Information Notice No. 84-06: Steam Binding of Auxiliary Feedwater Pumps

                                                           SSINS No.:  6835 
                                                           IN 84-06        

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                          WASHINGTON, D.C. 240555 
                                     
                              January 25, 1984 

Information Notice No. 84-06:   STEAM BINDING OF AUXILIARY FEEDWATER 
                                   PUMPS 

Addressees: 

All pressurized water reactor (PWR) facilities holding an operating license 
(OL) or construction permit (CP). 

Purpose: 

This information notice provides notification of a problem pertaining to 
steam binding in the auxiliary feedwater (AFW) pumps due to leakage from the
main feedwater system. It is expected that addressees will review the 
information provided for applicability to their facilities. No specific 
action or response is required. 

Description of Circumstances: 

On April 19, 1983, Carolina Power and Light reported that the two 
motor-driven AFW pumps started automatically on low steam generator level 
following a manual scram at the H. B. Robinson nuclear plant. After two 
minutes, the B train AFW pump tripped. The trip was attributed to a signal 
from low discharge pressure. 

The discharge piping from the motor-driven AFW train is connected to the 
main feedwater piping near the steam generator. (See Figure 1.) Hot water, 
about 425F, from the main feedwater system leaked back through the 
first check valve, the motor-operated valve, and the second check valve to 
the pump and flashed to steam because of the lower pressure in the AFW 
system. (A significant amount of steam was vented from the pump casing 
during the testing to determine the cause of the trip.) When the 
motor-driven pumps started, the instrumentation sensed a low discharge 
pressure. The steam binding reduced flow and prevented discharge pressure 
from increasing above the low pressure setpoint in the 30 seconds before the 
instrumentation tripped the pump. Condensation could have further lowered 
the pressure to the sensors. 

Robinson had experienced leakage through valves in the discharge piping and 
consequent trips of the A train AFW pump on June 11 and 16, 1981. On July 
21, 1983 the steam-driven pump was declared inoperable because of potential 
steam binding caused by leakage from the feedwater system. Crystal River 3 
reported two steam-voiding events which caused the emergency feedwater 
system train B to be declared inoperable. Two similar events were reported 
at D.C. Cook Unit 2  


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                                                         IN 84-06          
                                                         January 25, 1984  
                                                         Page 2 of 2       

in 1981. (Reference LERs 50-261/83-044, 83-016, and 81-016; 50-301/82-076, 
and 83-045; and 50-316/81-032 and 81-063.) 

A special interim procedure at Robinson calls for the venting of all three 
pumps once each shift, monitoring of the casing temperatures, and operating 
the pumps as required to prevent saturation conditions in the system. Cook 
also monitors the AFW system temperature. Robinson is exploring a design 
change or replacement of the check valves as a long-term solution. 

The safety implication of these events is that leakage into the AFW from the
feedwater system constitutes a common mode failure that can lead to the loss
of all AFW capability. Further, there is the potential for water hammer 
damage if an AFW pump discharges relatively cold water into a region of the 
piping system that contains steam. Since the design of the AFW at Robinson 
is typical of other PWRs, the potential for backleakage exists in other 
operating plants. Routine monitoring of the AFW system temperature would 
detect backleakage so that the system could be periodically vented to 
prevent steam binding until an appropriate long-term solution is developed. 

No written response to this notice is required. If you have any questions 
regarding 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 Contacts: M. S. Wegner, IE 
                    301-492-4511 

                    J. J. Zudans, IE 
                    301-492-4255 

Attachments: 
1.   Figure 1, "Simplified Schematic of Feedwater and Auxiliary Feedwater 
       Systems" 
2.   List of Recently Issued IE Information Notices