Information Notice No. 86-43: Problems with Silver Zeolite Sampling of Airborne Radioiodine
SSINS No: 6835
IN 86-43
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
June 10, 1986
Information Notice No. NO 86-43: PROBLEMS WITH SILVER ZEOLITE SAMPLING OF
AIRBORNE RADIOIODINE
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP)
Purpose:
This information notice is provided to alert licensees of the potential
hazards associated with sampling for radioiodines with silver zeolite (AgZ)
in the presence of hydrogen (H2) and oxygen (02) It is expected that
recipients will review the information for applicability to their radiation
monitoring and survey program and consider actions, if appropriate, to
preclude similar problems at their facility However, suggestions contained
in this notice do not constitute NRC requirements; therefore, no specific
action or written response is required
Description of Circumstances:
During a recent surveillance test, the LaCrosse Boiling Water Reactor (BWR)
experienced a hydrogen ignition while sampling the plant offgas system The
high H2 concentration in the offgas system was caused by a faulty H2-02
recombiner An excerpt from the NRC regional followup inspection report in
Attachment 1 gives a detailed account of the event
The licensee believes that the ignition source during this event was the AgZ
cartridge used in the offgas sampling rig The cartridge contained
dehydrated (less than 1% water content) AgZ The heat of hydration released
when the AgZ absorbed moisture from the sample stream may have added enough
heat to allow catalytic recombination of H2 and 02 Catalysis also releases
heat that would have rapidly increased the temperature of the AgZ to the H2
ignition point
Discussion:
Silver zeolite cartridges are used for sampling radioiodines in radioactive
gas mixtures The AgZ chemically traps the iodines while letting the noble
gasses (which would interfere with subsequent radioanalysis) pass through
The LaCrosse event highlights the following two properties of AgZ that are
apparently not well understood by the industry
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IN 86-43
June 10, 1986
Page 2 of 3
1 Hydration State
The noble gas retention efficiency of AgZ is a function of its water
content (hydration) Dehydrating AgZ activates its surface, freeing-up
sites for noble gas adsorption Data presented in NUREG/CR-3445 (March,
1985), "A Comparison of Iodine, Krypton, and Xenon Retention
Efficiencies for Various Silver Loaded Adsorption Media," show that the
retention efficiency for bound noble gas (gas not residing in the void
fraction of AgZ) of dehydrated AgZ is an order of magnitude higher than
AgZ hydrated to 9% by weight Because AgZ is used to minimize noble gas
retention and interference, the use of dehydrated AgZ is
self-defeating
The heat of hydration can also effect AgZ's iodine retention Heat
released, if dehydrated AgZ is used to sample humid air, increases its
temperature This elevated temperature can reduce the retention
efficiency for iodine
For the above reasons, when sampling for airborne radioiodine in the
presence of noble gases, use of dehydrated or activated AgZ is not
appropriate
2 Catalytic Properties
Silver zeolite will act as a catalyst to recombine H2 and 02 into H20
(water) One manufacturer's study, run with 2% H2 in moderately dry
(10% humidity) air, indicated a threshold temperature at 150F
before the catalytic reaction becomes significant Although the
dependence of this threshold temperature on such parameters as moisture
content and H2 concentration are not known, it is assumed that higher
H2 concentration will lower the threshold temperature This catalytic
property of AgZ makes its use in sampling from explosive or potentially
explosive atmospheres (such as BWR offgas or waste gas storage tanks)
unadvisable
Note that for sampling the offsite environment or other onsite areas where
there is no possibility of having a significant H2 concentration, the
catalytic properties of AgZ are of no concern
In addition to returning the offgas recombiner to an operable state, the
licensee is considering the following actions:
o using only charcoal cartridges for sampling the offgas radionuclides
o using a portable H2 analyses and checking for H2 concentration before
sampling the offgas
o using only partially hydrated (or noncatalyzing cartridges) for
post-accident sampling of stack effluents
o not using AgZ for post-accident containment sampling unless H2
concentration is less than 4 percent
IN 86-43
June 10, 1986
Page 3 of 3
