ABSTRACT (Limit to 1400 spaces, i.e.,
approximately 15 single-spaced typewritten lines)
On Friday February 8, 2013, at 2117
hours with the reactor initially at 85% core thermal power, Pilgrim Nuclear
Power Station (PNPS) experienced a loss of off-site power (LOOP) resulting in a
load reject and a reactor scram. All rods fully inserted and the Emergency
Diesel Generators automatically started and powered safety-related buses A5 and
A6. All other safety systems functioned as required. The plant stabilized in
Hot Shutdown. At the time of the event a significant winter storm (Nemo) was
buffeting Southern New England. At 2200 hours PNPS in conjunction with the
local grid operator determined off-site power sources were not reliable and
efforts to restore off-site power were temporarily suspended. At 2200 hours,
PNPS declared a Notification of Unusual Event. On February 10, at 1055 hours,
one of two off-site power supplies was restored, all safety buses were powered
from the startup transformer and the Unusual Event was exited. Later on
February 10, at 1402 hours with the plant in Cold Shutdown, ice bridging on a
startup transformer insulator caused its 345 KV supply breaker to open
resulting in a second LOOP. Again the EDG's started and powered safety-related
buses. All other safety systems functioned as required. Shutdown cooling was
restored at 1426 hours.
On February 10, at 2020 hours, this
occurrence was reported to the USNRC as documented in EN# 48739.
The severe winter storm which caused
extensive generalized geographical damage to the electrical distribution network
was root cause of the LOOP events.
These events posed no threat to public
health and safety.
NARRATIVE
BACKGROUND:
Pilgrim Station is connected to the
transmission lines through a 345KV ring bus located within the station's 345KV
switchyard. The 345KV ring bus connects the output of the main transformer, the
startup transformer (SUT), Line 355, and Line 342. There are four gas circuit
breakers which comprise Pilgrim's 345KV ring bus: ACB-1 02, ACB-1 03, ACB-104
and ACB-105.
Line 355 is a two terminal line which
connects Pilgrim to National Grid's Bridgewater Station and is connected to ACB-102
and ACB-1 05. Line 342 is a three terminal line, which connects Pilgrim to the
Canal Power Plant's Switchyard in Sandwich, MA and to Auburn Street Station
Switchyard in Whitman, MA. The Canal Switchyard is owned and operated by NSTAR
and Auburn Street Station Switchyard is owned and operated by National Grid.
Pilgrim's ACB-103 and ACB-104 connect
Line 342 to the plant's switchyard. The 345KV system is the Pilgrim Station
output power connection and is the preferred off-site power source via the SUT.
The 345KV ring bus design locates the
power transmission lines such that a failure of any one line will not result in
the loss of the other line. Specifically, with both transmission lines in
service, a failure of either 345KV line will not result in a main generator
trip, a SUT trip, or a failure of the other 345KV line. Either of the two 345KV
lines is capable of carrying full station output and supplying station loads
via the SUT.
The 345KV protective relay system is
designed and coordinated to isolate system disturbances and minimize the impact
to the overall transmission system. The protective systems are comprised of a
primary and secondary protection scheme and are divided into four zones of
protection.
·
The
main transformer (bounded by ACB-104 and ACB-105)
·
The
SUT (bounded by ACB-102 and ACB-103)
·
Line
355 (bounded by ACB-102 and ACB-105 and Bridgewater Station)
·
Line
342 (bounded by ACB-103 and ACB-104 and Auburn Street Station Street and Canal
Stations)
When ACB-104 and ACB-105 open, the
main transformer is isolated from the 345KV transmission system thus resulting
in a generator full load reject event.
In addition to the preferred 345KV
off-site power lines, Pilgrim has a secondary off-site power source, a 23KV line
from NSTAR's Manomet Substation that provides power to a shutdown transformer
(SDT).
During normal station start-ups and
shutdowns, the station's 4160V demands are supplied by the SUT. Once the
station main generator is synchronized to the 345KV transmission system, the
station unit auxiliary transformer (UAT) supplies all station 4160V demands,
with the SUT maintained in standby, ready to provide 4160V power if necessary.
