Information Notice No. 90-50: Minimization of Methane Gas in Plant Systems and Radwaste Shipping Containers
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
WASHINGTON, D.C. 20555
August 8, 1990
NRC INFORMATIONNO. 90-50: MINIMIZATION OF METHANE GAS IN PLANT SYSTEMS
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).
August 8, 1990
Page 2 of 4
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
August 8, 1990
Page 3 of 4
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.
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
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
- 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.
August 8, 1990
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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
Joe Wang, NRR
1. List of Recently Issued NMSS Information Notices
2. List of Recently Issued NRC Information Notices
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