05000335/LER-2017-001
| 05-02-2017 | On January 31, 2017, while St. Lucie Unit 1 was shut down in Mode 3, technicians identified reactor coolant pressure boundary leakage within the 1B2 reactor coolant pump (RCP) lower seal heat exchanger. At 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br />, St. Lucie Unit 1 entered Technical Specification 3.4.6.2 Action a. and the plant was maneuvered to Mode 5 to affect repairs.
The most probable cause was determined to be a deficiency in the lower seal heat exchanger design which permitted stresses that approached or exceeded the yield strength of the assembly tubing during torqueing of the CCW flanges. The resultant plastic deformation and associated flaw formation caused low stress high cycle fatigue failure of the weld joint. The flaw was removed and the weld repair was completed. St. Lucie Unit 1 was subsequently returned to service on February 7, 2017. All remaining in-service and a spare RCP lower seal heat exchangers have since been inspected and no defects have been found. This event had no impact on the health and safety of the public. Reported lessons 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. Description On January 31, 2017, St. Lucie Unit 1 was shut down in Mode 3 for a maintenance outage to investigate and repair the source of reactor coolant system (RCS) [EIIS:AB] leakage coming from the vicinity of the 1B2 reactor coolant pump [EIIS:P] (RCP) seal [EIIS:SEAL] package. At 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br />, technicians determined that the leak was located in the RCP lower seal heat exchanger [EIIS:HX] and that the leakage was classifiable as reactor coolant pressure boundary leakage. St. Lucie Unit 1 entered Technical Specification (TS) 3.4.6.2 Action a. and the plant was maneuvered to Mode 5 to affect repairs. The 10 CFR 50.72(b)(3)(ii) notification was made at 1539 hours0.0178 days <br />0.428 hours <br />0.00254 weeks <br />5.855895e-4 months <br />. The flaw was removed and the weld repair was completed. St. Lucie Unit 1 was subsequently returned to service on February 7, 2017. ContentsCause of the EventThe most probable cause was determined to be a deficiency in the lower seal heat exchanger design which permitted stresses that approached or exceeded the yield strength of the assembly tubing during torqueing of the CCW flanges. The resultant plastic deformation and associated flaw formation caused low stress high cycle fatigue failure of the weld joint. All of the in-service and spare St. Lucie Unit 1 and 2 RCP lower seal heat exchangers have been inspected. These inspections did not find any deficiencies. Analysis of the EventThis condition is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as an operation or condition prohibited by TSs, and 10 CFR 50.73(a)(2)(ii)(A) as a degraded or unanalyzed condition. Remote video analysis at power was inconclusive whether the leak was RCS pressure boundary. However, close visual inspection following unit shutdown determined the leak to be a small flaw in a RCS pressure boundary component (i.e. RCP seal cooler) which was a degraded condition prohibited by Technical Specifications. The rotating assembly, the pump cover, and integral lower seal heat exchanger for the 1B2 RCP had been replaced during the previous refueling outage in the fall of 2016. The 1B2 RCP has an integral tube-in-tube heat exchanger which is permanently attached to the pump cover. This heat exchanger surrounds the labyrinth seal and provides cooling of the RCS water prior to entering the seal. This heat exchanger is comprised of two rows of six coils circling the RCP seal. The inner tube of the tube-in-tube configuration carries the high pressure RCS water. The outer tube carries the low pressure component cooling water (CCW) [EIIS:CC]. RCS fluid enters the coils at the bottom of the assembly and exits the coils at the top of the assembly (one from the inside coil and one from the outside coil). The outlet of the coils is directed thru a machined elbow fitting welded to a short length of 1.5 inch diameter pipe, which carries the RCS flow to the seal housing and seal cartridge The leak was located in the tube material near the toe of the partial penetration weld that joins the seal cooler inner tube and ring. A review of the Unit 1 containment atmosphere particulate monitor and reactor cavity leakage flow instrument data indicates that RCS leakage from the 1B2 RCP lower seal heat exchanger was initiated on November 9-10, 2016, approximately 1 week after the 1B2 RCP had been started during startup from the fall 2016 refueling outage. The Unidentified RCS leak rate was closely monitored while a maintenance outage was planned to repair or replace the newly installed RCP seal package. Reported lessons 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. Safety SignificanceThis condition was determined to be of very low safety significance for the following reasons:
Therefore, this event had no impact on the health and safety of the public. Corrective Actions1. The 1B2 RCP lower seal heat exchanger leak was repaired during the maintenance outage. 2. All remaining St. Lucie Unit 1 and 2 RCP lower seal heat exchangers were inspected and no other flaws were identified. 3. FPL is developing methods to reduce the stress on the RCP lower seal heat exchanger tubing during installation activities. Failed Components Identified Flowserve supplied RCP lower seal heat exchanger Additional InformationThe weld repair required relief from ASME Code requirements, and those details are documented in FPL letter L-2017-017 dated Feb 2, 2017, titled “In-service Inspection Plan Fourth Ten-Year Interval Unit 1 Relief Request No. 14, Revision 0,” ADAMS accession number ML17033A151. |
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| St. Lucie Unit 1 | |
| Event date: | 1-31-2017 |
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| Report date: | 05-02-2017 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| LER closed by | |
| IR 05000335/2017002 (2 August 2017) | |
| 3352017001R01 - NRC Website | |
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