In anticipation of a major snow storm
impacting Pilgrim Station (PNPS) on February 8, 2013, Operations entered Procedure
2.1.37 (Coastal Storm Preparations). Procedure 2.1.42 (Operation During Severe
Weather) and ENEP- 302 (Severe Weather Response, currently, EN-FAP-EP-010) at
0800 hours on February 7, 2013. At 1021 hours on February 8th, Station Risk was
elevated to YELLOW due to the winter storm warning (severe weather).
During the storm on February 8,
meteorological instruments at PNPS recorded sustained wind speeds between 42
and 49 mph through 2338 hours at which time the plant information (PI) system
stopped recording weather data until 1840 hours the following day. The wind
direction was predominantly from the ocean toward the switchyard during the
storm.
EVENT DESCRIPTION:
On Friday, February 8, 2013, at 2018
hours, the shutdown transformer (SDT) was declared inoperable due to repeated
off-site 23KV Trouble/Trip Bypass alarms and reports from NSTAR regarding the
power loss and restoration events on the Line via the Manomet Substation.
On February 8th, two line faults
occurred on both 345KV transmission lines connected to the PNPS ring bus. At 2102
hours a major fault occurred on off-site Line 342 which remained de-energized
for the remainder of the storm. At 2117 hours a fault on Line 355 occurred
resulting in a full load reject of the PNPS generator, a subsequent reactor
scram, and loss of the SUT. Emergency diesel generators (EDGs) automatically
started and provided power to safety buses A5 and A6. Groups 1, 11, and
VI isolations went to completion. Reactor Core Isolation Cooling (RCIC) system
was placed in service to maintain reactor vessel water level. High Pressure
Coolant Injection (HPCI) system was placed in service to control reactor
pressure. All systems performed as designed to bring the reactor to Mode 3,
including initiation of reactor water cleanup isolation, reactor building
isolation system, and standby gas treatment system.
At 2200 hours, an Unusual Event was
declared (EAL SU 1.1) for loss of off-site power to Emergency Busses.
At 2211 hours, off-site line 355 was
restored and ACB-102 was closed manually to reenergize the SUT.
At 2340 hours, a 'B' phase fault on
the SUT bus work tripped the SUT bus lockout relay. Walk downs of the switchyard
were conducted by Maintenance and Engineering to assess the condition of the
SUT bus. The relays that initiated the bus trip indicated the fault was within
the SUT protection scheme, but external to the SUT. Breaker ACB-1 02 was closed
at 1809 hours on February 9th.
At 0813 hours on February 9th, NSTAR
reenergized Line 355 and the SUT was energized and non safety related buses Al,
A2, A3, A4 were energized from the SUT commencing at 1815.
At 0400 on February 10, off-site power
was restored to safety-related 4160V bus A5 through the SUT via a single
345KV line. At 0830 hours, off-site power was restored to safety-related 4160V
bus via A6 through SUT.
The EDGs were secured and were on
standby. Residual heat removal was in shutdown cooling mode maintaining the
reactor in cold shutdown. Fuel pool cooling was in service with fuel pool
temperatures trending down.
Effective at 1055 hours on February
10, 2013, Pilgrim terminated the Unusual Event and transitioned to recovery.
At 1401 hours on February 10th, with
all control rods fully inserted and the reactor in cold shutdown conditions, the
plant experienced a second loss of off-site power with a flashover fault on the
B phase bus work of the SUT due to salt contaminated ice bridging on the phase
insulator. This resulted in the tripping of the breaker ACB- 102 and loss of
power to 4160V busses. Both EDGs auto-started as designed and provided power to
the emergency buses. This loss of off-site power resulted in de-energization of
both reactor protection system (RPS) channels resulting in a reactor scram
signal and loss of Shutdown Cooling. At 1426 hours, Shutdown Cooling was
returned to service. All other plant systems responded as designed. Station
personnel established back-up power to A5 and A6 buses in accordance with
plant procedures. On February 12, at 0405 hours off-site power was restored to
the non safety buses via the Main/ Unit Aux Transformers. At 0601 hours on
February 12th, offsite power was restored to safety bus A6 through the
Main/Unit Aux Transformers. Off-site power was restored to all 4160V buses
through the SUT as of 2147 hours on February 12. The plant returned to power
operation on February 15, 2013.
CAUSE:
The coincident loss of transmission
Lines 342 and 355 from faults external to the Pilgrim Nuclear Power Station switchyard
due to the winter storm resulted in the loss of offsite power, full load
rejection, and reactor scram. The subsequent flashovers in the switchyard were
due to snow and ice buildup on insulators and electrical distribution components.
