05000286/LER-2015-002
| Indian Point 3 | |
| Event date: | 2-27-2015 |
|---|---|
| Report date: | 7-7-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2862015002R01 - NRC Website | |
Note: The Energy Industry Identification System Codes are identified within the brackets {}.
DESCRIPTION OF EVENT
On February 27, 2015, while at approximately 96 percent power, during Surveillance Testing of the Main Steam Safety Valves (MSSV) in accordance with procedure 3-PT- R006A, three main steam safety valves (MSSV) (MS-46-2, MS-45-4 and MS-47-4) failed their As-Found lift set point test. Per the test, the MSSVs must lift at +/- 3% of their required setting. At 0833 hours0.00964 days <br />0.231 hours <br />0.00138 weeks <br />3.169565e-4 months <br />, MSSV MS-45-4 on Steam Generator (SG) 34 failed its As-Found set-point pressure. Valve MS-45-4 lifted at 1120.2 psig, 23.2 psig outside its acceptance range of 1034 to 1097 psig and 5.2% above its 1065 psig set point. Consequently, MS-45-4 was declared inoperable and Technical Specification (TS) 3.7.1 (Main Steam Safety Valves) Condition A was entered. Two subsequent tests were performed without any adjustments to the set-point adjusting bolt and the valve lifted within its required +/- 1% band and the valve was declared operable and TS 3.7.1 exited at 0843 hours0.00976 days <br />0.234 hours <br />0.00139 weeks <br />3.207615e-4 months <br />.
On February 27, 2015, at approximately 0935 hours0.0108 days <br />0.26 hours <br />0.00155 weeks <br />3.557675e-4 months <br />, during MSSV surveillance testing, valve MS-46-2 on SG 32 failed its As-Found set-point pressure. MSSV MS- 46-2 lifted at 1161.3 psig, 41.3 psig outside its acceptance range of 1048 to 1120 psig and 7.5% above its 1080 psig set point. MS-46-2 was declared inoperable and Technical Specification (TS) 3.7.1 (Main Steam Safety Valves) Condition A was entered. Two subsequent tests were performed without any adjustments to the set- point adjusting bolt and the valve lifted within its required +/- 1% band and the valve was declared operable and TS 3.7.1 exited at 0943 hours0.0109 days <br />0.262 hours <br />0.00156 weeks <br />3.588115e-4 months <br />.
On February 27, 2015, at approximately 0951 hours0.011 days <br />0.264 hours <br />0.00157 weeks <br />3.618555e-4 months <br />, during MSSV surveillance testing, MSSV MS-47-4 failed its As-Found set-point pressure. MSSV MS-47-4 on SG 34 lifted at 1156.7 psig, 61.7 psig outside its acceptance range of 1063 to 1127 psig and 5.6% above its 1095 psig set point. MS-47-4 was declared inoperable and Technical Specification (TS) 3.7.1 (Main Steam Safety Valves) Condition A was entered. Two subsequent tests were performed without any adjustments to the set- point adjusting bolt and the valve lifted within its required +/- 1% band and the valve was declared operable and TS 3.7.1 exited at 1002 hours0.0116 days <br />0.278 hours <br />0.00166 weeks <br />3.81261e-4 months <br />.
During the surveillance test, seven (7) other MSSVs tested passed their As-Found test criteria and were left within the +/- 1% set point criteria. After verifying the set-point pressure had become repeatable, adjustments were made to MS-46-2, MS-45-4, and MS-47-4 returning the valves to their required +/- 1% As-Left set- pressure band restoring their operability and allowing exit from the Action Statement for TS 3.7.1. On March 22, 2015, during power ascension with the plant in Mode 3, the remaining MSSVs were tested. At 21:41 hours MS-46-3 failed testing due to lifting 3.6% above its TS lift set point of 1080 psig and was declared inoperable. Two subsequent tests were performed without any adjustments to the set-point adjusting bolt and the valve lifted within its required +/- 1% band and the valve was declared operable and TS 3.7.1 exited at 21:51 hours.
TS 3.7.1, requires the MSSVs to be operable in accordance with TS Tables 3.7.1-1 and 3.7.1-2. TS Surveillance Requirement (SR) 3.7.1.1 requires that each MSSV be verified to lift per Table 3.7.1-2 in accordance with the In-service Testing (IST) Program. Operability of the MSSVs is determined by periodic surveillance testing in accordance with the TS and IST program. The conditions were recorded in the Indian Point Energy Center (IPEC) Corrective Action Program (CAP) as Condition Report CR-IP3-2013-00898.
As a result of the failure of MS-45-4, the valve was scoped into the refueling outage for disassembly and inspection. On March 12, 2015, MSSV MS-45-4 was disassembled and inspected by Engineering and the valve vendor.
NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION FACILITY NAME (1) DOCKET (2) LER NUMBER (6) Indian Point Unit 3 05000-286 2015 002 01 The valve spindle showed signs of wear and it was determined the spindle was not replaced during the previous cycle. The valve bronze wear sleeve showed significant transfer of material from the spindle. MSSV MS-46-2 was disassembled on March 10, 2015 and inspected. Fretting was noticed on the spindle rod. High vibrations were concluded to be the cause of fretting. Maintenance was performed and the valve retested March 22, 2015, on power ascension and failed its post maintenance test.
