of the Brunswick problem in particular and the Target Rock two-stage setpoint drift problem in general will continue until resolved satisfactorily As additional information is obtained, it will be disseminated to the industry No specific action or written response is required by this information notice If you have any questions about this matter, please contact the Regional Administrator of the appropriate regional office or this office Edward L Jordan Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contact: Mary S Wegner, IE (301) 492-4511 Attachments: 1 Wyle Lab Test Results: Brunswick 2/Hatch 1 2 List of Recently Issued IE Information Notices Attachment 1 IN 86-12 February 25, 1986 Page 1 of 1 1986 WYLE LAB TEST RESULTS: BRUNSWICK 2/HATCH BRUNSWICK 2 VALVE # SET PRESS (psig) N2 PRESS (psig) STEAM PRESS (psig) +/-1 percent DISC LIFT FIRST POP (1) 1091 1105 11 1131 1099 1105 200+ 1107 (2) 1101 1115 11 1134 1102 1115 82 1134 1103 1125 48 1134 1104 1125 200+ 1153 (2) 1105 1115 200+ (DISASSEMBLED) 1106 1105 5 1116 1107 1115 200+ (INSTRUMENT FAILURE) 1108 1125 200+ 1176 (2) 1109 1105 200+ 1219 (3) HATCH 1 VALVE# SET PRESS (psig) N2 PRESS (Psig) STEAM PRESS (Psig) +/-1 percent DISC LIFT FIRST POP (1) 313 1080 42 1092 1002 1080 5 1092 1003 1100 31 1132 1004 1090 5 1084 1006 1090 5 1119 1009 1080 5 1095 1011 1080 5 1086 1186 1100 5 1104 1187 1090 5 1101 1189 1090 (LEAKING) 1102 1190 1100 5 1127 (1) The steam pressure "first pop test" is performed after the nitrogen pressure disc lift Therefore, except for the Brunswick 2 valves that failed to lift with a nitrogen pressure of 200 psig, any corrosion bond that existed between the pilot disc and seat had been broken by the nitrogen test (2) These valves were opened by using the solenoid actuator before the steam pressure "first pop test", thus breaking any corrosion bond that existed between the pilot disc and seat (3) Since this valve was not opened by nitrogen pressure or by the solenoid actuator, this pressure approximates the "as-found" valve condition
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021