United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. NO 86-01: Failure of Main Feedwater Check Valves Causes Loss of Feedwater System Integrity and Water-Hammer Damage

                                                        SSINS No: 6835 
                                                        IN 86-01       

                                UNITED STATES
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT
                            WASHINGTON, DC 20555

                               January 6, 1986

Information Notice No. NO 86-01:   FAILURE OF MAIN FEEDWATER CHECK VALVES 
                                   CAUSES LOSS OF FEEDWATER SYSTEM INTEGRITY
                                   AND WATER-HAMMER DAMAGE 

Addressees: 

All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP) 

Purpose: 

This notice is provided to inform recipients of a recent event caused by 
five main feedwater (MFW) check valve failures at a 
pressurized-water-reactor (PWR) plant These failures resulted in a loss of 
MFW system integrity and significant water-hammer damage Recipients are 
expected to review the information for applicability to their facilities and 
consider actions, if appropriate, to preclude similar problems occurring at 
their facilities However, suggestions contained in this information notice 
do not constitute NRC requirements; therefore, no specific action or written 
response is required 

Description of Circumstances: 

On November 21, 1985, San Onofre Unit 1 was operating at 60% power when an 
auxiliary transformer failed, resulting in a loss of power to a vital bus 
and to the bus feeding the east (electric) MFW pump (A schematic of the MFW
system is enclosed as Figure 1) The west (electric) MFW pump remained 
energized from the unit main generator due to an abnormal electrical lineup
When the east MFW pump tripped, its discharge-check valve (FWS-438) failed 
to seat properly As a result of the failure of the east MFW pump 
discharge-check valve, the west MFW pump supplied feedwater backwards 
through this discharge-check valve and overpressurized the east feedwater 
heater-condensate train Several tubes apparently ruptured in the east 
feedwater train fifth stage (low pressure) feedwater heater as a result of 
the overpressurization, causing the shell side of this feedwater heater to 
rupture also In addition, several main turbine rupture discs failed 
Following the above events, the operators tripped the reactor and turbine by
procedure because of the loss of power to a vital bus This also caused the 
west MFW pump to trip Both 12-inch MFW pump discharge-check valves (FWS-438
and 439) were later found cocked open, supported by their disc antirotation 
lugs that had rotated under the check valve hinge arm 



8512310461 


                                                            IN 86-01       
                                                            January 6, 1986 
                                                            Page 2 of 3    

When the west MFW pump tripped, all three steam generator (SG) MFW 
regulating valve discharge-check valves (FWS-345, 346 and 398) also failed 
to seat Two of these 10-inch check valves were later found to have their 
flappers loose in the bottom of the valve body with their nuts missing The 
third 10-inch check valve was later found to have failed in the same mode as 
FWS-438 and 439 These check valve failures in the MFW system resulted in 
leak paths from the SGs backward through the MFW regulating valves and the 
east MFW pump to the ruptured east train FW heater In addition, the west 
MFW train may have been pressurized from the SGs The net effect of this is 
that the inventory in all three SGs began to blow steam and hot water back 
through the east MFW train 

The above reactor trip also caused level shrink in the SGs, causing SG level
to drop below the actuation level for the auxiliary feedwater (AFW) pumps 
The electric driven AFW pump received an actuation signal, but no longer had
electric power available The steam-driven AFW pump, after a 3-minute 
automatic warmup period, began to deliver relatively cold feedwater to the 
SGs at a point in each of the MFW lines between the MFW regulating valve 
discharge-check valve and the SG This AFW to the SG feedwater lines 
initially flowed backward through the failed check valves and forward 
through long horizontal runs of feedwater pipe in the primary containment 
Although the operators were unaware that the check valves had failed, they 
then closed all MFW regulating valves, FCV-456, 457 and 458 and their 
associated isolation valves, MOV-21, 20 and 22 in accordance with 
procedures (The effect of the closure of these MFW valves has yet to be 
determined) Contact between steam in the feedwater lines and the cool AFW 
in the horizontal pipe resulted in a water-hammer The water-hammer caused 
damage to the feedwater line pipe supports and stretched the bonnet bolts on 
the "B" feedwater regulating valve bypass-line check valve (FWS-378), 
causing the metal valve gasket to extrude The flapper on this 4-inch check 
valve was later found to have been damaged by the water-hammer impact The 
extrusion of the valve gasket resulted in a substantial steam-water leak 
from the "B" SG to the feedwater mezzanine area and the atmosphere that was 
not isolatable for some time because of the proximity of the associated 
isolation valve to the leak As a result, the "B" SG boiled dry since all 
"B" AFW flow was carried out through the leak Plant personnel were finally 
able to close valves FWS-342 and FWS-376 to isolate the leak and continue 
the plant cooldown about six hours after the event started 

Discussion: 

The NRC sent a five-member incident investigation team (IIT) to the San 
Onofre, Unit 1 site shortly after the above incident The licensee agreed to
hold in abeyance any work in progress or planned (as allowed by plant safety
considerations) until the licensee and the NRC had an opportunity to 
evaluate the event The licensee also has agreed to maintain Unit 1 shutdown 
until concurrence is received from the NRC to return to power The IIT has 
completed a preliminary investigation of this event and expects to issue a 
report in January 1986 


                                                            IN 86-01       
                                                            January 6, 1986 
                                                            Page 3 of 3    

No specific action or written response is required by this information 
notice If you have 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:  Henry Bailey, IE
                    (301) 492-9006

Attachments:
1   Figure 1, San Onofre Unit 1 Main Feedwater
       System Schematic Diagram
2   List of Recently Issued IE Information Notices

Page Last Reviewed/Updated Tuesday, November 12, 2013