Patent Number: 051714833
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a method for permanently storing radioactive waste materials in a salt bed such that long term crushing of the containers is minimized. In practicing the method of this invention, the material being stored is first placed into one or more storage containers, typically a 55 gallon steel drum or a rectilinear metal box. The manner of placement therein will depend upon the size and shape of the waste material and the level and type of radioactivity exhibited by it. The most usual storage method involves shredding the waste material, along with the original contaminated container thereof and mixing at least a portion of the shredded mixture with an amount of a cementitious mixture to form a solid, incompressible mass for subsequent storage. The ratio of waste to cementitious mixture will depend upon several factors including the level of radioactivity, and the compressibility of the waste material. Almost any standard cementitious mixture may be used for this purpose. Preferred however is Portland Cement which may have some gravel aggregate mixed with the cement to gain compressive strength. Similarly, any standard mixing technique may be used for this purpose, however to use of vibrators is preferred to remove any air bubbles from the mixed mass after it is poured into the final storage container. To avoid potential problems with excessive hydrostatic compression forces during subsequent long term storage, it is most important that the amount of material added be sufficient to substantially fill the entire void volume of the container before it is sealed. The amount of water used to mix and cure the concrete should be carefully monitored to avoid creating potential steam emission problems with any excess water remaining in the final mixed mass after curing. Lastly, the overall compressive strength of the drum may be significantly improved by preplacing reinforcing rods into the container, so that after the cement/radioactive mixture is poured thereinto, the cured structure will be essentially a column. There are no particular requirements for the size and shape of the storage containers used, other that they be adapted to withstand large compressive forces. Typically, however, 55 gallon drums are used. However, even larger capacity cylindrical drums, up to about 6 feet in diameter and 6 feet high, and rectilinear boxes up to about 4.times.4.times.6 feet in size may also be used. Containers of such size and weight can easily fit into and be transported by an NRC-certifiable Type B transportation container. Containers having other sizes and shapes may also be used. The only size and shape limitations are that they be able to fit inside the transportation container without difficulty. To avoid potential problems with corrosion, particularly chloride stress corrosion induced by any ground water reaching the salt contacting surfaces, it is preferred that the containers be made of a metal alloy resistant to such corrosive attack, such as type 316 stainless steel. Other suitable materials include copper, plastic coated sheet steel, various nickel alloys, fiber reinforced concrete and filament or tape wrapped fiberglass. After filling, these containers are sealed with a welded or plastic bonded top cover fitted with a particulate filter, usually of activated carbon, to equalize internal gas pressures. Also, the containers are preferably fitted externally with fixtures adapted to facilitate stacking of the containers to form a regularly spaced array in which, as shown in FIG. 1, a uniform distance between each of the containers in the array is maintained during backfilling operations. In addition, various measuring devices and telemetry systems to help keep track of such factors as external compressive forces, gas generation rates, internal gas pressures, corrosion rates and brine inflow into the room may be attached to the containers. Radiation monitors to help identify the occurrence and location of any leakage may also be included within the array of containers. The final step in performing the waste storage method of the present invention is the placement of a granular load distributing medium around each of the stored containers. First, approximately a 12 inch thick layer of such a medium is spread over the floor of the storage chamber onto which the sealed storage containers are placed. After the first layer of the array of storage containers is completed, the granulated medium is provided in sufficient quantity to substantially fill all of the void volume between and around the containers. Where the array comprises more than one layer of stored sealed containers the process is repeated. When the last container is placed in the room, all of the void volume remaining in the storage chamber is then completely filled with the granulated medium. The granules of the medium are sized to provide a uniform coupling of the compressive forces generated by the plastic flow of the salt bed towards the storage array in a manner approaching hydraulic force distribution. To achieve such a degree of utility, the particles must have a size not to exceed about 2,000 microns. Particles of such size are known to form beds which will accommodate themselves to annular spaces and to flow in approximately the same manner as a fluid under static forces. Beds of such medium are customarily identified as being "fluidized" beds and this term shall be used hereinafter to such beds. Fill materials used for this purpose include, but are not limited to, sand, bentonite, gypsum and mixtures thereof. One or more layers of such materials may also be used in forming the final fluidized bed. Bentonite is particularly preferred because it will both absorb any ground water entering the chamber and expand in so doing so as to form a dense barrier to water around each of the containers or around the array of containers. To assure maximum effectiveness of the bed packing, there should be about a three inch spacing between each of the containers and a twelve inch minimum clearance to the walls and ceiling of the chamber. In use, the bed should be compacted using a vibratory compactor to remove any air bubbles which have been trapped during pouring of the granulated particles around the containers and to achieve both a uniform spacing distribution and a maximum packing density. Remotely operated equipment to do this is well known in the art. With a granular load distribution system as described above, it is found that after the room is sealed with more of the granular medium used to surround each of the storage containers, it will only take a very short time for the loaded storage array to reach equilibrium with the lithostatic pressure of the salt formation. The expected appearance of the chamber after about 50 years of storage is shown as FIG. 2. Removal of the stored containers for subsequent inspection is very simple. All that is necessary is to first vent any gases which may have accumulated, then remove the sealing material from the entrance to the chamber, and then remove the packed granular material from around each of the containers. Since the granular materials is still freely flowing, such removal can be done using, for example, a remotely operated vacuum hose. The stored containers may now be removed in reverse of the order in which they were placed in the chamber. Since it expected that, little, if any, radioactive material will have escaped from within the containers, the dry granulated material will itself not be "hot" and can be used over again. It will be understood that various changes in the details, materials, arrangement and interrelationship of the various elements which have been described and illustrated in order to explain the nature of the method of the present invention, may be made by those skilled in the art without departing from the principles of the invention. Accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.