Information Notice No. 90-02: Potential Degradation of Secondary Containment
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
January 22, 1990
Information Notice No. 90-02: POTENTIAL DEGRADATION OF SECONDARY
CONTAINMENT
Addressees:
All holders of operating licenses or construction permits for boiling water
reactors (BWRs).
Purpose:
This information notice is intended to alert addressees to potential
problems involving degradation of secondary containment as a result of
unforeseen interactions with various normal plant ventilation systems and
inadequate surveillance testing of secondary containment integrity. 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:
Duane Arnold Energy Center
During a reactor building exhaust ventilation inspection, an Iowa Electric
system engineer discovered a large hole in the duct work (see Figure 1).
This hole allowed the main plant ventilation system to communicate directly
with the reactor building ventilation system, thus bypassing the standby gas
treatment system (SGTS) and providing a direct path for an untreated release
of radioactive effluents to the environment. This pathway would have
existed even with an automatic Engineered Safety Feature (ESF) isolation of
the reactor building ventilation system and an autostart of the SGTS. The
licensee discovered that its normal secondary containment integrity
surveillance test (which requires a measurement of 0.25 inch water vacuum
with one train of SGTS operating) was still apparently satisfied with the
main plant ventilation fans running. This has been the test configuration
used at the plant since initial startup in 1974. However, upon conducting
the test with the SGTS operating as designed and the main plant ventilation
secured, the 0.25-inch vacuum could not be achieved because of numerous
secondary containment leaks. A 10 CFR 50.72 4-hour notification was made to
the NRC upon this discovery.
The licensee determined that during the performance of the secondary
containment integrity surveillance as it was previously conducted, the main
plant
9001160420
.
IN 90-02
January 22, 1990
Page 2 of 3
ventilation, turbine building ventilation, and the radwaste building
ventilation exhaust fans had aided the SGTS in drawing the required vacuum.
Because surveillance tests performed since initial unit startup usually
indicated that the required negative pressure had been obtained, numerous
secondary containment integrity deficiencies went undetected and
uncorrected. The maintenance to correct this overall degradation of the
secondary containment required more than 3 weeks of effort to locate and
repair numerous door seals, electrical penetrations, steam tunnel boot
seals, dampers, building seals, and duct systems. These repairs resulted in
an improvement in the measured secondary containment vacuum from less than
0.08 inch of water to greater than 0.25 inch of water, as determined by
retesting with a new surveillance test procedure. This test now requires
all major plant exhaust ventilation fans that potentially communicate with
secondary containment to be secured.
Continued testing after repairs has shown that one main plant exhaust
ventilation fan can still draw an approximate 500 to 1000 standard cubic
feet-per-minute (SCFM) volume from the reactor building ventilation system
(the secondary containment boundary) when the ESF-required isolation is in
effect. As this circumstance could result in an untreated ground-level
release under certain accident scenarios, the licensee developed alarm
response procedures designed to secure the main plant ventilation fans when
secondary containment isolation is initiated and main plant exhaust
ventilation radiation monitors reach specified values. Iowa Electric is
also considering long-term corrective action involving hardware
modifications that would prevent main plant ventilation from drawing air out
of the reactor building under accident conditions.
Monticello
As a result of the secondary containment deficiencies identified at Duane
Arnold, the NRC resident inspector contacted the licensee for the Monticello
plant concerning the procedures and practices for testing the secondary
containment system.
The licensee had normally tested the secondary containment integrity with
the main plant exhaust fans running and isolated both supply and exhaust
fans in the reactor building. However, on October 14, 1989, the licensee
conducted the secondary containment integrity test with the main plant
exhaust fans secured and was not able to maintain the 0.25-inch water vacuum
required by the technical specifications. The vacuum reached in the reactor
building was only 0.1 inch of water. Preliminary investigation revealed
that failure of the test was due to leaks in the reactor building supply and
exhaust system dampers and various other secondary containment leaks. The
licensee also believes that the operation of the main plant exhaust fans
assisted the SGTS in drawing the required vacuum on the secondary
containment, thus establishing a path that bypassed the SGTS. A
10 CFR 50.72 notification was made to the NRC.
Discussion of Safety Significance:
Plant heating, ventilation, and air conditioning (HVAC) systems are diverse
among boiling water reactors. Interactions between these systems and the
standby gas treatment system could mask degradation of the secondary
containment boundary during surveillance testing and could exacerbate such
degradation by diverting flow from the SGTS following an accident.
.
IN 90-02
January 22, 1990
Page 3 of 3
For those plants discussed above, a portion of the secondary containment
boundary is formed by the ductwork and isolation dampers of the reactor
building ventilation system. This system exhausts to a common plenum area
within the reactor building, but outside secondary containment, where
ventilation flows from other plant HVAC systems are also directed. The
high-capacity main plant ventilation exhaust fans take suction from the
plenum area and discharge the flow through the reactor building roof stacks.
Because of the high capacity of the main plant exhaust fans and their
proximity to the reactor building ventilation system components that form
part of the secondary containment boundary, the potential exists for air
flow to bypass the SGTS even when the secondary containment is isolated.
Further, if the main plant ventilation fans are operated during surveillance
testing, degradation of secondary containment could go undetected as the
main plant fans and the SGTS fans could collectively draw the required
vacuum.
In view of the possibility of the main plant ventilation systems masking
deficiencies in SGTS and/or secondary containment integrity and of creating
possible untreated release paths under accident conditions, licensees for
other BWR plants may wish to review the design and operation of major plant
ventilation systems and to review their secondary containment integrity
surveillance procedures.
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 NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: William L. Axelson, Region III
(708) 790-5574
John A. Kudrick, NRR
(301) 492-0871
James R. Hall, NRR
(301) 492-1391
Attachments:
1. Figure 1 - Reactor Building Ventilation
2. List of Recently Issued NRC Information Notices
.ENDEND
Page Last Reviewed/Updated Tuesday, March 09, 2021