Designing a container for safe, long-term storage

A primer on the DOE 3013 Standard

In the decades between the Manhattan Project and the end of the Cold War, the DOE's mission was to support nuclear weapons development and production. Plutonium metal needed to be stored only for short periods before it was recycled into new weapons components. The end of the Cold War and the ratification of the Strategic Arms Reduction Treaties (START) dramatically decreased the needed inventory of nuclear weapons and significantly increased the quantity of special nuclear material being returned to DOE for custody. These excess materials would need longer-term storage.

In collaboration with researchers from Los Alamos National Laboratory, Savannah River Site, and Rocky Flats, the DOE established safety criteria for long-term storage of plutonium materials. In April 1993, the Rocky Flats contractor notified DOE that periodic inspections of stored plutonium metal had not been performed as required. Several concerns were raised: Is this a problem? Is it restricted to Rocky Flats? How should plutonium metal and oxide be stored?

In March 1994 the DOE secretary commissioned a study of plutonium vulnerabilities throughout the complex. The site assessments and evaluations were performed during May and June 1994 with members of the Defense Nuclear Facilities Safety Board serving as observers. The final report, "Plutonium Working Group Report on Environmental, Safety and Health Vulnerabilities Associated with the Department's Plutonium Storage," was issued in November 1994. The plutonium vulnerability assessment identified numerous storage issues and concerns. It also answered the questions raised at Rocky Flats: Inspecting the stored plutonium metal was a problem, and the problem was not restricted to Rocky Flats.

A crimp-sealed "Food-Pack Can" was used as a convenient packaging arrangement for plutonium-bearing solids not conforming to the 3013

Consequently, the DOE issued the 3013 Standard in December 1994 and made significant revisions in 1996, 1999, and most recently in 2000. The standard and the container that evolved from it are meant to ensure the safe packaging and storage of plutonium metals and stabilized plutonium oxides for up to fifty years. The current version of the 3013 Standard and container evolved from the need to accommodate changes and refinements in requirements over the last ten years. The original 3013 container as conceived in 1994 was intended to be used both for storage and for transportation. However, by the time the next version of the standard was issued in 1996, the concept had changed so that the 3013 container was to be used only for storage.

The container design and the stabilization requirements in the standard represent a balance between consistently achieving low moisture content in the processed material and containing the resultant "worst-case" pressure. Lower moisture content would reduce the design pressure requirement but would be more difficult to achieve. Higher moisture content might be more easily achieved but would increase the pressure containment requirement dramatically.

Container design:the "safety envelope"

The 3013 Standard addresses design criteria for the "safety envelope" of the containers; requirements for storage facility design, safeguards and security, and transportation are specified in other policies and regulations.

The assembly consists of a minimum of two individually welded, nested containers to isolate the stored materials from the environment. The outer container provides the pressure boundary to prevent release of the contents. The inner container provides an additional isolation boundary and an internal pressure indicator. Each container is etched or engraved with a unique, permanent identification marking. The storage package has been designed to be maintenance free and compatible with existing or planned qualified shipping containers without the need for additional reprocessing or repackaging.

A sealed container design rather than a container design with a gas filter was selected for two reasons. First, gas filters allow moist air to enter, which could interact with salts and other impurities in the stored materials. Second, if the container was not always oriented properly, stored powder could plug the filters and later "blow out" causing, at a minimum, a local spread of contamination. Full-penetration weld closures provide the highest integrity and longest-life seals possible. Welds eliminate gaskets, which may degrade and leak. Mechanical seals using bolts or screwed connections are susceptible to wear, creep relaxation, seizure, or other mechanical failure. A welded closure is preferred over other closure types because it may provide the best combination of features such as design qualification test performance, ease of assembly under production conditions in a glovebox, container (package) payload capacity, and achievement of a fifty-year life.