The valve was adjusted and as-left set points set to within 1% of required set point.
MSSV MS-47-4 was disassembled on March 10, 2015, and all valve parts and internals were inspected. The inspection showed minimal wear on the spindle rod, but there were signs of interference wear on the guide bearing between it and the valve guide.
The interference wear is believed to be caused by high, vibrations of the spindle rod over the last cycle.
There are five code safety valves (MSSVs) and one atmospheric dump valve (ADV) (RV} on each main steam (MS) line outside the Reactor Containment {NH} and upstream of the MS isolation valves {ISV). The MSSVs consist of four 6-inch by 10-inch and one 6-inch by 8-inch valve per SG on each of four MS lines for a total of 20 valves.
The MSSV's also provide a heat sink for the reactor coolant system if the Main Condenser is unavailable and the Atmospheric Dump Valves cannot relieve steam line pressure. The five valves on each steam line are nominally set to open at 1065, 1080, 1095, 1110, and 1120 psig. The operability of the MSSVs is defined as the ability to open within the set point tolerances, relieve SG overpressure, and reset when pressure has been reduced. The applicable accident/transient analyses require five MSSVs per SG to provide overpressure protection for design basis transients occurring at 102% reactor thermal power. The MSSVs are ASME Code relief valves, manufactured by Crosby Valve and Gauge Company {C710}. Valve MS-45-4 is a 6-inch by 8-inch Crosby Model .HC-65W 6Q8 Safety Valve. MS-46-2 and MS-47-4 are 6-inch by 10- inch Crosby Model HC-65W 6R10 Safety Valves.
In accordance with the IST program, all MSSV's (20) were tested in the last four years.. Due to the amount of failures experienced in 2015, all MSSV's were tested in refueling outage 3R18.
An extent of condition (EOC) was performed to determine where potential conditions with similar valves and environments could occur. The review determined that EOC found in the failure of MSSV MS-45-4, MS-46-2 and MS-47-4 is restricted to the other 17 MSSVs at unit 3 and the 20 MSSVs at unit 2 due to the valve design. All MSSVs are exposed to high vibrations during their operating cycle during which wear can occur. Previous failures of MSSVs have included wear due to spring skewing and set point drift. Spring skewing can occur in any of the MSSVs and cause side loading frictional forces which prevent the valve from lifting. Set point drift can occur due to age of the components and the operating cycle it is exposed to (e.g., changes in temperature, pressure and vibrations).
The Cause of Event The direct cause of the failure of these valves was severely worn spindle rods. The apparent cause for the failure of MS-47-4 and MS-46-2 was internal friction due to spindle vibration. The spindle rods had severe wear from the internal friction between the spindle rod and spring washer and adjusting bolt. The internal friction was due to the high vibrations of the spindle rods when the spindle rods come in contact with the spring washers and adjusting bolts. The contact areas cause fretting of the spindle rod over the course of the cycle. The apparent cause of the failure of MS-45-4 was reuse of a worn spindle. The valve failed to lift within its set pressure range due to binding on the worn spindle rod. The spindle rod was binding due to fretting wear found on the rod from previous wear.
FACILITY NAME (1) DOCKET (2) LER NUMBER (6) PAGE (3) MSSV's MS-46-2 and MS-46-3 were outside the Code requirement of +/- 3% of the set point during testing on startup from the March 2015 refueling outage (3R18). Because MS-46-2 was modified during the outage, the test was considered a post modification test and not a Code test failure since the valve had not yet been declared operable.
MSSV MS-46-2 was adjusted to within +/-1% and placed in service. The first of three test lift pressures for MSSV MS-46-3 was 1119.6 psig (+3.6%) which was outside the Code required range of 1048 to 1112 psig. However, the second and third tests resulted in lift pressures within the required +/- 1%. Since the second and third tests showed MS-46-3 was back to within the required range of +/-1%, no valve adjustments were performed prior to returning it to service. The apparent cause of the initial test failure of MS-46-3 was likely foreign material introduced during valve modification performed during the previous refueling outage (3R17). Because the valve's spindle and all other parts that could have impacted the valve lift pressure were replaced, other failure modes were considered not likely. Engineering judged that the first lift of MS-46-3 dislodged the foreign material.
Corrective Actions
The following corrective actions have been or will be performed under the Corrective Action Program (CAP) to address the causes of this event.
- All 20 MSSVs were tested and their set point adjusted to be within +/- 1% of design set pressure.
- Installed new spindles and bronze wear sleeves on valves MS-46-4, MS-46-2, MS- 47-4, MS-48-2, MS-49-2, MS-49-1, MS-49-3
- _Replaced the old spindle on valve MS-45-4.
- The MSSV test frequency will be changed for Unit 3 from 4 years to 2 years until all modifications are implemented and IPEC is confident that the cause of the failures has been corrected.
