United States Nuclear Regulatory Commission - Protecting People and the Environment

Morning Report for August 17, 2001

                       Headquarters Daily report

                         AUGUST 17, 2001

                    REPORT             NEGATIVE            NO INPUT
                    ATTACHED           INPUT RECEIVED      RECEIVED
HEADQUARTERS                           X
REGION I            X
REGION II                              X
REGION III                             X
REGION IV                              X

Licensee/Facility:                     Notification:

Entergy Nuclear Generation Company     MR Number: 1-01-0016
Pilgrim 1                              Date: 08/13/01
Plymouth,Massachusetts                 ENS Call
Dockets: 50-293


Reportable Event Number: 38206


At 5:53 PM on August 13, 2001, the Pilgrim nuclear power plant
automatically shutdown (scrammed) (see Event Report 38206).  Prior to the
event the unit had been operating at full power.  The cause of the scram
was due to the loss of both recirculating water pumps at power, which
resulted in an actuation of the flow-biased, high power flux trip of the
neutron monitoring system, average power range monitors (APRMs) and a
loss of power to one Reactor Protection System (RPS).  No actuations of
SRVs or ECCS resulted from the plant scram. Operators successfully
stabilized the plant condition in accordance with plant procedures.
Subsequently, on August 14, 2001, the unit was placed in cold shutdown to
commence a short duration maintenance outage.

The cause of the loss of both operating recirculating water pumps was
separate, but related, events that were a result of a loss of the 4160
Volt Emergency Bus A5.  Emergency Bus 5A was de-energized as a result of
an error in a test procedure that was in progress at the time.  The error
resulted in bus logic opening the normal feeder breaker to the bus.  In
addition, the A RPS de-energized when Bus 5A was lost, resulting in a
"half scram" condition.  Throughout the event, the redundant 4160 Volt,
Emergency Bus A6 remained operable and energized.

During a swap of power supplies from the A5 bus to the A6 bus, the
"swing" Bus B6 de- energized for about 1 second.  This caused a momentary
loss of power to a regulating voltage circuit for the B recirculating
water pump motor-generator (M-G) set.  This, coupled with a previously
undetected, high resistance fuse block assembly in the same control
circuit, resulted in tripping the M-G set and the B recirculating water
pump.  Also as a result of Bus A5 de-energizing, the power supply to the
A recirculating water pump M-G set oil pumps was lost, causing the A
recirculating water pump to trip.  With both recirculating waters pumps
tripped, total core flow and reactor power decreased rapidly causing a
flow-biased neutron power trip actuated on the B RPS logic.  Since the A
RPS was already tripped, the reactor was shut down.

Subsequent to the reactor scram operators noticed a slow upward trend on
one of the indicated reactor vessel water level channels.  Operators had,
subsequent to the scram and by procedure, isolated the control rod drive
(CRD) charging flow path in order to promote the settling of one control
rod to the full-in position.  During the time that the CRD charging flow
path was isolated (about 30 minutes), operators diagnosed that this was a
result of an indication problem and that actual water level was not
increasing.  After restoring the CRD charging flow path to its normal
configuration the indicated level returned to its expected condition.
Subsequent testing of this instrumentation led Pilgrim to conclude that
the reactor vessel water level reference leg keep-fill system, supplied
by CRD charging flow, allowed back leakage from the vessel water level
reference leg.  Such back leakage is designed to be prevented by two, in
series, spring-loaded, check valves.  These valves apparently did not
fully close and, as reference leg water leaked back into the keep-fill
system, the indicated reactor vessel water level deviated from expected

On August 17, 2001, the licensee initiated a plant restart following
completion of the post-scram evaluation, including repairing the fuse and
fuse assembly that contributed to the loss of the B recirculating water
pump, replacing the check valves for the reactor vessel water level
keep-fill system, and ensuring that the test procedure error was fully
understood in order to prove that Bus A5 was fully operable.  The
licensee will maintain the reactor vessel water level keep-fill isolated
(for a period up to two weeks) in order for additional engineering
evaluation of the system to be completed to determine if further
modification to the system is required to avoid similar problems.

Regional Action:

The resident inspector was dispatched to the site on August 13, 2001, to
monitor the licensee's post-scram actions to place the station in a
stable condition.  A region-based inspector was dispatched to the site on
August 16, 2001, to assist the resident inspector in event followup, as
well as to review the licensee's actions regarding the reactor vessel
water level indication problem.  The NRC will continue to monitor the
licensee's actions relative to the isolated keep fill system along with
reviewing plant performance and determining overall significance of the

Contact:  R. Laura                   (508)747-0565
          R. Summers                 (610)337-5282
Page Last Reviewed/Updated Thursday, March 29, 2012