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Effects on Plant Operation - std10580051
Guide to Good Practices For Developing and Conducting Case Studies
Lessons Learned - std10580053

When reactor power was affected by
from Unit 2. During this period, the reactor
an unexplained runback of the reactor
water level dropped from its normal 201
recirculating pumps, the shift engineer
inches to 48 inches above the top of the
instructed  the  operator  to  reduce
active fuel. To recover water level, water
recirculating pump loading and scram the
was added to the reactor through the
reactor. While this was being done, the
feedwater bypass valve from the condensate
recirculating pumps tripped off. The reactor
booster pump. Water level went too high
was scrammed by the operator 30 minutes
because control of the bypass valve had
after the fire started.
been lost, so an auxiliary unit operator was
sent to manually throttle the valve. Nearly
Operators confirmed that the reactor
four hours after the fire started, four
control rods were fully inserted. Thirty-five
auxiliary unit operators working in pairs
minutes  after  the  fire  started  the
were successful in isolating faulted residual
turbine-generator was manually tripped.
heat removal circuits to get the residual heat
One minute later, all capability to monitor
removal system aligned. The residual heat
core power was lost as the vital power
removal system was not started until early
supply electrical boards were lost.  In
the next morning, 13 hours after the fire
addition, all emergency core cooling
started, because it could not be confirmed
systems  were  lost  because  their
that the system was filled with water.
motor-operated valves lost power and could
Sixteen hours after the fire started,
not be operated remotely.  All of the
shutdown cooling had been established,
outboard main steam isolation valves shut,
suppression pool cooling continued, and
isolating one main condenser as a heat sink.
essential plant instrumentation had been
Because reactor pressure increased rapidly
restored. Nearly all of these activities were
to 1100 psig, the control room operators
accomplished as a result of operator actions
took manual control of the main steam relief
locally. Temporary power supplies, manual
valves to reduce pressure by cycling
valve operation, and use of temporary
pressure between 850 and 1080 psig.
procedures were typical conditions because
Owing to the almost constant blowdown of
of the fire damage. The effects on Unit 2
reactor pressure that added heat to the
were less severe; however, the reactor
suppression pool, suppression pool cooling
depressurized because of a suspected stuck-
became essential 40 minutes after the fire
open relief valve and some vessel level
started, but the residual heat removal
instrumentation was lost. Unit 2 reactor
system, which is normally used, was
was placed in shutdown cooling about
unavailable because of the electrical board
11 hours after the fire started.
losses. The next 30 minutes were spent
trying to get the shutdown buses powered
either by the running diesel generators or

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