Patent Number: 048448400
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to the field of hazardous waste containment and disposal. In particular, the invention relates to a method and structure suitable for both short and long term storage and permanent disposal of hazardous wastes, particularly nuclear wastes, where the containment structure resists failure from earth movement and allows rapid location and isolation of any leaks which should occur. Hazardous wastes include a variety of toxic and radioactive substances which would cause harm if released directly into the environment. The various types of hazardous wastes are generally defined by the United States Environmental Protection Agency (EPA) and the Nuclear Regulatory Commission (NRC). With respect to hazardous wastes of a radioactive nature, the NRC presently identifies the following classes of low level wastes with respect to containment and disposal. Class A: Wastes for which there are no stability requirements but which must be disposed of in a segregated manner from other wastes. These wastes, termed class "A", or "segregated wastes, are defined in terms of maximum allowable concentrations of certain isotopes and certain minimum requirements on waste form and packaging that are necessary for safe handling. Class B: Wastes which need to be placed in a stable form and disposed of in a segregated manner from unstable waste forms. These wastes, termed class "B", or "stable" wastes, are also defined in terms of allowable concentrations of isotopes and requirements for a stable waste form as well as minimum handling requirements. Class C: Wastes which need to be placed in a stable form, disposed of in a segregated manner from nonstable waste forms, and disposed of in such a manner that a barrier is provided against potential inadvertent intrusion after institutional controls have lapsed. These wastes are termed class "C", or "intruder protected" wastes, and are also defined in terms of allowable concentrations of isotopes and requirements for disposal by deeper burial or some other barrier. A fourth class of wastes (mixed wastes) which is not included in the NRC's current listing comprises radioactive wastes contaminated by chemical components classified as hazardous wastes by the EPA. In general, hazardous wastes cannot be completely eliminated, but rather must be contained in a manner which prevents their release into the environment for very long periods of time, which in practical terms would be considered permanent. A variety of approaches have been proposed for such long term containment, including burial of primary containers holding the waste, disposal of such containers at sea, incorporation of the waste and/or containers of the waste in a solid matrix, such as cement, and the like. In general, most of the proposed approaches suffer from certain drawbacks. The disposal of wastes in primary containers, such as barrels or drums, either by burial or at sea, depends on the integrity of the container. Encapsulation of the containers in concrete, in contrast, would appear to provide long term stability and containment, but in fact can be subject to failure from a variety of causes, particularly earth movement resulting from earthquakes, earth subsidence and the like. Such earth movements frequently can fracture even the most solid containment matrixes over a long period of time. An additional problem relates to the detection of leaks in a long term containmnent facility. Although a number of leak detection systems have been proposed, generally they do not allow for precise location of the leak. As many of these containment facilities are quite large, knowledge that a leak has occurred without knowing the precise origin of the leak can be problematic. Moreover, even after the leaks are detected, most storage facilities do not provide for any convenient manner for the leak to be cleaned and facility restored. For the above reasons, it would be desirable to provide a method and structure for the secure containment of radioactive and other hazardous wastes for very long time periods. In addition, it would be desirable to provide methods and systems for leak detection in such containment structures which would allow the immediate location and isolation of leaks which might occur in the structure, and further allow for correction of the leak and restoration of the structure. 2. Description of the Background Art The use of nested containers for the storage of nuclear and hazardous wastes has been proposed. See, e.g., U.S. Pat. No. 4,229,316 which describes the use of an inner metal container for storing liquid wastes, where the container is housed in an outer concrete receptacle, and excess space within the receptacle is filled with a radiation-absorptive substance; U.S. Pat. No. 4,588,088 which describes an inner container, typically a metal drum, housed within an exterior container 20, where the interstitial space between the two containers is filled with a sealant material; U.S. Pat. No. 4,453,857, which discloses the sealing of a plurality of containers, typically steel drums, in a solid concrete block; and U.S. Pat. No. 4,513,205, which discloses the use of nested concrete vaults to hold a plurality of inner containers, typically metal barrels. Monitoring of possible leakage of nuclear and hazardous wastes from underground storage systems is known. See, e.g., U.S. Pat. No. 4,513,205, where an inner vault is placed within an observation vault to allow for monitoring of leakage; U.S. Pat. No. 4,464,081, where an underlying pipe and manifold system is placed beneath an underground storage facility to collect seepage and monitor for hazardous wastes; and U.S. Pat. Nos. 4,624,604 and 4,543,031, both of which disclose a leakage detection system located beneath a landfill site. U.S. Pat. No. 4,362,434 discloses a lined basin for the collection of hazardous wastes in bulk. Monitoring of leakage can be done through a sump system. U.S. Pat. No. 4,375,930 also discloses a hazardous waste site having a