United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-06: Design and Testing Deficiencies of Tornado Dampers at Nuclear Power Plants

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                         WASHINGTON, D.C.  20555-0001

                               January 25, 1996

                               DAMPERS AT NUCLEAR POWER PLANTS


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for inoperability of tornado
dampers because of either inadequacies in damper testing or deficiencies in
damper design.  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 March 2, 1994, the licensee for South Texas Project, Unit 1, reported a
condition to the NRC that could result in the rapid depressurization of
heating, ventilation and air conditioning (HVAC) systems and buildings in the
event of a tornado (Licensee Event Report No. 94-003).  The licensee
discovered an interference between the plant exhaust vent tornado damper and a
gusset (duct work structural stiffener).  The gusset limited the closing
travel of the damper linkage in one damper section to 40 percent of full
stroke, thus preventing the damper from closing completely.  The interference
was discovered during stroke testing in response to an NRC inspector
observation that there was no testing evidence to show that tornado dampers
would actuate as required.  In response to that observation, the licensee
initiated changes to existing preventive maintenance work instructions for all
tornado dampers (15 per unit) to include manual stroking on a periodic basis. 
Only one of the 30 dampers subsequently tested was found to be inoperable.   
The originally planned periodic maintenance of tornado dampers at 10-year
intervals did not include any stroke or motion testing.  

South Texas stated that rapid depressurization could result in the collapse of
the HVAC duct work and could prevent cooling of safety-related equipment.  The
rapid depressurization could also cause unanalyzed internal pressure
differential conditions that might result in damage to safety-related
structures, systems, or components.   

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                                                            Page 2 of 3

The licensee revised the preventive maintenance program to require preventive
maintenance on the tornado dampers every 2 years or at each refueling outage,
depending on the specific damper function.  The 2 year or refueling outage
check includes inspection, cleaning, lubricating, and stroking of the damper
while taking dynamometer measurements of the spring forces.    

On October 22, 1993, the licensee for the River Bend Station reported a design
condition to the NRC (Licensee Event Report No. 93-020) that could result in
loss of ventilation to several buildings after passage of a tornado.  The
licensee determined through static pressure testing, that the air-exhaust
dampers for several buildings would not reopen after passage of a tornado. 
The failure to reopen results from inadequate spring force on dampers to
overcome exhaust fan shutoff pressure.  This condition was applicable to the
diesel generator building, the auxiliary building, the control building, the
fuel building, and the standby cooling tower.

The licensee evaluation at the River Bend Station revealed that the postulated
tornado event could cause loss of offsite power and could prevent the
air-intake dampers for the diesel generator from reopening.  Other plant areas
were less susceptible to damage because of design features and/or alarms,
followed by operator actions that could mitigate the consequences.  The
air-exhaust tornado dampers in the diesel generator building were modified by
blocking them open.  The licensee evaluated the equipment in the diesel
generator building and concluded that it would not be affected by
depressurization resulting from the tornado.

The licensee purchase specification for dampers required the dampers to be
capable of automatically returning to their normal operating position (open)
after the tornado passed.  However, the specification did not specify the
design-bases conditions on the inlet side of the exhaust tornado dampers.  The
static pressure that the damper counterweights would have to overcome was not

Equipment such as tornado dampers may have been overlooked in development of
surveillance testing programs as tornado dampers are not generally considered
part of the HVAC system in which they are required to function.  Tornado
dampers are more likely to be considered part of the structure in which they
are installed.  Surveillance tests in the technical specifications may involve
damper operations during HVAC systems testing.  However, specific NRC
requirements or guidelines for damper testing in general are limited and do
not extend to tornado dampers.  Likewise, industry standards and guidelines
for testing of tornado dampers are also limited.  Unlike other active safety-
related dampers that are generally tested along with the HVAC system in which
they function, the licensees considered tornado dampers as passive devices
until they were called upon to perform their function.  
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When called on, certain dampers must close to prevent rapid depressurization
within the structures and HVAC ductwork and then reopen after the tornado
passes.  In cases for which no redundancy exists, a single failure of a
tornado damper to perform its safety function could result in a loss of safety
system function.  Therefore, absence of appropriate surveillance testing for
tornado dampers could lead to multiple failures in safety systems.

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 (NRR) project manager.

                                          signed by

                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management 
                                    Office of Nuclear Reactor Regulation

Technical contacts:  William T. LeFave, NRR
                     (301) 415-3285

                     Thomas Koshy, NRR
                     (301) 415-1176
Page Last Reviewed/Updated Thursday, November 21, 2013