Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No: 6835
IN 86-04
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
January 31, 1986
Information Notice No. NO 86-04: TRANSIENT DUE TO LOSS OF POWER TO
INTEGRATED CONTROL SYSTEM AT A
PRESSURIZED WATER REACTOR DESIGNED BY
BABCOCK & WILCOX
Addressees:
All nuclear power facilities holding an operating license (OL) or a
construction permit (CP)
Purpose:
This notice is to inform recipients of a recent event at an operating
pressurized water reactor resulting from loss of power to the integrated
control system Recipients are expected to review the information in this
notice for applicability to their facilities and consider actions, if
appropriate, to preclude similar problems from occurring at their
facilities However, suggestions contained in this notice do not constitute
NRC requirements; therefore, no specific action or written response is
required
Description of Circumstances:
On December 26, 1985, Rancho Seco was operating on automatic control at a
constant power level of 710 MWe (76% of licensed power) At 4:14 am, power
to the integrated control system (ICS) was lost The annunciator alarm for
"Loss of ICS or Fan Power" sounded As designed, ICS demand signals went to
midscale The main feedwater valves closed to 50%, and the atmospheric dump
valves, turbine bypass valves, and one set of auxiliary feedwater valves
opened to 50% The main feedwater pump speed was reduced to minimum Low
discharge pressure at the main feedwater pump caused the motor-driven
auxiliary feedwater pump to start automatically The net decrease in
feedwater flow caused the reactor to trip on high reactor coolant system
(RCS) pressure
After the reactor trip, the above ICS valves remained at 50% (ie, could
not be operated from the control room) causing excessive cooling of the RCS
which was exacerbated by autostarting of the dual-drive auxiliary feedwater
pump During the 26 minutes required to restore ICS power, operators acted
to minimize the resulting transient However, difficulties were experienced
with manipulation of valves, operation of pumps, and control of various
liquid levels, pressures, and temperatures RCS pressure decreased to a
minimum of 1064 psig at 4:21 am At 4:40 am, the lowest RCS temperature
(386F) during the cooling transient was reached RCS pressure at that
time was 1413 psig Eventually, a senior reactor operator discovered that
witches which supplied
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IN 86-04
Page 2 of 3
January 31, 1986
power to the ICS dc power supplies were in the off position and set them to
the on position Although manual (ie , hand) operation was now possible in
the control room, the valves initially received a 100% demand signal
Operators quickly shut the valves At 5:00 am , RCS pressure and
temperature were stabilized at 716 psig and 433F and maintained there
for 3 hours This unusual event, which was declared at 4:30 am, was
terminated at 8:41 am
Before the event was terminated, a large number of problems were
experienced, including:
* The RCS was cooled 180F in 24 minutes violating the technical
specifications limit of 100F in l hour
* Recommended pressure/temperature limits for pressurized thermal
shock were exceeded; however, nil ductility temperature limit in
the technical specifications was not violated
* Pressurizer level was low and off scale
* After loss of ICS power ICS controlled valves could not be
manually operated from the control room
* One auxiliary feedwater isolation valve could not be closed
* One auxiliary feedwater flow control valve was overtorqued using
the manual handwheel, and the manual operator failed
* Operators had considerable difficulty determining (locally) the
position of the auxiliary feedwater flow control valves
* One steam generator was overfilled
* A main feedwater flow recorder in the control room failed at
midscale because of the loss of ICS power although main feedwater
flow was essentially zero
* An RCS makeup pump was run without water (ie, suction valve
shut) and severely damaged, specifically, seals for the makeup
pump failed and approximately 450 gallons of water were spilled in
the auxiliary building
* A containment radiation monitor was damaged because it continued
to run after the suction valve had been shut by a Safety Features
Actuation Signal
Four senior reactor operators were present during the event At 5:01 am,
one of them collapsed from exhaustion in front of a control panel He was
transported by ambulance to a local hospital and subsequently released in
satisfactory condition at 7:00 am
IN 86-04
Page 3 of 3
January 31, 1986
Discussion:
The NRC sent an incident investigation team (IIT) to Rancho Seco shortly
after the event The licensee has agreed to hold in abeyance any work in
progress or planned (except as required by plant safety considerations)
until the licensee and the NRC have had an opportunity to develop detailed
troubleshooting plans for failed equipment Further, the licensee has agreed
to maintain the unit in a shutdown mode until NRC concurs with the licensee
that the unit can be returned to power safely
Review by the IIT is continuing As additional information about the event
is obtained, this notice will be supplemented, if appropriate
No specific action or written response is required by this information
notice If you have any questions about this matter, please contact the
Regional Administrator of the appropriate NRC regional office or this
office
Edward L Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: R W Woodruff, IE
(301) 492-8597
Attachment: List of Recently Issued IE Information Notices
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