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 Report dateSiteEvent description
05000289/LER-2017-0026 February 2017Three Mile Island
Three Mile Island Unit 1
On December 7, 2016 Three Mile Island Unit 1 was in the hot shutdown condition following a planned maintenance outage that replaced the Reactor Coolant Pump (RCP) 1A seal package when a reactor coolant system (RCS) leak was discovered on a welded connection of the Reactor Coolant Pump 1A thermal barrier. The identified leak was determined to be approximately 0.5 gpm, located on the RCS pressure boundary and nonisolable. Operators returned Unit 1 to a cold shutdown condition to repair the leak. The most probable cause for the leak is a latent weld defect that reduced the fatigue strength of the connection, coupled with RCP-1A startup vibration leading to failure. Immediate corrective action involved a weld repair modification to the leak location. Extent of condition applied to five similar locations that were examined with no weld defects identified. Additional corrective actions are planned to implement a weld repair modification to five similar connections on the RCPs during the Fall 2017 maintenance & refueling outage (T1R22). This event is reported as a degraded condition pursuant to 10 CFR 50.73(a)(2)(ii)(A). This event had no effect on public health and safety.
05000289/LER-2015-0015 October 2015Three Mile IslandOn August 6, 2015 NRC provided TMI-1 the final response to Task Interface Agreement 2015-01 "Assessment of Three Mile Island Nuclear Station's Use of a Non-Seismic Qualified Cleanup Path for the Borated Water Storage Tank." It concluded that opening the boundary valve between the safety related and seismically qualified Borated Water Storage Tank (BWST) and the non-seismically qualified clean-up system piping during power operation mode renders the BWST inoperable. A review of operator logs determined that the BWST was connected and recirculated via the clean-up system while in the power operation mode within the past three years and for a period of time greater than the applicable Technical Specification completion time allowed. This represented a condition prohibited by Technical Specifications and is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B). This event had no effect on public health and safety.
05000289/LER-2014-00211 September 2014Three Mile IslandOn July 13, 2014 at 17:31 Three Mile Island, Unit 1 exceeded a 72 hour limiting condition for operation (LCO), after being granted a period of enforcement discretion, due to a through-wall leak found on the "A" train of the High Pressure Injection (HPI) system piping at the 1/2" socket weld coupling to root valve MU-V-1034. The flaw resulted in declaring the "A" train of HPI inoperable. This event was reportable under 10 CFR 50.73(a)(2)(i)(B) as an operation or condition which was prohibited by the plant's technical specifications. A Notification of Enforcement Discretion (NOED) was granted by the NRC at 17:19 on July 13, 2014. The apparent cause leading to the NOED request was inadequate procedure guidance for applying freeze seals which resulted in delays in achieving isolation of the leak in order to establish proper conditions to perform the repair. The through-wall leak most probable cause was determined to be from stress corrosion cracking driven by fatigue. The corrective action was to grind out the weld and repair (re- weld). This event had no adverse effect upon the health and safety of the public.
05000289/LER-2012-00519 November 2012Three Mile Island

On September 20, 2012, Reactor Coolant Pump RC-P-1 C at Three Mile Island (TMI) Unit 1 tripped on an apparent differential overcurrent condition 55 seconds after Vital Bus C was secured in support of inverter maintenance. An automatic runback was initiated, but the Reactor Protection System (RPS) actuated on flux to flow imbalance and the reactor tripped at 14:17:34. The RC-P-1C trip was attributed to the differential overcurrent condition due to a differential overcurrent relay (ITH 87 relay) for the RC- P-1C, "C" phase operational indicator being flagged. Complex troubleshooting was performed with an emphasis on an RC-P-1C Motor or RC-P-1C87 Differential Relay failure as the probable cause; however, the troubleshooting did not reveal a definitive cause of the initiating RC-P-1C trip. An operational decision evaluation was performed to assess plant restart without identifying the cause of the RC-P-1C trip during the complex troubleshooting. A subsequent investigation was performed to determine why RC-P-1C tripped but could not determine a definitive root cause. The most likely causes were addressed by the replacement of the relay, adjusting the trip setpoint, and eliminating this type of vital bus outage at power.

This LER is being submitted pursuant to the requirements of 10 CFR 50.73 (a)(2)(iv)(A).

05000289/LER-2012-00322 October 2012Three Mile Island

On August 22, 2012 Three Mile Island (TMI) Unit 1 discovered an unisolable leak in the upper pressurizer heater bundle diaphragm plate and shut down the reactor. The cause of the leak was Primary Water Stress Corrosion Cracking (PWSCC). The root cause was determined to be "The use of Alloy 600 materials in high temperature locations was a design weakness in the construction of the TMI station.

The corrective actions include replacement of the upper pressurizer heater bundle (completed September 2012) and planned replacement of the remaining Alloy 600 susceptible pressurizer heater bundle. The leak was not a threat to the safety of the reactor and did not represent a reduction in the public health and safety.

A previous PWSCC condition was reported in LER 50-289/2003-003-00.

This LER is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(A).

05000289/LER-2008-0019 June 2008Three Mile IslandOn April 8, 2008, it was discovered that the "B" Decay Heat River Water System Pump (DR-P-1B) was inoperable from January 25, 2008 till February 13, 2008. This conclusion was reached following evaluation of test data on a failed X relay coil for the circuit breaker of DR-P-1B. At approximately 2250 on February 12, 2008, the "B" Decay Heat River Water System Pump (DR-P-1B) was being started in accordance with procedure OP-TM-533-402, "Operating DR-P-1B for Other Than Decay Heat Removal Operations," and the pump failed to start. DR-P-1B was declared inoperable, placing TMI-1 in an unplanned 72 hour shutdown Limiting Condition of Operation (LCO). Troubleshooting determined that the coil for the DR-P-1B breaker control X relay was open circuited. The function for the control X relay is to control the operation of the breaker closing solenoid and to provide the breaker anti-pumping control feature. The anti-pumping feature allows only one closing attempt for the breaker as long as a close signal is present. The control relay for the breaker was replaced and DR-P-1B was successfully returned to service at approximately 1032 on February 13, 2008. The apparent cause of the X relay open circuit was a coil manufacturing defect. This relay failure is considered to be an isolated occurrence. The submittal of this LER constitutes reporting to the NRC in accordance with 10 CFR 50.73 (a)(2)(i)(B), "a condition prohibited by Technical Specifications.