Information Notice No. 90-50: Minimization of Methane Gas in Plant Systems and Radwaste Shipping Containers

                                UNITED STATES
                              WASHINGTON, D.C.    20555

                               August 8, 1990

                              AND RADWASTE SHIPPING CONTAINERS


All holders of operating licenses or construction permits for nuclear power 


This information notice is being provided to inform addressees of the 
detection of methane gas in plant radwaste systems and shipments of resins 
from nuclear power plants and of preventive measures being taken by 
licensees to prevent such recurrences.

It is expected that addressees will review the information described herein 
for applicability to their activities and consider actions, as appropriate, 
to avoid similar problems.  Addressees are encouraged to distribute the 
notice to responsible radiological staff and waste handling personnel.  
However, suggestions contained in this information notice do not constitute 
NRC requirements; therefore, no specific action or written response is 

Description of Circumstances:

Methane gas was discovered in a radwaste package shipped February 6, 1990, 
from the Grand Gulf Nuclear Station to the Barnwell burial site.  The 
radwaste was filter-demineralizer resin that had been put into liners, 
dewatered and dried. The sound of escaping gas (subsequently identified as 
methane) was detected when the shipping cask was opened on February 8, 1990, 
at the Barnwell site. The lid of one of the liners in the cask was distorted 
and ballooned outward. Although the 3/8-inch passive vent had released gas 
into the cask, the outwardly distorted liner lid indicates that the vent 
became plugged, thus pressurizing the liner.  The Mississippi Power and 
Light Company, the licensee, investigated and believes the source of the 
methane gas is bacteria interacting with the cellulose of the 
filter-demineralized resins from the radwaste system.

Because the U.S. Department of Transportation Regulations prohibit transport 
of hazardous materials that could cause a dangerous evolution of heat or 
gas, the South Carolina Department of Health and Environmental Control sent 
the licensee a notification of infraction (Warning).


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The licensee believes that the methane-producing bacteria were introduced 
into the radwaste system during activities such as the recent cleaning of a 
turbine building heat exchanger.  The plant service water from the heat 
exchanger, known to contain bacteria, was washed through the floor drains 
into the radwaste system.  This system consists of water storage tanks, 
filter-demineralizers, pumps, and piping.  Gases from the radwaste system 
are normally released to the radwaste building exhaust.  Resins are used in 
the filter-demineralizers to decontaminate liquids in the radwaste system.  
These resins are sent periodically to disposal.  The resins are dewatered, 
placed in liners, and air dried.  Each of the liners containing these spent 
resins has a 1 psig relief valve, which is intended to prevent 
over-pressurization of the liners.  In this instance, the 1 psig relief 
valve allowed methane gas to be vented to the building exhaust.

Liners containing spent resins are placed in outer shipping cask shields 
which are sealed.  The licensee also had two more liners that were filled 
with resin. These liners were stored in the radwaste building ready for 
shipment.  As part of its post-incident evaluation, the licensee fitted 
temporary caps onto the two liners to collect methane gas samples.

The production of methane gas was observed to have ceased after five days 
and no further gas buildup was detected after two weeks.  Nevertheless, 
these liners will be sampled by the licensee for methane (that is, 
pressurization) before they are shipped.  The licensee uses a standard 
detection instrument to sample gas.  This instrument determines the 
percentage of oxygen and flammable gas and displays the percentage of the 
lower explosive limit (LEL).

The recent Grand Gulf event is but one example of the pressurization that 
can result from chemical reactions in low-level waste (LLW) shipping 
containers storing dewatered synthetic organic materials (such as resins).  
Earlier incidents are described in a report of the Brookhaven National 
Laboratory (BNL), issued in May 1986 and in NRC Information Notices 83-14, 
"Dewatered Spent Ion Exchange Resin Susceptibility To Exothermic Chemical 
Reaction," March 21, 1983, and 84-72, "Clarification of Conditions for Waste 
Shipments Subject to Hydrogen Gas Generation," September 10, 1984.  The BNL 
report evaluated three pressurization incidents that occurred in 1983 and 
1984 and that involved LLW from separate nuclear power plants.  The first 
incident took place at Arkansas Nuclear One (ANO) in January 1983 during the 
dewatering of ion-exchange resin wastes in a container.  An exothermic 
reaction apparently occurred, heating the resin wastes and causing them to 
release smoke and/or steam.  The exothermic reaction was stopped or 
suppressed by adding 150 gallons of water to the container.  The other two 
events occurred during the unloading of containers at the Barnwell Disposal 
Site.  The LLW involved in these two incidents consisted of dewatered filter 
media from the Millstone Nuclear Station (MNS) and from the James A. 
Fitzpatrick Nuclear Power Plant (FNPP). One waste container from each of 
these two power plants was found to have become pressurized during transport 
in September 1983 and September 1984, respectively, causing the containers 
(made of high-density polyethylene) to deform.  These early containers were 
not equipped with relief valves or passive vents, which are used in the 
current containers.

