December 9, 2016
On September 28, 2016, while performing the pre-start checks
prior to running the Emergency Diesel Generator (EDG)-A •or the monthly
Technical Specification (TS) surveillance, the oil level in the EDG radiator
fan right angle gearbox was found ~o be low and additional checks found the
gearbox oil pressure relief valve in a loose state which provided a pathway for
gear oil to be pumped out of the gear box while the EDG was operating. EDG-A
was declared inoperable, the relief valve was repaired, pressure tested and the
pressure adjusting threaded union was staked to eliminate any risk from
vibration induced motion in the future, the gearbox oil was replaced and the EDG
run for a post-maintenance test.
A Functional Failure Determination completed on October 11,
2016 determined that the EOG would not have been able to run for its stated
mission time of 30 days. This condition existed for a period of 28 days since
the last surveillance test on August 31, 2016 which is greater than the TS
Allowed Out of Service Time (AOT) of 72 hours. However, the Station Black Out
Diesel Generator was available during this time frame. This issue is reportable
under 10 CFR 50. 73(a)(2)(i)(B) as an Operation or Condition which was
Prohibited by the plant's TSs. On September 15, 2016 EDG-B was made inoperable
to perform its monthly operability run. This created a situation where •or a
brief period of time both EDGs were inoperable which is a condition that could
have prevented the fulfillment of the safety function of a system needed to
shut down the reactor and maintain it in a safe condition, remove residual
heat, and mitigate the consequences of an accident which is reportable in
accordance with 50. 73(a)(2)(v)(A), 50. 73(a)(2)(v)(B), and 50.73(a)(2)(v)(D).
EDG-B remained available and could quickly have been restored by manual action
to an operable condition if needed during the operability run.
The safety objective of the emergency diesel generators
(EDGs) is to provide a source of on-site AC power adequate for the safe
shutdown of the reactor following abnormal operational transients and
postulated accidents assuming a complete loss of off-site power, as described
in Pilgrim Nuclear Power Station (PNPS) Updated Final Safety Analysis Report
(UFSAR). Two EDGs and their associated fuel supply systems provide a single
failure proof source of standby AC power. Pilgrim EDGs are 2600 KW ALCO 251-F
type diesel generators. These EDGs are designed to automatically start upon
receiving a valid signal, and come to operating speed ready to assume load.
Each generator is sufficient to power all loads on its emergency bus upon
failure of all off-site power. Each generator has the ability to pick up loads
in sequence within a specified time period. The two EDGs at PNPS are cooled by a
self-contained system consisting of radiators and a fan that is driven through
a right angle gearbox.
The standby AC power source provides two independent diesel
generators as the onsite sources of AC power to ~he emergency service portions
of the station Auxiliary Power Distribution System. Each onsite source provides
~C power to safely shut down the reactor, maintain the safe shut down
condition, and operate all auxiliaries necessary for station safety.
Historical review revealed that this fan drive gearbox was
replaced on the EOG-A in the May 2000 time frame. At ~hat time, the original
gearbox, Cotta Transmission Model Number J1327-2 was replaced with an upgraded
Model Number J1327-3. Correspondence with the OEM vendor indicated the inside
of the cases were identical with the only major physical change on the outside
which was the addition of a relief valve in the oil circuit. Changes to the
gearbox inspections were not updated to include any inspections or preventive
maintenance for the relief valve.
EVENT DESCRIPTION
On September 28, 2016, while performing the pre-start checks
prior to running the Emergency Diesel Generator (EDG)-A monthly Technical
Specification (TS) surveillance, the oil level in the EOG radiator fan right
angle gearbox was found low and additional checks found the gearbox oil
pressure relief valve was loose.
The two EDGs at PNPS are each cooled by self-contained
systems consisting of radiators and fans that rotate through a right angle
gearbox. At the time of discovery, even though the oil level was low, the EOG
would have started on a valid start signal. However, it would have been losing
gearbox oil and we conservatively assumed it would have overheated due to
failure of the cooling fan from gearbox damage. ~Functional Failure
Determination completed on October 11, 2016 conservatively determined that the
EOG would not have been able to run for its stated mission time of 30 days.
This condition existed for a period of 28 days, which is greater than the TS
Allowed Out of Service Time (AOT) of 72 hours. However, the Station Black Out
Diesel Generator was available during this time frame.
CAUSE OF THE EVENT
The failure was determined to be low oil level found in the
EOG-A radiator fan right angle gearbox due to the external relief valve
pressure setting screw and the cap which goes over it being disconnected from
the valve body. This allowed a pathway for oil to escape the gearbox when the
EOG was running.
