Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 28, 1991
NRC INFORMATION NOTICE 91-68: CAREFUL PLANNING SIGNIFICANTLY REDUCES
THE POTENTIAL ADVERSE IMPACTS OF LOSS
OF OFFSITE POWER EVENTS DURING SHUTDOWN
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to describe the positive benefits that were derived by two licensees
when their outage activities were carefully planned, scheduled, and
performed. These licensees did not rely solely on technical specifications
to identify the minimum equipment that should be available under various
shutdown configurations. Rather, these licensees made additional equipment
available and developed special procedures, as appropriate, to mitigate the
consequences that could result from forced equipment outages. It is
expected that recipients will review the information for applicability to
their facilities and consider appropriate actions. However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances
In November 1990, the licensee for the Turkey Point Plant began an 11-month
dual unit outage to conduct a major electrical system upgrade including
installing two additional emergency diesel generators (EDGs). On March 13,
1991, while both units were defueled and the safety-related EDGs were
inoperable, offsite power for Unit 4 was being provided through the startup
transformer. Spent fuel pool cooling pumps were being powered from the "C"
load center which was being supplied by the "A" vital bus. Electrical power
for the component cooling water (CCW) pumps and the intake cooling water
(ICW) pumps was being provided from vital bus "A". At the time, spent fuel
pool cooling was the most significant nuclear safety function.
An alternate source of power supply to the CCW, ICW, and spent fuel pool
cooling pumps was available from four, non-safety-related (black start)
diesel generators. Other backup sources of power to the pumps included
interconnection with the Unit 3 startup transformer and transfer to another
Unit 4 transformer (after defeating circuit breaker interlocks). An
alternate method of
9110230028
.
IN 91-68
October 28, 1991
Page 2 of 3
spent fuel pool cooling, which was available, involved the use of a trailer-
mounted diesel generator and a diesel powered fire pump or a non-safety
grade screen wash pump. Detailed procedures for use of the alternative
cooling methods were available.
At 3:30 p.m., a lockout occurred on the Unit 4 startup transformer resulting
in a total loss of all vital AC power to Unit 4. A notification of an
unusual event (NOUE) was declared as a result of the loss of vital offsite
power. The licensee manually started, but did not load, two of the black
start diesel generators. The other black start diesel generators were not
started. All of these sources were available to supply power to the cooling
pumps for the spent fuel pool if the startup transformer could not be
returned to service.
The licensee inspected the startup transformer and associated circuit
breakers and found no electrical fault indications. At 4:35 p.m., the
licensee energized the startup transformer. At 5:27 p.m., power was
restored to the CCW, ICW and spent fuel pool cooling pumps and forced
cooling to the spent fuel pool was resumed. The NOUE was then terminated.
The temperature of the spent fuel pool had increased from 84øF to 87øF
during the 2-hour duration of the event.
On March 20, 1991, another event of interest occurred at the Indian Point
Station. Before the event, Indian Point, Unit 2, (IP2) was in cold shutdown
with all fuel removed from the vessel and stored in the spent fuel pit.
Indian Point, Unit 3, (IP3) was operating at 100 percent power in a normal
plant configuration. At 7:15 p.m., a potential transformer on a 345kV bus
section at a remote substation failed catastrophically, creating a ground
fault on the 345kV system. Fault isolation circuitry opened the main
generator output breaker for IP3, which initiated a reactor trip. The
reactor trip was uncomplicated and offsite power to IP3 vital buses was not
interrupted.
However, an explosion and subsequent fire in the substation caused circuit
breakers to electrically isolate the 138kV feeder circuit supplying offsite
power to the IP2 6.9kV buses. As a result, the 6.9kV non-vital and 480V
vital buses that were in service were deenergized. EDG 23 started
automatically on bus undervoltage. At the time, the other two IP2 EDGs were
tagged out of service for maintenance. As designed, EDG 23 did not load to
the buses because all logic requirements for automatic loading were not
satisfied. EDG 23 was manually loaded to two vital 480V buses. This action
allowed a service water pump and a component cooling water pump to be
started, a battery charger to be placed in service, and lighting to the
auxiliary and containment buildings to be restored.
At the time of the event, offsite power to some vital loads was being
supplied directly from temporary connections from an Indian Point Unit 1
feeder bus as opposed to being supplied from normal sources. These loads
included the spent fuel pit cooling pump, the auxiliary building exhaust
fan, and a battery charger. The licensee established these temporary
connections to mitigate the consequences of a potential loss of the normal
offsite source.
Since two EDGs were out of service, a temporary EDG was started as a precau-
tion. However, the unit was not needed during the event. The licensee had
.
IN 91-68
October 28, 1991
Page 3 of 3
imported this temporary EDG unit to the site specifically for use as an
alternate onsite source of emergency power during the shutdown in view of
the significant number of planned equipment outages. The licensee provided
this unit even though there were three gas turbines and a second 138kV
feeder that were available from off site and that could have been manually
aligned to energize the IP2 vital buses.
The licensee restored normal offsite power to IP2 within 30 minutes. During
the event, component cooling water and service water were lost for only 2
minutes limiting the interruption to spent fuel cooling.
Discussion
These events illustrate the benefits of carefully planning equipment outages
during shutdown. Before each event, the licensees had made additional power
sources available to augment normal supplies. Furthermore, the licensees
had developed written procedures to address anticipated equipment failures.
These efforts allowed operating personnel to have adequate time to perform
deliberate, well informed actions and minimized the safety concerns
associated with each event. If the licensees had been less careful in their
planning, these events could have been more significant. The technical
specifications allow licensees considerable flexibility in removing
equipment from service and altering normal system lineups during shutdown.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please call the
technical contact listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contact: N. Fields, NRR
(301) 492-1173
Attachment: List of Recently Issued NRC Information Notices
.