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In two of the incidents (FNPP and ANO) biological activity was the cause or 
was a significant contributing factor.  In these two incidents at least some 
of the wastes involved had been stored in holding tanks for several years.  
In the MNS incident, the pressure buildup in the container was attributed to 
contamination with a volatile chemical, Freon-113.  However, the analysis of 
the gas from the liner showed levels of carbon dioxide and methane higher 
than expected for air, which could have been the result of biological 

To prevent future pressurization incidents MNS procedures were changed to 
provide for storage of containers onsite after they are ready for shipment, 
in order to monitor pressure increase.  The containers are sealed with a 
modified lid which incorporates a pressure gauge, and the pressure is 
monitored for 3 to 5 days before the container is shipped.

Explosive Hazard:

The licensee has performed a hazard analysis on the explosiveness of methane 
gas in plant systems.  This analysis was performed to determine the areas of 
vulnerability and to establish proper administrative and engineering 
requirements to control the hazard.  From this hazard analysis, the licensee 
concluded that during a normal operating mode, any methane that can escape 
into the environment of the radwaste facility (that is, outside the 
immediate vicinity of the radwaste tank) will be diluted to a safe 
concentration level. The lower explosive limit (LEL) for methane in air is 5 
percent by volume. However, the methane concentration in the radwaste tank 
could reach explosive levels of 5 to 15 percent before the tank is purged.  
Thus, stringent controls are required to ensure the safety of activities 
performed around the tank. Present engineering controls consist of 
ventilation systems and an associated fire suppression system.  Additional 
administrative controls which have been implemented by the licensee include 
the following:

     o    The licensee issued a special safety bulletin to station 
          personnel. This bulletin directed personnel entering or performing 
          work within the radwaste tank rooms to adhere strictly to certain 
          precautions, including:

          -    Perform no work that creates sparking, arcing, or other 
               source of ignition.

          -    Use only electronic instrumentation that is intrinsically 
               safe for use in hazardous (explosive) atmospheres.  If 
               intrinsically safe, the instrumentation will have a 
               manufacturer's identification to that effect on the 

          -    Use only electric tools or equipment that are spark proof.  
               This includes portable equipment items which are battery 
               operated, such as flashlights.

     o    Planning and Scheduling personnel stamp a precautionary statement 
          onto work orders for radwaste areas where methane concentrations 
          may be present.

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     o    Health Physics personnel monitor radwaste areas where the methane 
          hazard exists.  Monitoring is performed before and during work.  
          Work is allowed only after explosiveness reaches zero percent of 
          the lower explosive limit.

To control the generation of methane at its source, the licensee considers 
the first step to be the cleanup of the microbiological-contaminated 
compounds of the radwaste system, followed by prevention of possible 
microbiological recontamination.  Because the bacteria are considered 
anaerobic, aeration (emptying) of the tanks is the first step under 
consideration.  Should that fail, other actions would be considered.  Future 
flushings of equipment that may contain bacteria will be controlled, as far 
as possible, to prevent non-radiological plant service water that may contain 
bacteria from entering the radwaste system through floor drains.

No specific action or written response is required by this information 
notice. If you have any questions regarding this information notice, please 
contact either or both of the technical contacts listed below or the 
Regional Administrator of the appropriate regional office.

                              Richard L. Bangart, Director 
                              Division of Low-Level Waste Management
                                and Decommissioning, NMSS

Technical Contacts:  Michael Tokar, NMSS
                     (301) 492-0590

                     Joe Wang, NRR
                     (301) 492-1848

1.   List of Recently Issued NMSS Information Notices
2.   List of Recently Issued NRC Information Notices

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