CONTRIBUTING CAUSES:
PNPS procedure 2.1.42, Operation
During Severe Weather, provides limited guidance for Operations to determine
which severe snow storms are most likely to challenge Pilgrim switchyard
reliability.
Corrective actions to preclude
recurrence taken in response to prior LER 2008-006-00 and LER 2008-007-00, Loss
of SUT on line 355 flashover and ACB-1 05 flashover, and plant trip did not
prevent recurrence.
Previously identified internal
Operating Experience (OE) was not successfully utilized to direct the removal
of snow and ice from insulators on the SUT prior to reenergizing.
CORRECTIVE ACTIONS:
The following corrective actions were
completed to address the LOOP and flashover events prior the restart of Pilgrim
Station:
The component of the capacitor coupled
voltage transformer (CCVT) that was damaged due to the flashover fault was
repaired.
Pilgrim reviewed LOOP events with
NSTAR to improve reliability of service to the SUT.
Additional corrective actions included
in the corrective action program are as follows:
·
Procedural
guidance containing a systematic process for removal and restoration of bus
sections during and following blizzard conditions will be developed and
implemented. This includes all 345KV and 23KV power lines.
·
Procedural
guidance for de-icing insulating material susceptible to a flashover event in
the switchyard following blizzard conditions will be developed and implemented.
SAFETY CONSEQUENCES:
The Loss of Off-Site Power (LOOP)
event is analyzed in the Updated Final Safety Analysis Report (UFSAR) and assumes
loss of both 345 KV and 23 KV (preferred and secondary) sources coincident with
a design basis accident (DBA). The design imposes a 10 second delay in
re-energizing the 4160V Emergency Buses required to mitigate the DBA. In cases
where coincident loss of an EDG presents a bounding condition, the affected safety
bus is not assumed to be picked up by the shutdown transformer (SDT). The
bounding condition in which all off-site power and onsite AC (EDGs) sources
would be lost is a Station Blackout (SBO) transient event (10 CFR 50.63).
Pilgrim is designed to recover from the SBO event by having a separate SBO
diesel generator capable of providing power to the required safety buses to
shutdown the plant and maintain it in a safe condition. Thus, the LOOP events
experienced by Pilgrim are within the analyzed conditions and plant systems responded
as desiqned.
During and following the storm,
operators were able to maintain safe shutdown conditions (reactivity control, reactor
water inventory, decay heat removal, etc.). While loss of power to non-safety
related spent fuel cooling was a key consideration, time-to-boil never became
an overriding concern with respect to reenergizing buses and there was no
freshly discharged spent fuel in the pool. The most recent freshly discharged
fuel was almost 23 months old, and the time to boil was approximately seven
days upon loss of fuel pool cooling. The spent fuel pool temperature was less than 105
degrees F.
The EDGs started and loaded as
expected following each loss of SUT event. The amount of fuel onsite initially was
sufficient to operate the EDGs for 7 days (under LOCA conditions) and the SBO
DG was always available.
Based on the challenge to safety
systems during and following the event, Conditional Core Damage Frequency (CCDF)
of the event was estimated to be 1.8E-5.
Throughout these events there was no
adverse impact on the public health and safety.
PREVIOUS EVENTS:
The most recent LOOP events at Pilgrim
Station reported as LER are as follows:
LER 2008-006-00, Automatic Scram
Resulting from Switchyard Breaker Fault During Winter Storm, dated February 12,
2009.
LER 2008-007-00, Momentary Loss of all
345KV Off-Site Power to the Startup Transformer from Switchyard Breaker Fault,
dated February 12, 2009.
ENERGY INDUSTRY IDENTIFICATION SYSTEM
(EIIS) CODES:
COMPONENTS CODES
Transformer (Startup AC) Bus XFMR
SYSTEMS
Switchyard System (Startup
Transformer) FK
ESF Actuation (RPS, PCIS, RBIS) JE
Main Generator Output Power System EL
Medium Voltage Power System- Class 1 E
(4KV) EB
REFERENCES:
Condition Report CR-PNP-2013-0798,
Loss of Off-Site Power
NRC FORM 366A (10-2010)