The radiator fan right angle gearbox oil level is checked
prior to every monthly EOG run and the last time it was performed was August
31, 2016 with no problem identified during the pre-start checks. The last
maintenance performed on the EOG radiator fan right angle gearbox was part of
the routine examination and checks during the 2 year Preventive Maintenance
(PM), which was completed on March 7, 2015. The inspection includes draining
and changing the oil, performing internal inspection of the drive gears and
bearings and performing backlash measurements of the drive and driven gears.
However, the two (2) year PM does not perform any maintenance on ihe gearbox
oil pressure relief valve. The relief pressure of 15 psig was set at the time
of installation in 2000. Changes to the gearbox inspections were not updated to
include any inspections or preventive maintenance for the relief valve.
The failure is attributed to minimal engagement of the
pressure adjusting threaded union for the relief valve setting of 15 psig, and
there is some contribution from either engine vibration or possibly human error
which makes the cause indeterminate.
CORRECTIVE ACTIONS
EOG-A was declared inoperable, the relief valve was
repaired, pressure tested and the pressure adjusting threaded union was staked
to eliminate any risk from vibration induced motion in the future, the gearbox
oil was replaced and •he EOG run for a post-maintenance test.
PNPS conducted an extent of condition review for EDG-B by
performing an inspection to ensure that a common mode failure did not exist. 1
-,
The following are additional corrective actions to address
this issue which are being processed through the PNPS Corrective Action
Program:
1. Update station procedure 8.9.1, "Emergency Diesel
Generator and Associated Emergency Bus Surveillance," to identify the plug
that is used to check the oil level and visually inspect the reli~f valve to
ensure the cap is appropriately aligned before and after each EOG run.
2. Incorporate a vendor manual change to capture the
upgraded EDG's Cotta Transmission gear box. The gear box was updated in the
2000 time frame but the drawings/vendor manual was never updated.
3. Establish PM's for both EOG radiator fan right angle
gearbox relief valves.
SAFETY CONSEQUENCES
There were no consequences to the safety of the general
public, nuclear safety, industrial safety, and radiological safety due to this
event.
The safety objective of the EDGs is to provide a source of
on-site AC power adequate for the safe shutdown of the reactor following
abnormal operational transients and postulated accidents assuming a complete
loss of off-site power, as described in PNPS UFSAR.
At the time of the event, the preferred AC and the secondary
AC power sources were Operable and available to perform their intended safety
function. In addition, the Station Blackout AC Power Source was Functional and
available as the onsite source of AC power to the emergency service portions of
the Auxiliary Power Distribution System.
REPORT ABILITY
In a determination completed on October 11, 2016 it was
conservatively determined that EOG-A would not have been able to run for its
stated mission time of 30 days. This condition existed for a period of 28 days,
which is greater than the TS Allowed Out of Service Time (AOT) of 72 hours.
However, the Station Black Out Diesel Generator was available during this time
frame. This issue is reportable under 10 CFR 50.73(a)(2)(i)(B) as an Operation
or Condition which was Prohibited by the plant's Technical Specifications.
In addition, on September 15, 2016 EDG-B was made inoperable
to perform its monthly operability run. This created a condition that could
have prevented the fulfillment of the safety function of a system needed to
shut down the reactor and maintain it in a safe condition, remove residual
heat, and mitigate the consequences of an accident which is reportable in
accordance with 10 CFR 50.73(a)(2)(v)(A), 50.73(a)(2)(v)(B), and 50.
73(a)(2)(v)(D).
The date the condition was discovered was September 28,
2016. As such, this 60-day 10 CFR 50.73 Licensee Event Report was due to the
NRC Staff on November 27, 2016. This LER is being submitted late, and PNPS is
addressing this through the Corrective Action Program.
PREVIOUS EVENTS
Events involving LERs where both EDGs were inoperable were
reviewed. One related LER was identified and is summarized as follows:
LER 2016-001, "Both Emergency Diesel Generators
Inoperable," dated June 9, 2016 stated while EDG-B was out for
maintenance, EOG-A was declared inoperable due to a 130 dpm leak from the
jacket water pressure boundary.
There were no other LERs involving EOG inoperability at PNPS
identified in a search of the past five (5) years.
REFERENCES:
CR-PNP-2016-7443
CR-PNP-2016-9552
CR-PNP-2016-9653
CR-PNP-2016-9831