sump which allows observation of leakage and leachate. U.S. Pat. No. 4,428,700, describes a particular filler which can be used for sealing hazardous waste, either in containers or otherwise, in underground storage locations. SUMMARY OF THE INVENTION The present invention provides a method and structure for the permanent or temporary containment of hazardous and toxic wastes, particularly nuclear wastes. The method allows for the collection of such wastes over an extended period of time, typically years, in conventional containers, such as metal drums, liners and dry active waste boxes. The containers are collected and sealed within larger canisters, typically concrete boxes. The remaining interstitial space within the canisters will typically be sealed with a curable fluid sealant, such as grout, to provide both mechanical stability and radiation shielding. The individual canisters are shaped in a particular geometry which allows them to be stacked in an interlocking manner to form an integrated monolithic structure in which canisters are held firmly in place by gravity, but are able to shift relative to one another in response to earth movements, such as earthquakes, ground shifting, settling, sliding, and the like. The use of such interlocking canisters which are held in place by gravity has particular advantages. First, the relative shifting of canisters will not generally result in fracturing or other damage to the individual canisters. Thus, the integrity of the canister seal is maintained even when the disposal site is subject to earthquakes, subsidence, and other earth movement. Equally important, should a leak occur in one of the canisters, the integrated structure may be disassembled to allow for removal and repair of the defective canister without disturbing the seal of the other canisters. Finally, the assembly of the integrated structure is greatly simplified as there is no need to tie or otherwise connect the individual canisters together. The integrated disposal structure of the present invention is suitable for above ground, partially buried, and underground disposal sites. In all cases, a barrier layer will be prepared over the site to seal the ground surface to inhibit penetration of water which passes through the structure (to prevent ground water contamination should a leak occur in one of the canisters). Usually, a system will be provided for collecting the water which has passed through the structure, which system may be adapted to monitoring for the presence of hazardous wastes in the water from the structure. In the preferred embodiment, the collection system is zoned to provide for segregated collection of water from a plurality of sections beneath the integrated structure. In this way, the area where a leaking canister is located and from which the hazardous waste is originating may be quickly identified. The integrated structure can then be disassembled in the area of the leak, leaving the remainder of the structure intact. After removal of the defective canister, the integrated structure can then be restored. In the case of particularly hazardous wastes, e.g., the Class C wastes and mixed wastes defined above, the present invention allows for verification of the integrity of the integrated structure prior to and after the final closure of the structure with a permanent earthen cover. Prior to the final closure, the structure may be allowed to remain without the earthen cover for a certain period of time so that the leak tightness of the structure is verified. Before the final earthen cover is placed, it is desirable to expose the structure for an interim period to conditions that are the same or more severe than the conditions the structure will be exposed to while buried under the earthen cover during the long-term isolation period. In this interim period, the structure will be exposed to rainwater, snow, freeze and thaw cycles, and other harsh environmental conditions which will verify the performance expected from the structure. These conditions will test the integrity, leak tightness, stability, strength and other related properties. During this interim period, the existence of a failure in the integrity of the structure, which usually means the existence of a faulty canister, can be easily detected by periodically collecting water from the integrated structure and sampling for contamination. In the event that the existence of a failed canister is indicated by these sampling operations, such canister can be easily remedied by removal from the structure followed by repair and replacement in its original position in the integrated structure. If no failure in the integrity of the structure is detected after a predetermined number of years, the integrated structure can be closed in a permanent manner, typically by placement of an earthen cover over the integrated structure. The present invention is also capable of monitoring the integrity of the structure for a number of years after the final closure with the earthen cover. The method and structure of the present invention has a number of additional advantages. A wide variety of waste streams may be accepted for short-term, long-term or permanent isolation in surface, partially buried, and underground storage. The structure may be adapted to a wide variety of natural features at the disposal site, and employs common construction materials, such as reinforced concrete, which are readily available and known for their long-term reliability and performance. The initial placement of waste containers into the protective canisters is usually handled remotely in an enclosed area for the protection of the workers. All remaining operations, however, can be handled by workers without additional protection as the canisters provide sufficient shielding for worker safety. The monitoring and recoverability of the individual canisters may be performed over extended periods, exceeding one hundred years, and a substantially permanent sealing minimizes contact between the waste and water at all times.