United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-64: Potential for Common-Mode Failure of High Pressure Safety Injection Pumps or Release of Reactor Coolant Outside Containment During a Loss-Of-Coolant Accident

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C. 20555

                              October 4,  1990


Information Notice No. 90-64:  POTENTIAL FOR COMMON-MODE FAILURE OF HIGH
                                   PRESSURE SAFETY INJECTION PUMPS OR 
                                   RELEASE OF REACTOR COOLANT OUTSIDE 
                                   CONTAINMENT DURING A LOSS-OF-COOLANT 
                                   ACCIDENT 


Addressees:

All holders of operating licenses or construction permits for 
pressurized-water reactors.

Purpose:

This information notice is being provided to alert addressees to the 
potential for the common-mode failure of the high-head safety injection 
pumps (charging pumps) due to gas binding or the release of reactor coolant 
outside of contain-ment during a loss of coolant accident (LOCA), depending 
upon the manner in which the vent line isolation valves have been installed.  
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: 

On July 9, 1990, with the plant in cold shutdown in preparation for 
refueling operations, the Haddam Neck licensee had the Volume Control Tank 
(VCT) discharge and vent flow paths isolated in the Chemical and Volume 
Control System (CVCS). The charging pumps were aligned to the refueling 
water storage tank (RWST), with a train of the residual heat removal (RHR) 
system in operation.  The Haddam Neck licensee subsequently discovered that 
water was draining from the VCT through the high-point vent isolation valves 
(see Figure 1).  The 3/4-inch vent line, containing two ASCO 
solenoid-operated isolation valves, provides for continuous venting of 
hydrogen from the high points of the charging pump suction piping to the 
VCT, during normal CVCS letdown opera-tions.  Further investigation by the 
licensee revealed that the ASCO valves would not isolate in the reverse flow 
direction because the valve disc unseats with a differential pressure across 
the valve seat of 15-30 psid.  The licensee indicated, as a result of 
discussions held with ASCO, that no ASCO-manufactured solenoid-operated 
valves were designed to provide isolation in the reverse flow direction.  

Initiation of a safety injection actuation signal (SIAS) causes the normal 
discharge path from the VCT to isolate by automatic closure of the VCT 
discharge isolation valves.  The suction of the charging pumps is then 
switched 


9009280049 
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                                                            IN 90-64, 
                                                            October 4, 1990 
                                                            Page 2 of 3 


from the VCT to the RWST.  The ASCO vent line isolation valves are shut 
automatically upon closure of the VCT discharge isolation valves and 
manually de-energized.   With the valves in the closed position, a 
differential pressure in the reverse flow direction could allow gases in the 
VCT to pass through the high-point vent line and cause gas binding of the 
pumps.  Haddam Neck responded to this concern by adding two check valves as 
shown in Figure 1.

Discussion:

At Haddam Neck, the licensee identified a possible common-mode failure to 
both high-point vent isolation valves in the suction vent line that connects 
the charging pumps to the VCT.  The common mode failure occurs if there is 
leakage flow through both valves after they have been shut.  From their 
discussions with the valve manufacturer (ASCO), the licensee learned that 
these valves are designed to provide isolation in one flow direction only.  
The ASCO valves are identified as solenoid-operated, two-way, 1/2 inch 
isolation valves with a maximum pressure rating of 400 psig and are 
commerical grade.  The model numbers of the valves are 6109R and L8211D89.  
The installed configuration of the ASCO valves at Haddam Neck was such that 
failure of these valves to isolate during a LOCA (i.e., during the ECCS 
injection phase) could drain down the VCT and allow hydrogen gas in the VCT 
(and any gas that had accumulated in the vent line) to be transported to the 
suction of the charging pumps by way of the high-point vent charging pump 
suction line.  

A second scenario, was identified by the licensee involving the 
recirculation phase of a small-break LOCA, when the RHR pumps supply reactor 
coolant from the sump to the suction header of the charging pumps.  If the 
ASCO valves are installed in the reverse direction (i.e., to isolate venting 
from the VCT to the suction of the charging pumps), then the discharge 
pressure from the RHR pumps would be enough to increase pressure in the vent 
line to the ASCO isolation valves so as to lift the valve discs off of their 
seats and pressurize the VCT in excess of the 75 psig VCT relief setpoint.  
This would allow a release of reactor coolant outside containment.  

In addition, if the two ASCO vent line valves are installed in opposite 
directions, a single failure of a vent line isolation valve could lead to 
either gas binding or a release of reactor coolant outside of containment.  
Licensees with similar vent lines and ECCS pump arrangements may wish to 
review the above information for applicability to their plants.

The effectiveness of the Haddam Neck high point vent configuration (as shown 
in Figure 1) was not evaluated in conjunction with the development of this 
information notice.  It is important that any plant modifications made to 
alleviate hydrogen buildup concerns do not introduce other ways for gas to 
be ingested into the charging pump suction or other safety concerns. 

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                                                            IN 90-64, 
                                                            October 4, 1990 
                                                            Page 3 of 3 


Information notice 88-23, "Potential for Gas Binding of High Pressure Safety 
Injection Pumps During A Loss-Of-Coolant Accident" (including Supplements 1 
and 2) also describes situations that could result in gas binding of high 
pressure safety injection pumps. 

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 NRC regional office.                       




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


Technical Contact:  John Thompson, NRR
                    (301) 492-1171


Attachments:  1.  Figure 1, Haddam Neck High Point Vent Line
              2.  List of Recently Issued NRC Information Notices

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