No specific action or written response is required by this information
notice If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or this office
Edward L Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contacts: Roger L Pedersen, IE
(301) 492-9524
Edward F Williams, IE
(301) 492-7611
Attachments:
1 Hydrogen Ignition in an Offgas Sampling Rig
2 List of Recently Issued IE Information Notices
Attachment 1
IN 86-43
June 10, 1986
Page 1 of 2
HYDROGEN IGNITION IN AN OFFGAS SAMPLING RIG
(Excerpted from Referenced Inspection Report, Edited Version)
On March 6, 1986, a senior health physics technician at the LaCrosse Boiling
Water Reactor set up a portable sampling system to sample particulate and
radioiodine activity in the operational offgas line both upstream and
downstream of the on-line offgas system final high-efficiency particulate
air (HEPA) filter and charcoal adsorber bank The sampling was being
conducted to test the collection efficiency of the HEPA filter and charcoal
adsorber using a differential radionuclide concentration method
The sampling equipment consisted of new tygon tubing, a flow meter, an
aluminum filter paper holder containing an acetate filter paper, and a dual
cartridge holder containing two plastic cartridges containing silver zeolite
(AgZ) The AgZ had been dehydrated by the manufacturer to less than 5
percent moisture Past sampling of the offgas line was performed using two
standard 2-inch charcoal cartridges instead of AgZ cartridges AgZ
cartridges were substituted because they adsorb less noble gaseous activity
than charcoal cartridges and, therefore, reduce sample analysis
interferences
Two minutes after starting the sampling equipment (flow rate of 06 cfm),
the technician noted that water droplets had formed on the inside of the
tygon tubing downstream of the filter holders One minute later, the
technician heard a loud bang and saw a bluish flash inside the sample
tubing The technician then turned off the sampling pump, isolated the
sampling lines, and inspected the sampling equipment The tygon tubing was a
burnt brown color; the inside of the flow meter was covered with a
brown-colored oil film; the cartridge holder was hot, difficult to open, and
had a burned odor when opened; plastic parts of the zeolite filters and
holder displayed some melting and were fused; the plastic retaining screens
in the zeolite holders were no longer intact; and the HEPA filter had
disintegrated
The licensee believes that an ignition began in the sample holder and that
sufficient H2, must have been present in the sample lines to sustain an
ignition Sufficient H2 could be present if the offgas system catalytic
recombiner, located upstream of the filters being tested, were not operating
optimally There is no H2 monitor downstream of the catalytic recombiner
The licensee's Radiation Protection Engineer discussed the potential of H2
and O2 recombination catalysis by AgZ at ambient temperatures with two
outside suppliers It was determined that dehydrated AgZ will absorb
moisture from the sample stream until it reaches hydration During this
hydration, which creates a slightly exothermic reaction, the AgZ granules
will heat up During this heating, additional H2 and 02 alignment with the
AgZ molecules may occur, and some controlled catalytic recombination also
may occur, which may further increase the AgZ granules temperature At about
150 F, the AgZ will reach its threshold temperature for H2 and 02 catalytic
recombination At this threshold temperature, the AgZ may cause a rapid
recombination (ignition) of H2 and 02 if the H2 concentration is above 4
percent This may rapidly generate temperatures inside the AgZ in excess of
1065F
Attachment 1
IN 86-43
June 10, 1986
Page 2 of 2
The offgas system continued to operate after the ignition There were
fluctuations in effluent noble gas, particulate, and halogens for several
hours after the ignition; however, no alert setpoint was reached The
licensee noted an increase in the temperature of the offgas storage tanks
and the catalytic recombiner (monitored parameters) The system slowly
returned to near normal except that an identifiable increase in halogen and
particulate effluent remained These remaining increases indicate a possible
degradation of the final HEPA filter and charcoal adsorber The licensee
plans to inspect the filter bank for damage and replace the HEPA and
charcoal as necessary during this outage
Reference:
LaCrosse Boiling Water Reactor Inspection Report No 50-409/86003 (DRSS),
March 28, 1986
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