Patent Number: 048427748
Section: summary

This invention generally relates to an improved, above-ground waste disposal site and method of construction thereof. It is particularly adapted for the safe disposal of large quantities of low-level nuclear waste in a site of limited acreage. As new nuclear plants are put on line, and as the existing nuclear power facilities in the United States have matured, the amount of radioactive waste produced in general has expanded. The largest volume of this waste is what is known as "low-level" waste in the industry, which may be loosely defined as waste having a surface radiation intensity of about 11/2 rem per hour or less. Such low-level waste generally comes from waste treatment plants, which separate low-level from high-level wastes, or equipment, cleaning solvents and clothing used in nuclear maintenance operations. As greater volumes of these wastes are produced, the need for safe modes of disposal has increased. Burial systems for burying nuclear waste are known in the prior art. In the earliest of these systems, such wastes were merely packed into 55-gallon steel drums, dropped into a simple, earthen trench by a long-boom crane, and buried. Unfortunately, such "kick and roll" burial systems turned out to be generally unsatisfactory for the land disposal of nuclear waste. The loose soil with which these trenches were filled in was much more permeable to water than the densely packed soil which formed the sides of the trench, or the dense strata which typically formed the bottom of the trench. Consequently, the relatively loose soil which surrounded these drums encouraged the collection of large amounts of standing water around the drums in what is known as the "bathtub effect". This standing water ultimately caused the steel walls of the drums buried within the trenches to corrode and collapse. The collapsing drums and compaction of the soil over time resulted in a downward movement or subsidence of the soil, which caused a depression to form over the top of the trench. This depression in turn collected surface water and hence worsened the tendency of the trench to collect and maintain a pool of standing water over the drums. The resulting increase in standing water resulted in still more soil subsidence and accelerated the corrosion and collapse of the drums buried therein. The end result was the occurrence of radioactive contamination of the ground water flowing therethrough. To solve the soil subsidence and water accumulation problem associated with such "kick and roll" disposal sites, a variety of alternative burial systems have been developed. These alternatives include earthen vaults having structurally rigid walls, and container burial sites in which the spaces between the waste containers are filled in with concrete or some other hardenable grout. While these alternative systems constitute clear advances over the trenches used in the simple, "kick and roll" disposal systems, various shortcomings are associated with both. For example, the rigid walls used in earthen vaults are apt to crack and break in response to a seismic disturbance. Once the integrity of the vault walls is gone, ground water can flow in and accumulate around the waste packages. If any of these packages has metallic walls, the standing water surrounding them causes the walls to corrode and leak radioactive waste into the ground water. Because such vaults typically have only one access opening, the recoverability of a single, leaking package would be extremely difficult, if not impossible. While burial sites in which a hardenable substance is poured over a large group of waste containers to form a solid, integral monolith may be more resistant to cracking or breakage due to seismic disturbances, this particular type of disposal site would tend to apply very high, localized stresses on the waste containers located in the paths of any faults or cracks which develop in the monolith. Moreover, this type of site has an even worse problem with recoverability when a seismic disturbance does succeed in rupturing only one or a few of the containers encapsulated within the ground. A relocation of the site might be the only solution if such a cracking or breaking of the inaccessible containers occurred. To eliminate or at least minimize the tendency of individual containers to crack and break in response to stresses caused by seismic disturbances, nuclear waste disposal sites have been developed by the Westinghouse Electric Corporation which are formed by a solidly packed array of waste-containing modules which are arranged to be flexibly conformable with changes in the shape of the site brought about by seismic events or other natural disturbances. However, while this particular land disposal site represents a clear advance in nuclear waste disposal technology, it, too, has its limitations. For example, because it is a below-ground burial system, it is not well adapted for use in geographical areas having medium to high water tables, which includes much of the eastern United States. In such areas, the burial trench would have to be relatively shallow since the floor of the trench must be constructed well above the level of ground water in the earth. The relatively shallow depth of such a burial site would substantially limit the amount of nuclear waste that a given acreage of the site could hold. As every state government would like its particular toxic waste sites to be as few and as of small an area as possible, the limited amount of wastes such a shallow trench-type burial site could hold poses a significant problem for the use of this kind of site in many areas of the United States. Additionally, any type of underground waste disposal site is always prone to collect significant amounts of standing water as a result of the "bathtub effect" which in turn sets the stage for the possible radioactive contamination of ground water. Sites for disposing nuclear wastes above ground are also known in the prior art. In such sites, the waste is typically placed in containers, such as 55-gallon drums, and stacked in a compact array above ground in the site. A tumulus, or hill-shaped structure having walls formed by a combination