United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-06: Potential Bypass Leakage Paths Around Filters Installed in Ventilation Systems

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

                               January 22, 1993

                               FILTERS INSTALLED IN VENTILATION SYSTEMS 


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 potential problems resulting from missing or
deteriorated seals around shafts that penetrate fan or filter housings and
inadequately sealed ducting seams used in engineered safety feature (ESF)
ventilation systems.  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

As a result of a review of a nonconforming condition involving the standby gas
treatment system (SGTS), the licensee for Grand Gulf identified leakage paths
associated with ventilation system ducting and housings, including fan
plenums.  These leakage paths resulted in reduction of the removal capability
for radioactive material of the standby gas treatment and control room air
systems which are engineered safety features.  The affected ventilation system
ducting serves as either part of the secondary containment boundary or an
extension of the control room environment.  The effect of such bypass leakage
and the associated radiation doses was not considered as part of the facility
design or the licensing review.  As a result of these findings in June 1992,
the licensee determined that the facility had been operating in a condition
outside the facility design basis.  

The licensee assessment of the safety significance of bypassing the SGTS
radioactivity removal function (both adsorption and filtration), based on
estimated inleakage rates and licensing methodology, initially indicated that
potential calculated accident exposures could exceed the guidelines of 
10 CFR Part 100 and the values in General Design Criterion (GDC) 19 of
Appendix A to 10 CFR Part 50.  The licensee did a second assessment,
characterized as conservative but more realistic, which indicated that the
potential exposures would be within the guidelines of Part 100 and within 
GDC 19 values.


                                                            IN 93-06
                                                            January 22, 1993
                                                            Page 2 of 3


With respect to the SGTS, the licensee determined that an opening, or gap,
between the fan hub and the hole in the fan housing could result in air being
drawn into the suction plenum of the SGTS downstream of the charcoal adsorber
and filter (HEPA).  The paths were around a motor shaft, through a slotted
opening to an actuator for a damper, and at a transition piece of ducting. 
These paths could allow radioactive gases that may have leaked from
containment following a design-basis accident to be sucked into the ducting
and discharged to the environment without the anticipated adsorption and
filtration assumed in the design-basis analyses for the SGTS.  Bypass leakage
associated with the SGTS affects all calculated dose consequences that involve
the use of the SGTS to mitigate the consequences of an accident.  If the
actual inleakage is not within the amounts assumed in the design-basis
analyses, the facility may not be operating as intended and may be operating
outside of the design basis with calculated accident exposures exceeding
either 10 CFR Part 100 guideline values or GDC 19 values or both.

A subsequent investigation by the licensee of other filter trains at 
Grand Gulf disclosed that the ventilation system for the control room also had
bypass paths.  Air from the area around the fan plenum would be sucked into
the ducting and discharged directly into the control room.  This deficiency
would result in unfiltered, potentially contaminated air being supplied to the
control room.  This supply source of potentially contaminated air was not
incorporated in the design-basis analyses for the facility. 

The licensee reported that the apparent root cause for these deficiencies,
which included missing seals, was a failure to specify a leak-tight
construction for the fan housings.  At Grand Gulf, shaft seals were installed
and other leak paths were reworked to reduce bypass flow and consequent
potential release of radioactive materials.

Many licensees have designed these types of systems to the standards in the
American National Standards Institute and American Society of Mechanical
Engineers (ANSI/ASME) N509 and have committed to testing to the standards in
ANSI/ASME N510.  Testing in accordance with N510 can identify bypass leakage
if the leakage is a significant fraction of system flow.  Testing using tracer
chemicals, such as S-F6, can determine small inleakage rates such as those
identified at Grand Gulf.

The spread of contamination and potential for exposure of individuals can
occur from outleakage as well as inleakage.  There have been instances where
the circulation of contaminated air through ducting located in a clean area
has resulted in unfiltered leakage into the clean area.  In addition,
deficiencies identified in engineered safety feature ventilation systems may
also be present in those systems used to limit normal effluents..

                                                            IN 93-06
                                                            January 22, 1993
                                                            Page 3 of 3

Related Generic Communications

IN No. 86-76, "Problems Noted in Control Room Emergency Ventilation Systems,"
August 28, 1986.

IN No. 90-02, "Potential Degradation of Secondary Containment," 
January 22, 1990.

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.

                                      ORIGINAL SIGNED BY

                                   Brian K. Grimes, Director
                                   Division of Operating Reactor Support     
                                   Office of Nuclear Reactor Regulation

Technical contacts:  J. Hayes, NRR
                     (301) 504-3167

                     J. Carter, NRR
                     (301) 504-1153

Attachment:  List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Tuesday, November 12, 2013