United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-77: Inadvertent Reactor Vessel Overfill

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C. 20555

                               September 22, 1988

Information Notice No. 88-77:  Inadvertent Reactor Vessel Overfill


All holders of operating licenses or construction permits for boiling-water 
reactors (BWRs).


This information notice is being provided to alert addressees to potential 
problems resulting from inadvertent overfilling of the reactor vessel.  It is 
expected that recipients will review the information for applicability to 
their facilities and consider actions, as appropriate, to avoid similar 
problems.  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 January 20, 1988, Nine Mile Point, Unit 2, personnel were performing start-
up testing; the reactor was in the natural circulation mode and at 42 percent 
power.  An auxiliary operator, who was performing maintenance without 
informing the control room, isolated the instrument air system, which 
eventually led to tripping both operating feedwater pumps and one of two 
operating condensate booster pumps.  All three condensate pumps and one 
condensate booster pump remained running.  After the reactor tripped because 
of low water level, the water level in the reactor vessel was restored to 
level 8 by the high-pressure core spray (HPCS) and the reactor core isolation 
cooling (RCIC) systems with the reactor pressure decreasing slowly from 655 to
625 psig. 

Meanwhile, the operator who isolated the instrument air system realized his 
error and restored the isolated air system.  This action reestablished the 
normal valve arrangement for the condensate/feedwater system.  Since the 
condensate and condensate booster pumps were still operating and the master 
feedwater controller had previously been put in manual, feedwater flow to the 
vessel resumed.  The reactor operator was unaware that feedwater was being 
injected.  When the operator noticed that the water level in the reactor 
vessel was starting to rise without HPCS or RCIC operating, he attempted to 
close the feedwater flow control valves.  However, the feedwater flow control 
valves locked up when they were approximately 20 percent closed.  This valve 
lock-up caused reactor vessel overfilling and flooding of the main steam lines
up to the turbine stop valves.  It was later found that information regarding 
the potential for valve lock-up and corrective actions had not been provided 
to the operators prior to the event.  A detailed discussion of this event can 
be found in NRC Inspection Report No. 50-410/88-01, dated March 8, 1988, 
prepared by an Augmented Inspection Team.  


                                                            IN 88-77 
                                                            September 22, 1988
                                                            Page 2 of 3 

On March 22, 1987, Washington Nuclear Power, Unit 2, was operating at approxi-
mately 71 percent power when the operators manually scrammed the reactor fol-
lowing a loss of both feedwater pumps.  The water level in the reactor vessel 
was restored to level 8 by the HPCS and RCIC systems.  Because of an erroneous
report that the RCIC system had tripped on overspeed, the operators were 
relying on the condensate/feedwater system to control the water level, while 
the safety relief valves (SRVs) were used to control pressure.  Additionally, 
the plant was operating with an abnormal feedwater/condensate system lineup 
for which temporary procedure deviations had not been incorporated into the 
scram procedures.  The operators failed to complete the valving sequence 
required to establish shutdown level control by failing to close two 
high-pressure heater block valves.  The reactor vessel overfilled and water 
entered the steam lines. 


Although the events at Nine Mile Point, Unit 2, and Washington Nuclear Power, 
Unit 2, followed different scenarios, there are common factors that 
contributed to overfilling the reactor vessel, such as inadequate procedures 
and training, poor or no communication, lack of updated information for the 
operators, and failure of the operators to observe the water level in the 
reactor vessel after it reached level 8.  

Both plants had previously experienced an overfill event:  Washington Nuclear 
Power, Unit 2, on July 25, 1986, and Nine Mile Point, Unit 2, on October 1, 
1987.  Neither plant considered the filling of the steam lines with water as 
significant and neither reported the details of the events in their 10 CFR 
50.72 notifications.  The NRC regards a reactor vessel overfill event as a 
significant safety concern and has identified the following four safety issues
with these events: 

(1)  Hydrodynamic effects of water or two-phase fluid being discharged through
     the SRVs.  This process could damage the SRVs.

(2)  Stressing of the vessel, steam line nozzles, steam line snubbers, pipe 
     supports, and hangers as a result of: 
     (a)  the thermal transient caused by colder water flowing into the hot 
          main steam line and reactor vessel;

     (b)  the weight of water in the main steam lines; and

     (c)  the dynamic transient loads caused by water flow in the main steam 

(3)  Potential for MSIVs not to close if the steam lines are filled with 

(4)  Placing the plant in a condition that has not been analyzed in the final 
     safety analysis report (FSAR).

Although not a reactor safety concern, water could damage the main turbine. 

                                                            IN 88-77 
                                                            September 22, 1988
                                                            Page 3 of 3 

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional

                              Charles E. Rossi, Director 
                              Division of Operational Events Assessment 
                              Office of Nuclear Reactor Regulation 

Technical Contact:  T. Greene, NRR 
                    (301) 492-1176 

Attachment:  List of Recently Issued NRC Information Notices 
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