|Joseph M. Farley Nuclear Plant, Unit 2|
|Reporting criterion:||10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications|
10 CFR 50.73(a)(2)(v), Loss of Safety Function
|3642017002R00 - NRC Website|
|Person / Time|
|From:||Madison D R|
Southern Nuclear Operating Co
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML17353A931 (5)|
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3. LER NUMBER
On November 1, 2017, while in Mode 6 and at 0% power level for refueling outage 2R25, with the Reactor Coolant System (RCS) at atmospheric pressure and 84 degrees Fahrenheit, one of the C Loop Main Steam Safety Valves (MSSV) as-found lift pressure did not meet the acceptance criteria of +/- 3% of setpoint (1129 psig) as required by Technical Specifications (TS) Surveillance Requirement (SR) 18.104.22.168 when tested by an off-site testing facility per their testing guidelines and in accordance with plant procedures. The MSSV lifted high at 1171 psig which is 9 psig outside of its acceptance range of 1096 to 1162 psig and 3.72% above its setpoint. The +1- 3% as-found lift pressure requirement is an ASME Section III, 1971 edition, and Farley Technical Specification (TS) requirement to ensure that the MSSV provides adequate protection by preventing the steam pressure from exceeding 110 percent of the main steam system design pressure.
EVENT CAUSE ANALYSIS:
Evaluation of failures of a B Loop MSSV in 2010 and 2013 concluded there was inadequate preventive maintenance inspections on the MSSVs. As a result, a 10.5-year inspection and refurbishment preventative maintenance task was created with a comprehensive plan to inspect and replace the subcomponent in all MSSVs. Each MSSV was scheduled for inspection and replacement of valve disk/material and spindle compression screw assembly with dampened vibration.
The C Loop MSSV was the last remaining valve to be inspected and rebuilt as part of this comprehensive plan. The valve removed from the C Loop MSSV location had been tested satisfactorily prior to refueling outage 2R24 and was not part of the In-Service Testing (1ST) scope for 2R25. It was removed and tested in 2R25 as part of the comprehensive plan. The apparent cause of exceeding the MSSV upper acceptance limit is degradation of the valve spring and/or valve spindle compression screw. The as-found settings remained within analytical bounds; therefore, operation of the facility in this condition had no impact on the health and safety of the public.
REPORTABLITY AND SAFETY ASSESSEMENT:
This event is reportable in accordance with 10CFR50.73(a)(2)(i)(B). The applicable accident/transient analyses requires five MSSVs per Steam Generator (SG) to provide overpressure protection for design basis transients occurring at 102% Rated Thermal Power. The MSSVs also provide a heat sink for the RCS if the main condenser is unavailable and the atmospheric dump valves cannot relieve steam line pressure. Operability of the MSSVs is defined as the ability to open within the setpoint range, relieve SG overpressure, and re-seat when pressure has been reduced, and is determined by periodic surveillance testing. On November 1, 2017, a C Loop MSSV was found outside of its required setpoint range; therefore, it failed its as-found testing criteria and was declared inoperable. The apparent cause determined that the failure was due to degradation of the valve spring and/or valve spindle compression screw. This degradation is not normal drift; therefore, the valve may have been inoperable during past operation. As it is not possible to determine when the valve would have exceeded the setpoint range, the C Loop MSSV was determined to be inoperable for greater than the TS allowed completion time. Based on the MSSV as-found lift setpoint being less than 110% of design Steam Generator pressure (1194 psig), this one MSSV failure would not have resulted in a loss of safety function. Therefore, this condition is not reportable under 10CFR50.73(a)(2)(v) as a safety system functional failure.
The C Loop MSSV was replaced on November 5, 2017, while in Mode 5.
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PREVIOUS SIMILAR EVENTS:
One of the B Loop MSSVs lifted low outside of the +1- 3% lift pressure requirement in 2010 and lifted high outside of +1- 3% lift pressure requirement in 2013. An analysis of both failures identified inadequate preventive maintenance inspections on the MSSVs. Refurbishment of the C Loop MSSV was initiated as a corrective action from the analysis of the previous events on the B Loop MSSV.
OTHER SYSTEMS AFFECTED:
No other systems were affected by this event.