The containers are fabricated of ductile, corrosion-resistant materials, such as 300-series stainless steel (typically 304L or 316L). Typical container dimensions are an inside diameter of 126 millimeters (4.961 inches) and an internal height of 255 millimeters (10.030 inches). The outer container is sized to fit into existing certified or currently proposed shipping containers (primarily the 9975 and SAFKEG packages).

Pre-DOE 3013 Standard packaging of plutonium dioxide in an unopened (left) and partially opened (right) container system. The package was originally prepared at Rocky Flats, shipped to Hanford, and ultimately opened at Los Alamos inside a glovebox at TA-55's PF-4. Note the stage of decomposition of the wrapping tape and materials around the exterior of container.

Criteria for possible damage, change in temperature, and pressurization

The 3013 container design criteria take into account the possibility of mechanical damage, abnormal temperature excursions, and pressurization from a number of mechanisms. A maximum storage height of 30 feet has been set, and the outer container must be able to withstand a drop from that height without releasing any material, that is, it must remain leak-tight. The containers can be used in a range of external environmental conditions, including ambient temperatures up to 52 degrees Celsius (125 degrees Fahrenheit), relative humidity up to 100 percent, and atmospheric pressures down to the equivalent of 8,000 feet above mean sea level.

Conditions are specified for intake air to a storage vault. The vault atmosphere may depart from this specification depending on the heat loading in the vault and any facility-specific equipment that may be in use. Containers are loaded in a glovebox with either an inert atmosphere such as nitrogen or possibly argon (as practiced at Los Alamos) or air (as practiced at Hanford).

Plutonium-bearing solids packaged for analytical work at CMR and awaiting DOT-approved packaging for shipment. These do not conform to the 3013 Standard.

Facility analyses indicate that loss of cooling isn't uncommon in the operation of a storage vault. The temperature transient depends greatly on the details of the facility system and the ambient conditions, but early estimates were that a container gas temperature as high as 204 degrees Celsius could be reached, which is less than the normal maximum temperatures that might be reached during transportation. Under normal transportation conditions, the average gas temperature within the 3013 container can be as high as 211 degrees Celsius and plutonium metal temperatures might exceed 200 degrees Celsius. This value corresponds to a 3013 container packaged into a 9975 transportation container that is sitting in the sun in an air temperature of 100 degrees Fahrenheit.

The pressure containment function addresses three primary sources of gases that contribute to overall pressurization. These include pressure increases attributed to ideal gas behavior (e.g., the pressure is directly proportional to absolute temperature for a fixed concentration in a sealed fixed volume), helium in-growth due to buildup from the alpha decay of the radionuclides in the package, and pressurization from radiolytically derived hydrogen (or other evolved gaseous species) from water or other species that were initially adsorbed onto the surface of process plutonium (oxide)-bearing solids.

Pressurization analysis assumes that the volume occupied by the gas includes the annular space between the inner and outer containers, the annular space between the inner and convenience containers, the head space in the convenience container, and the interstitial spaces between the grains of oxide material, including any porosity connected to the container atmosphere. Convenience containers, for instance "Food-Pack Cans," are frequently used to transfer plutonium-bearing material to reduce the potential for contamination. Convenience containers are not required under the 3013 Standard, but if they are used they are placed inside the inner container. No labels are allowed on convenience cans under the 3013 Standard.

Pressurization analysis also considers the effect that material density has on available gas volume. In general, the less pure the oxide material (or the lower the plutonium plus uranium content), the lower the density. As the density is reduced a point is reached at which the convenience container is filled and a minimum gas volume is reached producing the maximum pressure. Further reductions in the contained material density reduce the amount of material that can be loaded into the container, thereby reducing the amount of possible adsorbed moisture available for decomposition and reducing the pressure. (For a more extensive discussion of the pressure design function, see Appendix B of the 2000 revision, DOE-STD-3013-2000.)

A "Food-Pack Can" has been opened to display its contents inside a glovebox.

