United States Nuclear Regulatory Commission - Protecting People and the Environment

.
PRIORITY ATTENTION REQUIRED  MORNING REPORT - REGION IV  FEBRUARY 13,
1995

Licensee/Facility:                     Notification:

Nebraska Public Power District         MR Number: 4-95-0014
Cooper 1                               Date: 02/13/95
Brownville,Nebraska                    Senior Resident Inspector          
Dockets: 50-298
BWR/GE-4                              

Subject: UNPLANNED SHUTDOWN                                              
                                                                         
Reportable Event Number: N/A

Discussion:

At 10:40 a.m. (CST) on February 10, 1995, Nebraska Public Power District 
declared a Notification of Unusual Event (NOUE) after declaring three    
relief valves in the automatic depressurization system (ADS) inoperable. 
The Cooper Nuclear Station (CNS) was being restarted after an extended   
shutdown that began on May 25, 1994.  With reactor power at approximately
5 percent and RCS pressure at 350 psig, a functional test of the ADS     
failed when three of six relief valves would not manually open.  The NOUE
was declared and a shutdown to cold conditions was initiated.  The unit  
achieved cold shutdown and the NOUE was exited at 10:13 p.m. (CST) on    
February 10.                                                             
                                                                         
The unit has eight safety relief valves. Of the eight, six are associated
with the ADS function and two with the low-low set relief logic.  All    
eight valves were replaced during the recent outage.  Licensee testing   
confirmed the valves were inoperable due to the malfunction of the       
solenoid control valves in the safety relief assemblies.  One of the     
three solenoid control valves was disassembled and it was identified that
internal corrosion had caused binding of the solenoid operating          
mechanism.  The other two control valves were not disassembled in order  
to preserve them for future analysis.                                    
                                                                         
In consultation with the control valve vendor, Target Rock Corporation,  
it was identified that the valves may not have been appropriately dried, 
following hydrostatic testing with water, at the vendor's facility.  The 
licensee and the vendor were able to conclude that the affected defective
components were isolated to the three failed relief valves.  However,    
there may have been other defective units supplied to other licensees.   
The licensee will begin a significant hazards analysis process to        
determine reportability pursuant to 10 CFR Part 21.  The Vendor Branch   
has been briefed on this issue for further followup.                     
                                                                         
The licensee has replaced the defective solenoid control valves in the   
safety relief assemblies.  A functional test of all valves will be       
reperformed when the RCS reaches 300 psig.                               
                                                                         
In addition, on February 11, 1995, operations personnel were in the      
process of swapping trains of RHR and a motor-operated valve (MOV) failed
to shut on demand.  The licensee's investigation identified that the stem
cap on the valve operator had threaded itself, due to vibration when the 
valve was operated, into the MOV casing, thus preventing the valve stem  
from moving.  During followup of this issue, it was identified that: (1) 
the licensee had replaced some stem caps and the replacement caps were   

.
REGION IV  MORNING REPORT     PAGE  2                    FEBRUARY 13, 1995
MR Number: 4-95-0014 (cont.)

not constructed to the same tolerances as were the original caps (e.g.,  
the tolerance on the cap threads was less for the replacement caps than  
was used for the original caps), and (2) no actions (such as staking) had
been taken by the licensee to prevent self-threading of the stem cap into
the valve operator housing.  The licensee has inspected the valves in the
drywell and found three valves with similar problems.  No inspections    
have been performed in other areas of the plant.  The licensee is        
currently evaluating whether or not these inspections must be performed  
prior to restart of the unit.                                            
                                                                         
Finally, on February 11, while removing the ADS solenoid valves, the     
licensee noted by visual observation that one of the vacuum breakers was 
not fully shut. The licensee's investigation determined that the disc for
this vacuum breaker was not constructed correctly in that the disc had a 
right angle face instead of a tapered face to allow it to seal in the    
valve seat properly.  The licensee also noted that the disc was slightly 
too large, which prevented the movement of the disc inside the pipe when 
the temperature of the disc increased (caused by steam being relieved in 
the pipe).  The increase in temperature caused the size of the disc to   
increase because of thermal expansion and caused the disc to not freely  
move inside the pipe.  The licensee is in the process of correcting these
problems.                                                                

Regional Action:

Region IV and NRR have been providing 24-hour per day coverage of        
licensee restart activities.                                             

Contact:  P. H. Harrell              (817)860-8250

Page Last Reviewed/Updated Thursday, March 29, 2012