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On May 15, 2016, with Millstone Power Station Unit 3 (MPS3) operating in MODE 1 at 74% power, the operators observed decreasing hydrogen pressure in the main turbine generator. Upon field investigation it was determined there was an active hydrogen leak from the main generator. The operators manually tripped the reactor and vented the hydrogen from the main generator. The reactor trip was uncomplicated. The auxiliary feedwater (AFW) pumps started as designed on low steam generator level and operators maintained steam generator level.

The active hydrogen leak was the direct cause of the manual reactor trip. The hydrogen leak was caused by a dislodged plug on a port on the main generator. MPS maintenance procedures did not contain adequate procedural guidance in that there was no specific direction for installation, i.e., torque value and verifications. The procedures will be revised to include specific direction to tighten the plugs and applicable verifications (i.e., torque value, peer checking). Additional corrective actions are being taken in accordance with the station's corrective action program.

The actuation of the RPS and the automatic start of the AFW pumps is being reported in accordance with 10 CFR 50.73 (a)(2)(iv)(A) as an event that resulted in manual or automatic actuation of systems listed in 10 CFR 50.73(a)(2)(iv)(B).

On January 25, 2016 an automatic reactor trip occurred on Millstone Power Station Unit 3 (LER 2016-001-00). Following the Millstone Power Station Unit 3 reactor trip, (Mode 3, 0% power), while operators were performing feedwater isolation actuation verification steps, it was identified that the 'C' Feedwater Isolation Valve, 3FWS*CTV41C, did not auto-close as expected. Operators subsequently closed 3FWS*CTV41C from the control room.

Troubleshooting identified that a temporary jumper, associated with a maintenance activity completed in November 2014, was inadvertently left installed due to a human performance error. This jumper bypassed the input from the Solid State Protection System to isolate 3FWS*CTV41C upon receipt of an actuation signal. This input is required by Technical Specification 3.3.2 and Table 3.3-3, Engineered Safety Features Actuation System Instrumentation. The function to automatically isolate 3FWS*CTV41C was thus defeated. However, the safety function to isolate feedwater was met due to redundant valves in series that operated correctly via the 'C' feed regulating valve and the 'C' feed regulating bypass valves. Also, the containment isolation function was not lost as the valve, 3FWS*CTV41C, was capable of remote operation from the control room.

This condition is reportable in accordance with 10 CFR 50.73 (a)(2)(i)(B) as any operation or condition that is prohibited by the plant's Technical Specifications. The jumper was removed and the valve was verified to be OPERABLE.

On February 19, 2015, with Millstone Power Station Unit 3 (MPS3) at 100% power and in operating mode 1, an individual on a fire watch rove processed through a dual train high energy line break (HELB) door normally and upon checking the door after passage the individual noted the door did not latch. The Control Room was promptly notified./ An operator was dispatched to investigate. The operator exercised the door lock-set mechanism freeing the latch allowing the door to properly latch. The door was inoperable for approximately 7 minutes. Technical Specification 3.0.3 was entered and exited appropriately.

Although no definite failure mechanism was identified, the door was experiencing high usage due to compensatory fire watch roves entering/exiting the door. The door lockset mechanism was manually manipulated and then tested several times satisfactorily by maintenance personnel. Further, the door design has the door swing such that the HELB event would act to open the door when the lockset mechanism fails. Engineering is evaluating the adequacy of the preventive maintenance frequency. Additionally, a design change to reverse the door swing such that the HELB event would cause the door to close and thus not rely on the lock-set mechanism is being considered. Additional corrective actions are being taken in accordance with the station's corrective action program.

This event is being reported pursuant to 10 CFR 50.73(a)(2)(v)(D), as a condition that could have prevented the fulfillment of a safety function for systems needed to mitigate the consequences of an accident.

While Millstone Power Station Unit 3 (MPS3) was operating at 100 percent power in Mode 1 on March 12, 2014, operations determined that a concern identified by Engineering as a result of recent industry operating experience (NRC Event Number 49889) could result in an unanalyzed condition. Engineering's review of the direct current (DC) circuits associated with the main turbine emergency lube oil and main generator emergency seal oil pumps determined the described condition to be applicable to MPS3.

This condition is reportable pursuant to 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition that significantly degraded plant safety. A prompt notification per EN49903 was submitted to the NRC on March 12, 2014.

The cause of the condition was a manufacturer's design error that was installed during plant construction.

Compensatory fire watches have been implemented for affected areas of the plant. Plant modifications are planned that will protect or isolate the affected circuits leaving the initial fire area.

On February 15, 2013, with Millstone Power Station Unit 3 operating in MODE 1 at 100 percent power, operators discovered that the insulating cover installed over 3MSS*PT526, "B" Steam Generator Pressure Transmitter (SGPT) Channel 3, was not properly secured. When found, it was incorrectly assessed as not impacting equipment operability. However, on February 19, 2013, upon further review by the Electrical Equipment Qualification program engineer, it was determined that the SGPT would not be adequately protected from a postulated high energy line break (HELB). Therefore, 3MSS*PT526 was declared inoperable per Technical Specification (TS) 3.3.2, Engineered Safety Features Instrumentation, on February 19, 2013. Information exists that 3MSS*PT526 was inoperable on February 15, 2013, when the problem was initially discovered. Since the condition existed for longer than allowed by TS 3.3.2, Action 20 (i.e., for more than six hours without the bistable being placed in trip), this is a reportable condition per 10 CFR 50.73 (a)(2)(i)(B) as an event or condition which was prohibited by the plant's Technical Specifications.

Lack of awareness of the purpose of the enclosure boundary combined with the lack of clear labeling identifying the HELB enclosure resulted in a delay in declaring the equipment inoperable. Operations created and installed a new label for the insulating box around each SGPT clearly indicating the box is a HELB barrier and is part of the environmental qualification boundary.