Pressure buildup in the inner container for high-purity plutonium dioxide (PuO2) under normal storage conditions is expected to yield internal pressures of less than 100 pounds per square inch gauge (psig) from all known pressurization mechanisms except water desorption and vaporization. An internal pressure of 100 psig is indicative of unexpected pressurization, yet far below the minimum design pressure of 699 psig for the outer container. The specified design pressure of 699 psig is sufficient to contain the pressure generated by the maximum oxide loading under "worst-case" conditions of 0.5 weight percent (wt %) moisture, 19 watts heat generation, and 211 degrees Celsius (412 degrees Fahrenheit) gas temperature.

One potential for container failure is over-pressurization from a loading error or from an unrecognized gas-generation mechanism. The nested container concept makes it impossible to directly measure gas pressure in the inner container. To overcome this difficulty, the inner container must have a pressure-indicating feature. The most common feature is a deflectable lid, in which changes can be detected by radiography.

Stabilization objectives, packaged materials,and processing conditions

As mentioned earlier, the overall intent of the 3013 Standard is to stabilize excess plutonium-bearing solids and ensure they are secure for up to fifty years. The stabilization objectives include eliminating reactive metal fines (particles in the powder smaller than a specified size) and oxides, eliminating organic materials, reducing water content to below 0.5 percent by reducing moisture (hydroxyls, hydrates, and adsorbed water), eliminating other gas-producing species, and minimizing moisture uptake after calcination and before sealing in the container.

Radiographs of non-3013-packaged solids showing a breach of an outer container (top) and a partial vacuum created in an outer container (bottom) following the reaction of plutonium-bearing material with atmosphere in the sealed containers.

The standard applies to plutonium-bearing metals and oxides containing at least 30 wt % plutonium plus uranium; there is no lower limit for uranium. It does not apply to materials destined for the Waste Isolation Pilot Plant in southern New Mexico, such as transuranic waste. The standard doesn't limit chloride content, but it recognizes that many materials have significant concentrations of chloride following calcination. Plutonium metals and alloys do not need to be stabilized provided pieces have a mass greater than 50 grams and do not include turnings (or briquettes of plutonium turnings) or wire. Metals must be free of nonadherent corrosion products, liquids, and organic material.

Oxides must be calcined at 950 degrees Celsius for at least two hours and have their stabilization verified by measuring moisture content with a technically appropriate method (loss on ignition for high-purity PuO2). The defining stabilization criterion is a weight loss of less than 0.5 percent at the time of packaging following calcination. In a 3013 container, the total plutonium or other fissile material mass is less than 4.40 kilogram (kg) with a total mass loading of less than 5.00 kg and a free volume of at least 0.25 liter per kg oxide.

Both the outer and interior containers allow for nondestructive verification, inspection, and surveillance (such as radiography and weighing) of the contents. Storage of plutonium-bearing material must comply with existing materials control and accountability (MC&A), safeguards and security, and audit and surveillance directives, which rely on nondestructive assay as a technique for validation. The MC&A requirements call for routinely assaying stored materials for process, accountability, and inventory controls.

Cutaway of the DOE 3103 nested container system demonstrating the fit of the ARIES outer, inner, and convenience containers. The DOE 3013 nested container system with (from left to right) a British Nuclear Fuels, Ltd., outer container, and an ARIES-designed inner container and convenience can.

Remaining issues

Although the standard explicitly calls for a surveillance program that captures essential elements, including indication of pressure buildup and weight gain (also indicating leakage), the possibility of corrosion of the container and breach of the container is a technical issue that remains under intensive study. The article on Page 18 discusses observations and a predictive methodology for assessing the lifetime probability of specific 3013 containers. Gas-generation mechanisms arising from simple corrected values for water adsorption/desorption and energetic particle (radiation)-assisted surface chemistry not accounted for in Appendix B (the pressure design function) have recently been identified. Their importance is discussed in the next article.

The article was adapted from "Gas Generation from Actinide Oxide Materials," G. Bailey, E. Bluhm, J. Lyman, M.T. Paffett, G. Polansky, G.D. Roberson, M. Sherman, K. Veirs, and L. Worl, LA-13781-MS, Chapter 3.

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