05000389/LER-2014-001
| St. Lucie Unit 2 | |
| Event date: | 07-25-2014 |
|---|---|
| Report date: | 01-30-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(vii)(B), Common Cause Inoperability 10 CFR 50.73(a)(2)(i) |
| 3892014001R01 - NRC Website | |
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2014 - 001 00
Description of the Event
Background
On March 17, 2014 Unit 2 2B1 safety injection tank (SIT) (EIIS:TK) discharge piping vent valve V3811 (EIIS:VTV) was replaced during the SL2-21 refueling outage. On June 22, 2014, the control room entered the Process Radiation Monitor Abnormal Operating Procedure, due to a rising trend on two containment particulate radiation monitors. Concurrently, plant data showed that 2B1 SIT was being filled daily beginning June 22, 2014. The frequent SIT replenishment requirement and radiation monitor trend was indicative of a leak from the safety injection system. Containment entries were made on June 24 and 25, 2014 and on July 23 and 25, 2014 to investigate the suspected leak from the 2B1 SIT piping.
On July 25, 2014 at 1129 EDT, detailed video imaging confirmed the leak was on a one inch pipe (ENS: PSF) between the safety injection system (EIIS:BQ) SIT and vent valve V3811. The valve is within the Quality Group A (ASME Class 1 equivalent), Seismic Class I portion of the safety injection line. The applicable Technical Specification Limiting Condition of Operation (LCO) required the SIT to be restored to operable status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or shut down to Mode 3 within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> with continuation to Mode 4 within the following 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
The unit was subsequently shut down to repair the leak. Reactor shutdown to Mode 3 was completed on July 26, 2014 at 0414 and was uncomplicated. Mode 4 was achieved at 1000. All plant safety systems functioned as designed. The cracked vent line and valve assembly were replaced and returned to service on July 28, 2014.
Cause of the Event
A root cause evaluation team for the SI pipe leak determined that the repair and replacement of vent valve V3811 was not performed as prescribed in the work order documents utilized by plant maintenance. This resulted in a number of adverse factors which ultimately resulted in the failure of the pipe nipple upstream of the vent valve due to outside diameter initiated, high cycle, low stress fatigue.
A contributing cause was that neither Maintenance nor the non-destructive examination (NDE) Inspector verified the dimensions of the field-cut inlet pipe nipple before the vent valve was welded in as required by procedures.
Analysis of the Event
For the purposes of evaluating impact on the affected systems, the flawed pipe was conservatively considered to have fully failed, resulting in a nominal one (1) inch diameter breach in the safety injection header at the location of the leak. A one (1) inch breach, at this location, would result in a depressurization of the 2B1 SIT and loss of inventory from the tank. As a result, the affected SIT was declared Inoperable. The affected safety injection header is one (1) of the four (4) cold leg injection points for the high pressure safety injection (HPSI) system. Flow from the A and B HPSI pumps combines upstream of the postulated breach before injection into the affected cold leg. This flowpath is also shared by B low pressure safety injection (LPSI) header.
St. Lucie Unit 2 05000389
Safety Significance
The identified leakage was determined to be less than 1 gpm. This flowrate is insignificant as compared to either the SIT discharge flowrate or the safety injection flowrate during LOCA events. Also, the SIT tank volume lost during a four hour station blackout (SBO) event is insignificant compared to the total volume of the four SIT's.
A review of the failure analysis for the piping shows that through-wall cracking extended —225° around the piping; partial through-wall cracking extended another —67.5° and only —67.5° remained completely intact. As a result, while the leakage was insignificant, the remaining intact portion of the pipe wall could not have been assured to maintain its overall structural integrity during events requiring safety injection, adversely affecting the 2B1 SIT. The leak existed in Class I piping common to the 261 SIT, trains A and B of HPSI and train B of LPSI resulting in technical inoperability of those systems.
For PRA analysis it was assumed that the V3811 pipe breaks leaving a one inch hole in the 2B1 injection line.
As a bounding condition, in the event of a Safety Injection signal, the 61 injection path is assumed to be unavailable. This is a conservative assumption, since it is likely that some of the HPSI and low pressure safety injection flow would enter the RCS. Also, injection flow exiting the break would be available in the reactor containment building sump for once through cooling.
A review of the station risk assessment indicates that the calculated change in Core Damage Frequency (CDF) and the limited exposure time result in a low safety significance.
This condition is reportable in accordance with the following requirements: 1) 10 CFR 50.73(a)(2)(ii)(A), An event or condition that resulted in the condition of the nuclear power plant, including its principal safety barriers, being seriously degraded; 2) 10 CFR 50.73(a)(2)(i)A, The completion of a nuclear plant shutdown required by the plant's Technical Specifications; 3) 10 CFR 50.73(a)(2)(i)B An operation or condition which was prohibited by the plant's Technical Specifications; 4) 10 CFR 50.73(a)(2)(v)(D) Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate an accident; 5) 10 CFR 50.73(a)(2)(ii)(B), An event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety; and 6) 10 CFR 50.73(a)(2)(vii)(B) Any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system designed to remove residual heat.
Corrective Actions
1. The "Weld Coordinator" software program will be modified to include hold points in weld travelers for dimension verification. For Class 1, 2 and 3 piping and Pipe Category 4 and 5 (NNS, high pressure), the non-destructive examination inspector will verify piping dimensions.
2. Welding work control procedures will be revised to ensure that the applicable weld travelers incorporate the requirement to use NDE procedure 4.10 "Component, Support & Inspection ASME Section III and ANSI B31.1 Butt and Fillet Welds Visual Examination" and that the procedure is clearly designated as a corrective action to prevent recurrence (CAPR).
3. Maintenance continuing training will be revised to emphasize the findings of the root cause evaluation, the impact to the plant of this event, the importance of using human performance tools when complying with work documents, and the importance of ensuring that condition reports are addressed by the appropriate work document.
Similar Events The failure of a Unit 2 main steam antenna type vent line MS-11-1 occurred at approximately 2355 on 5/15/11.
The one inch steam line vent MS-11-1 appeared to have broken off, creating a steam leak that was non-isolable without closing the main steam isolation valves (MSIVs). An inspection of the failed vent revealed it did not meet the design dimensions on the installation work order. During subsequent Unit 2 main steam walkdowns another vent line, MS-8-4, was found to have been installed without meeting design dimensions. Two high energy vent lines were not installed in accordance with the approved plant design; one of these lines subsequently failed resulting in a plant shutdown. The root cause was that the installation of the vent line was not performed as prescribed in the work order documents utilized by plant maintenance.
St. Lucie Unit 2 05000389