Event Analysis
The event is reportable under 10CFR50.73(a)(2)(i)(B). The licensee shall report any operation or condition which was prohibited by the plant TS. TS 3.7.1 (Main Steam Safety Valves), requires the MSSVs to be operable in accordance with TS Tables 3.7.1-1 and 3.7.1-2. TS Surveillance Requirement (SR) 3.7.1.1 requires each MSSV be verified to lift per Table 3.7.1-2 in accordance with the In-service Testing Program (IST). Operability of the MSSVs includes the ability to open within the set point tolerances. On February 27, 2015, MSSV valve MS-45-4 was found outside its limit therefore, it failed its As-Found testing criteria and was declared inoperable.
Subsequently, valve MS-46-2 was tested and failed its as-found set pressure and declared inoperable. Subsequently MS-47-4 was tested and failed its as-found set pressure and declared inoperable. These valves were disassembled and inspected and determined to have conditions preventing their proper operation. On March 25, 2015, the apparent cause determined that valves MS-47-4 and MS-46-2 failure was due to internal friction caused by spindle wear from vibration during the operating cycle.
The apparent cause of MS-45-4 to fail was determined to be binding due to use of a worn spindle. Spindle wear is not normal drift therefore, the valve may have been inoperable during past operation. The failure of valve MS-46-2 on power ascension was considered a post maintenance test failure. The failure of MSSV MS-46-3 on power ascension was most likely foreign material.
As it is not possible to determine when the valve would not have met its set point lift pressure range, the valve was concluded to be inoperable for greater than the TS allowed completion time. An evaluation of applicable accident/transient analyses was performed to determine the impact of four MSSVs with a higher opening set point. The evaluation concluded the condition would not result in loss of safety function. Therefore, this condition is not reportable under 10CFR50.73(a) (2)(v) as a safety system functional failure.
FACILITY NAME (1) DOCKET (2) LER NUMBER (6) PAGE (3)
Past Similar Events
A review was performed of Licensee Event Reports (LERs) for the past three years for any events reporting TS prohibited conditions due to multiple valve test failures. LER-2011-004 reported two MSSV's outside their as-found lift set point acceptance criteria due to spindle wear and spring skew. LER-2013-001 reported two MSSVs (MS-46-3 and MS-48-3) outside their as-found set point acceptance criteria.
The cause of MS-46-3 failure was galling around the circumference of the spindle rod as a result of vibration. MS-48-3 evidenced similar fretting on one side of the spindle consistent with what was found on valve MS-46-3. Failure cause was determined to be due to internal friction caused by foreign material between the guide bearing and spindle. The causes for the previous events reported in LER- 2011-004 are similar to this event.
The corrective actions for that event included implementation of a bronze wear sleeve modification. All the unit 3 MSSVs are scheduled to have the bronze sleeve modification implemented by the next refueling outage. LER-2011-004 corrective actions included replacement of the valve spindle for valves MS-47-4 and MS-48-4.
Corrective actions for this LER included replacement of the worn spindle for valve MS-45-4 and new spindles installed on valves MS-46-4, MS-46-2, MS-47-4, MS-48-2, MS-49-2, MS-49-1 and MS-49-3 during the bronze wear sleeve installation. LER-2013- 001 cause was foreign material which has not been identified in this LER or previous LER except LER-2013-001. Corrective actions for LER-2013-001 included replacement of the spindle rod for valve MS-46-3, installation of bronze wear sleeves in valves MS-46-3,MS-48-3, MS-45-4, MS-47-1, MS-47-2, MS-47-3 and MS-49-4, and the increase in the PM frequency from an 8 year cycle to a 6 year cycle. The increased frequency of testing is expected to identify degraded conditions so that corrective actions can be implemented to prevent valve lift failures.
Safety Significance
This event had no effect on the health and safety of the public. There were no actual safety consequences for the event because there were no accidents or transients requiring the MSSVs.
There was no significant potential safety impact of the condition under reasonable and credible alternate conditions. Had an accident or transient occurred during the condition of the out of tolerance MSSV, the condition would not have significantly affected accident mitigation capability and the MSSVs overpressure function would have been adequate. The design basis of the MSSVs is to limit the secondary system pressure to 110% of design pressure when passing 100% of design steam flow. Each MS line has an ADV capable of releasing steam to the atmosphere.
The ADVs have the capability to relieve approximately 10% of total steam.
The MSSV design basis is sufficient to cope with any anticipated operational occurrence or accident considered in the Design Basis Accident and transient analysis. The events that challenge the relieving capacity of the MSSVs, and thus Reactor Coolant System pressure, are those characterized as decreased heat removal events, whose evaluations are presented in UFSAR Section 14. Of these, the full power loss of external electrical load without steam dump is the limiting event.
The limiting UFSAR Chapter 14 transients which do not credit the ADVs and which are impacted by the higher MSSV set point were evaluated and it was concluded that the acceptance criteria for the transients would have been met with the out of tolerance MSSVs.