Information Notice No. 90-02: Potential Degradation of Secondary Containment

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                              January 22, 1990



All holders of operating licenses or construction permits for boiling water 
reactors (BWRs).


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 


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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.  


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.

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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 

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 

                              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

1.  Figure 1 - Reactor Building Ventilation
2.  List of Recently Issued NRC Information Notices

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