United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-04: Transient due to Loss of Power to Integrated Control System at a Pressurized Water Reactor Designed by Babcock & Wilcox

                                                            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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                                                           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 
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                                                           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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