05000335/LER-2016-002
| St. Lucie Unit 1 | |
| Event date: | 08-05-2016 |
|---|---|
| Report date: | 10-04-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3352016002R00 - NRC Website | |
learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduc or sponsor, and a person is not required to respond to,the information collection.
Description of the Event
On August 5, 2016, Unit 1 was in Mode 5 and in the process of restarting following a maintenance outage. On August 5, 2016, Unit 1 entered Mode 4 at 0943, Operators were conducting activities to cool down the shutdown cooling system after removing the system from service. With Unit 1 in Mode 4, Operators observed the Reactor Coolant System (RCS) [AB] Pressurizer level lowering when opening of HCV-3618, Safety Injection Loop 1A2 Check Valve Leakage Isolation Valve, as part of the procedure for system restoration. Based on the lowering Pressurizer level, operators closed HCV-3618. This action arrested the Pressurizer level reduction by isolating the internally leaking check valve V3217 [ISV:AB] and stopping RCS leakage to the Refueling Water Tank (RN/VT).
The flow path was promptly identified and isolated prior to any alarms or automatic system actuations.
Procedural guidance to quantify the seat leakage required establishing a differential pressure which did not exist at current conditions. A seat leakage test was planned when RCS pressure could be established to meet the required conditions. Unit 1 entered Mode 3 on August 6, 2016. On August 7, 2016 with the RCS at the normal operating pressure 2250 psia, test personnel were unable to achieve an appropriate differential pressure to allow the performance of a seat leak test of V3217, and the seat leak test was halted.
On August 7, 2016, at 1600, V3217 was declared inoperable, and Unit 1 entered The Technical Specification action statement (TSAS) 3.4.6.2.e, RCS pressure isolation valve leakage. The action statement required that leakage be reduced to within limits within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> or be in at least HOT STANDBY within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />. During the allowed 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, a second attempt was made to leak check V3217, but the required differential pressure could not be achieved.
Unit 1 entered Mode 4 on August 8, 2016, at 0233, and entered Mode 5 on August 8, 2016, at 2003.
Unit 1 remained in Mode 5 until repairs to V3217 were completed.
Cause
The internal leakage past V3217 was caused by inadequate contractor maintenance practices and procedures that did not ensure V3217 was within acceptable tolerances and correctly assembled. It was identified that maintenance performed in 2013 incorrectly assembled internal valve bushings, and bushing spacers were not installed per the design. This resulted in excessive internal wear and misalignment.
A second root cause identified that V3217 is susceptible to accelerated wear if differential pressure is not maintained across the seat. This was not recognized and incorporated into the overall valve maintenance strategy.
learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
Analysis of the Event
The increased internal leakage past V3217 resulted in the RCS pressure isolation valve initially being declared inoperable on August 7, 2016 at 1600. Since the internal seat leakage was not reduced to within limits, Technical Specifications allow 40 hours4.62963e-4 days <br />0.0111 hours <br />6.613757e-5 weeks <br />1.522e-5 months <br /> for the Unit to be in Mode 5, COLD SHUTDOWN. As a result of the internal inspection and known operational at-power conditions, the period of concern was from the initial achievement of Mode 4 on August 5, 2016 at 0943 to the declaration of entry into the Technical Specification action statement and entry into Mode 5 on August 8, 2016 at 2003. This time period exceeded the allowed 40 hours4.62963e-4 days <br />0.0111 hours <br />6.613757e-5 weeks <br />1.522e-5 months <br />, making this event reportable pursuant to 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by Technical Specifications.
As a result of internal leakage with V3217, the upstream Safety Injection piping had been maintained near RCS system pressures for the majority of the SL1-25 and SL1-26 operating cycles, but had been successfully tested during each cycle with less than 1 gpm leakage prior to the maintenance outage. FPL performed an internal valve inspection after the unsuccessful test following a maintenance outage. kreview of previous valve maintenance and testing history concluded that cumulative internal valve wear had occurred within the valve due in part to inadequate assembly tolerances introduced in 2013 and a lack of differential pressure. This wear was sufficient to cause acceptable leakage up to the point where the last forward flow through the valve was established with shutdown cooling flow, on August 5, 2016 during a plant shutdown for maintenance. Cumulatively, the disc seat wear and increased clearances allowed the disc to progressively move lower in the valve seat until shutdown cooling flow moved the disc off its remaining seat. The disc inside the seating surface is convex in shape. This convex shape would help to keep the disc seated until the valve was opened by flow. The flow induced opening of the disc caused the disc to drop due to the cumulative wear of the internal parts. The disc seating surface and the valve body seating surface did not subsequently align resulting in leakage exceeding Technical Specification limits.
Safety Significance
This condition was mitigated by the downstream RCS pressure isolation valves in series with V3217 satisfactorily performing their isolation function during the period described.
Safety injection system pressure isolation check valves leakage of reactor coolant through the safety injection system isolation check valves (V3217, V3227, V3237 and V3247) can be detected by an associated pressure increase on the low pressure side of the check valves. Pressure indicator alarm instruments (PIA-3319, 3329, 3339 and 3349) on the control board monitor system pressure. Safety injection system isolation check valves outside the containment (V3113, V3114, V3123, V3124, V3133, V3134, V3143 and V3144) are also periodically tested in accordance with the plant Technical Specifications, in order to reduce the probability of reactor coolant system leakage into the high and low pressure safety injection system pipe lines. These valves are within the scope of Generic Letter 87-06, "Periodic Verification of Leak Tight Integrity of Pressure Isolation Valves.
learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduc or sponsor, and a person is not required to respond to, the information collection.
The Conditional Core Damage Probability (CCDP) and Conditional Large Early Release Probability (CLERP) values were evaluated for the stated event and were found to be below the thresholds of 1.0E-6 and 1.0E-7 for CCDP and CLERP, respectively. Therefore, it is concluded that the risk impact of the stated event is not risk-significant. The reactor remained shut down during the period of concern. V3217 is one of four different piping branches for Safety Injection. Internal valve leakage does not impact this safety function. Redundant, in-series check valves to V3217 remained operable during the period of concern. Given the short period where Technical Specification time limits were exceeded, the health and safety of the public were not affected by this event.
Immediate Corrective Actions
V3217 was disassembled and repaired prior to unit restart using improved procedure guidance and vendor input. New components were installed including the valve disc, hinge pins, bushings and gaskets. The valve body seat was refurbished. A post maintenance test indicated zero leakage.
Corrective Actions to Prevent Recurrence
The corrective actions listed below have been entered into the site corrective action program. Any changes to the actions will be managed under the corrective action program.
1. New time based preventive maintenance is being developed for safety injection loop pressure isolation valves consistent with EPRI PMB Guidance for Critical, Low duty cycle, Mild service (CLM) check valves.
2. The Maintenance Strategy for safety injection loop pressure isolation valves is being revised to reflect that mild service condition requires the valve to be closed with differential pressure.
3. The station's maintenance procedure for Atwood-Morrill swing check valves is being revised to incorporate industry best practices described in the EPRI Check Value Maintenance Guide for assembly procedures (section 5.1.3 of EPRI Guide), seat inspection (section 7.4.7 of EPRI Guide), excessive clearance checks (section 7.4.8 of EPRI Guide) and refurbishment (section 7.4.9 of EPRI Guide).
Failed Component Unit 1 V3217 is an Atwood & Morrill Company, 12-inch, swinging disc, inclined seat type check valve.
Manufacturer Atwood & Morrill Company Previous Occurrence No previous similar LERs.