of either poured concrete, or compacted clay, gravel, etc. is then formed over the stacked array of waste canisters. However, while such above-ground waste disposal sites are not prone to the "bathtub effect", they are limited in the amount of wastes which they may contain by the requirement of relatively long tumulus walls that should not have a slope any sharper than about one to four. If the slope of the tumulus walls is made any sharper, the tumulus would become unacceptably vulnerable to the effects of either water erosion or seismic disturbances, which could eventually cause the tumulus walls to lose their integrity. Since the nuclear waste within such a tumulus may not decay radioactively down to a safe level for four to five hundred years, the tumulus walls must be built to last with a high degree of certainty for at least half of a millennium. Clearly, what is needed is a toxic waste site which is particularly adapted for the disposal of large volumes of low-level nuclear waste in as small an acreage as possible. Ideally, such a site should not be prone to the "bathtub effect", and should be usable in geographic areas where the water table is not unusually low. Finally, it would be desirable if such a site were easy and economical to construct, and resulted in little or no exposure of potentially harmful radiation to during its construction. SUMMARY OF THE INVENTION Generally speaking, the invention is an improved waste disposal site for the above-ground disposal of toxic waste, which may be radioactive, that is formed from a plurality of individual, waste-containing tumuli arranged in a pyramidal configuration. Each tumuli includes a central raised portion bordered by a sloping side portion. The site is formed by at least two ground-level tumuli having mutually adjoining side portions, and an above-ground tumulus disposed over the adjoining side portions. Each tumulus includes both a floor and a roof formed in part by a water-shedding deformable layer of material, and the deformable layer of material in the roofs of the ground-level tumuli form at least in part the floor of the above-ground tumulus. Additionally, the roof of each tumulus may include an intrusion barrier formed from a plurality of flexibly interlocking structures. In the preferred embodiment, each of the interlocking structures includes a plurality of legs that mutually interfit with the legs of adjoining structures. When the improved waste disposal site of the invention is used to dispose of radioactive waste, each tumulus may contain at least one shield wall means for protecting workers from potentially harmful radiation during the construction of the tumulus. The workers may stand behind the shield wall means while stacking an array of non-handleable radioactive waste packages in the center portion of the tumulus. Nuclear waste packages which are of sufficiently low radioactive intensity so as to be handleable may be stacked on the other side of the shield wall. In the preferred embodiment, the shield wall means is, itself, formed from nuclear waste packages whose surface radiation intensity is low enough to be safely handleable by the construction workers. The wall means is formed by simply stacking such low-level waste packages together in a solid array. The resulting wall means is advantageously flexibly conformable with any shifts in the terrain caused by seismic disturbances. The layer of water-shedding material that forms the floor of each tumulus is preferably circumscribed by a trench which holds a drainage gallery. A deformable layer of drainable material, such as gravel, is placed over the water-shedding layer of material. A plurality of draining troughs are also provided to guide any water which flows through the water-impermeable material into the drainage gallery. Zeolitic materials may be added to the drainable layer to retard the passage of radioactive elements such as cesium. Radiation detectors may be placed within the drainage gallery in order to monitor the level of radioactive material and any water which collects therein. A valve may further be provided in the drainage gallery for diverting any water which collects therein into a treatment plant in the event that the concentration of radioactive contaminants in the water surpasses a selected level. In the method of constructing the improved site of the invention, the floors of two ground-level tumuli are constructed close enough together so that the deformable, water-shedding layer of material in the roofs of each will adjoin when the two tumuli are completed. In the preferred embodiment, these water-shedding layers are formed from compacted clay. Next, at least one shield wall means is constructed on one side of the water-shedding material. In the preferred embodiment, two such shield wall means are constructed, and positioned so as to border the raised portion from the side-sloping portion of the tumuli. The tumulus is next loaded with packages of nuclear waste. The disposal operators first deposit the more-radioactive, non-handleable packages of radioactive waste in the center portion of the tumulus by means of a long-handled crane while they stay behind the shield wall means. After the center portion of the tumulus is completely filled, lower-level packages of handleable radioactive waste is stacked on the other side of the shield wall means in a sloping configuration to define the sloping sidewalls of the tumulus. Next, a ceiling layer of water-shedding material, such as compacted clay, is placed over the center and side portions of the tumulus. After this has been completed, an intrusion barrier formed from a plurality of flexibly interlocking structure, such as dolos, is placed over the ceiling layer of water-shedding material. To complete the tumulus, another layer of water-shedding material is placed over the intrusion barrier. The second tumulus is constructed the same as the first. Finally, the third tumulus is constructed above ground the same as the first two, with the notable exception that the deformable, water-shedding layer in the roofs of the first and second tumulus are used to form the deformable water-shedding layer of the floor of the third tumulus.