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
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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, March 09, 2021