Information Notice No. 85-35, Supplement 1: Failure of Air Check Valves to Seat

                               UNITED STATES 
                           WASHINGTON, D.C. 20555 

                                May 17, 1988 

Information Notice No. 85-35, SUPPLEMENT 1:   FAILURE OF AIR CHECK 
                                                  VALVES TO SEAT 


All holders of operating licenses or construction permits for nuclear power 


This information notice is being provided to alert addressees to potential 
problems resulting from failures in instrument air system check valves 
intended to isolate safety-related air accumulators under accident 
conditions. 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: 

In December 1985 the NRC staff conducted a safety systems outage 
modification inspection (SSOMI) at Fort Calhoun Station, Unit 1. As a result 
of the escalated enforcement action following the SSOMI, the licensee 
committed to test air check valves associated with the safety-related air 
accumulators that provide air for various control valves and instruments on 
loss of the instrument air system. On April 15, 1988, as a result of check 
valve testing, the licensee found excessive leakage past four accumulator 
check valves in the air bubbler instrumentation used for initiation of the 
recirculation actuation signal (RAS). The valves had not been previously 
tested or included in the plant surveillance program. The licensee found 
that the check valves were not designed for air service and replaced them 
with soft-seat, spring-loaded valves. 


At Fort Calhoun the level in the safety injection and refueling water tank 
(SIRWT) is determined from the pressure in six air bubbler lines that extend
to the bottom of the SIRWT. Two of the lines provide control room indication
of level along with high/low level alarms. Four of the lines are used in 
two-out-of-four logic circuitry to provide input into the recirculation 
actuation signal. This signal provides automatic emergency core cooling 
system (ECCS) pump suction switchover from the SIRWT to the sump on low 
SIRWT level. The instrument air system supplying all air bubbler lines is 
not safety-related, seismically qualified or supplied with vital power. 


                                                   IN 85-35, Supplement 1  
                                                   May 17, 1988            
                                                   Page 2 of 2             

The four lines that provide for automatic ECCS switchover are provided with 
safety-related accumulators designed to maintain air pressure following 
design-basis events for which instrument air is assumed to be lost. Check 
valves separate each of the four accumulators from the instrument air 
system. Failure of these check valves would cause the accumulators to fully 
or partially discharge through the instrument air system if instrument air 
pressure was lost. Loss of accumulator air could cause a premature 
switchover of the ECCS and containment spray pumps following a 
loss-of-coolant accident. If premature switchover occurred, the containment 
sump might be dry or might have an inadequate water level to provide net 
positive suction pressure to the pumps during a design basis event. This 
could lead to damage of the pumps and their subsequent unavailability. The 
RAS initiation also secures the low pressure injection pumps and isolates 
the miniflow recirculation lines for the SI pumps back to the SIRWT. In 
addition, control room indication of SIRWT level would not be available if 
the instrument air pressure were lost since these bubblers do not have air 

Although the specific event described here occurred on a pressurized water 
reactor, failure of air check valves to seat when required to ensure 
availability of air from safety-related accumulators could be a problem on 
any type reactor. 

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 office.  

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

Technical Contact:  John Thompson, NRR 
                    (301) 492-1175  

Attachment:    List of Recently Issued NRC Information Notices

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