United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-45: Potential Common-Mode Post-Accident Failure of Containment Coolers

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                          WASHINGTON, DC  20555-0001

                                August 12, 1996


NRC INFORMATION NOTICE 96-45:  POTENTIAL COMMON-MODE POST-ACCIDENT FAILURE OF
                               CONTAINMENT COOLERS 


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a potential common-mode post-accident failure of
containment coolers. 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 are not NRC requirements; therefore, no specific
action or written response is required.

Description of Circumstances

On February 13, 1996, the licensee for Diablo Canyon determined that the
containment fan coolers were susceptible to component cooling water flashing
in the cooler unit cooling coils during a design-basis loss-of-coolant
accident (LOCA) with a concurrent loss of offsite power or with a delayed
sequencing of safety-related equipment.  Diablo Canyon has five fan coolers
per unit.  The containment fan coolers are designed to remove heat from the
containment atmosphere during normal operation.  The fan coolers are also
automatically initiated engineered safety features, which help maintain
containment integrity by reducing post-accident containment pressure by the
condensation of steam and the removal of heat in conjunction with operation of
the containment spray system.  The fan coolers are cooled by component cooling
water under both normal and accident conditions.

During a postulated design-basis LOCA with a concurrent loss of offsite power,
the component cooling water pumps and the fan cooler fans would lose power. 
The component cooling water flow stops almost immediately, while the fans
coast down over a period of minutes.  The first fan cooler will restart on
slow speed approximately 22 seconds after the loss of offsite power.  The
component cooling water pumps will restart 26 to 30 seconds after the
accident.  In this scenario, the high temperature containment atmosphere would
be forced across the cooling coils for several seconds with no component
cooling water flow.  This process may cause the stagnant component cooling
water to boil, thereby creating steam voids in the cooling coils and the 


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component cooling water piping.  When the component cooling water pumps
restart, the pumped liquid would collapse the steam voids and may create
significant hydrodynamic loads (waterhammer).  The hydrodynamic loads may
adversely affect the integrity of the fan coolers or the associated component
cooling water piping.  This problem may cause the fan cooler pipes to rupture
and could cause a significant loss of component cooling water inventory, which
could threaten cooling to emergency core cooling system components.  The
licensee for Diablo Canyon evaluated this potential failure mechanism and has
installed a nitrogen pressurization system on the component cooling water head
tank to increase the margin to boiling.
 
On July 22, 1996, the licensee for Haddam Neck declared all four of its
containment air recirculation fans inoperable and initiated a plant shutdown. 
The air recirculation fans are the only credited heat removal system for post-
accident containment heat loads, and the associated cooling coils are cooled
by the station service water system.  The fans were declared inoperable after
the licensee review of the consequences of two-phase service water flow
following a LOCA concurrent with a loss of normal power.  The licensee
analysis predicted hydrodynamic loads that exceeded the service water piping
and support structural limits.     

Discussion

At many plants, containment fan coolers provide a significant safety function
in reducing containment pressure and removing heat from the containment
building.  The postulated failure scenario could cause a common failure of all
the containment coolers, thereby potentially challenging the integrity of the
containment building.  In certain cases, failure of the cooling water system
piping could cause a potential containment bypass flow path and may jeopardize
cooling to other safety-related loads.  

An individual plant vulnerability to these postulated failures is dependent on
a number of factors.  The coastdown rates of the cooling water flow and the
cooler fans, the operating pressure and pressure decay rate of the cooling
water system, the timing of cooling water pump restart, the containment
temperature profile during the design-basis accident, and other site-specific
parameters have an effect on facility vulnerability to this potential failure
mode.  This failure mode would also be applicable to the design-basis main
steam line break in containment.

Westinghouse issued Nuclear Safety Advisory Letter NSAL-96-003, "Containment
Fan Cooler Operation During a Design Basis Accident," to alert their customers
to this concern.  NSAL-96-003 is provided as an attachment to this information
notice.
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This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation project manager. 


                                          signed by

                                    Brian K. Grimes, Acting Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical Contacts:  James Tatum, NRR                 John Tappert, NRR
                     (301) 415-2805                   (301) 415-1167
                     Internet: jet1@nrc.gov           Internet: jrt@nrc.gov
                        
                     Alan Levin, NRR                  Howard Wong, Region IV
                     (301) 415-2890                   (510) 975-0296
                     Internet: ael@nrc.gov            Internet: hjw@nrc.gov   

                     William Raymond, Region I
                     (203) 267-2571
                     Internet: wjr@nrc.gov

Attachments:  
1.  Nuclear Safety Advisory Letter NSAL-96-003, dated 06/20/96
2.  List of Recently Issued NRC Information Notices
(Note Attachments 1 and 2 not included)
Page Last Reviewed/Updated Thursday, November 21, 2013