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Cleaning Up Sites Contaminated with Radioactive Materials: International Workshop Proceedings 13 Rehabilitation of Contaminated Groundwater Layers Near the Mayak Enterprise Using Deep Burial Technology* V. G. Skidanov, Ye. N. Kamnev, and A. I. Rybalchenko, Federal State Unitary Enterprise—All-Russian Research, Design, and Surveying Institute of Industrial Technology Contamination of near-surface underground water (groundwater) occurs at many nuclear industry enterprises in areas where there are surface radioactive waste repositories and at production complex sites. Groundwater contamination has also been noted at the Mayak enterprise, which could create a real threat to surface bodies of water and to surface and groundwater supply sources. A radical means of cleaning up contaminated groundwater is to set up a system whereby water is pumped out and then cleaned using sorption, membrane, and similar technologies. This produces clean or conditionally clean water, which is subsequently discharged or reused, as well as regenerators or concentrated solutions containing radioactive substances and salts, which are processed into solid form. The use of such a technology requires significant material expenditures, as the volumes of water that must be pumped to produce a significant effect will be in the several hundreds or even thousands of cubic meters per hour. At the nuclear industry enterprise Chepetsk Mechanical Plant (city of Glazov, Republic of Udmurtia), research was conducted on areas of salt-contaminated groundwater from an alluvial layer near a tailings repository. This research laid the scientific foundations for a geoinfiltration and geomigration model of the up- * Translated from the Russian by Kelly Robbins.
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Cleaning Up Sites Contaminated with Radioactive Materials: International Workshop Proceedings permost underground layers, the study of which made it possible to propose a system for rehabilitating a layer by pumping out contaminated water. Such measures are necessary because the alluvial layer drains into and possibly contaminated the Cheptsa River. The magnitude of seepage losses from the tailings repository was calculated, broken down by elements of the water balance and including three calculation stages: (1) calculation of initial data, (2) calculation of balance inputs, and (3) calculation of balance outputs. The maximum level of seepage losses from the tailings repository totaled approximately 300,000 m3 per year. The overall approach to rehabilitating the water-bearing alluvial layer is to prevent this layer from soaking up contaminated solutions caused by seepage from the tailings repository and subsequently to pump the contaminated water out of the area of its distribution and return it to the tailings repository. Various options for capturing the contaminated water were considered: installation of an antiseepage “wall-in-the-ground” barrier around the perimeter of the tailings repository and the pumping off of contaminated water; installation of an impenetrable barrier around the tailings repository (using the Sergeev method, Moscow State University); and reconstruction of the existing horizontal drainage system. All of these measures are aimed at limiting the influx of water from the tailings repository into the alluvial layer, but they do not address matters of how to clean the contaminated layer drained by the Cheptsa River or how to handle the contaminated water that is pumped out. Based on these models, a capture drainage system was designed, including boreholes down to the alluvial layer located near the tailings repository dam (option 1) and over the area of the alluvial layer (option 2). According to preliminary assessments, this technology is the most effective and the least costly. Under option 1, approximately 900 m3 per day (about 330,000 m3 per year) would be pumped out and transferred to an existing deep repository (storage site) for isolation in a collection layer located at a depth of 1,435-1,600 m. The total volume of industrial wastewater removed from operational production facilities and of water pumped out of the underground layer being rehabilitated will not exceed the designed processing capacity of the storage site, 2,500 m3 per hour. Under option 2, which is the more effective, the volume of water pumped out is 2-2.5 times greater, which exceeds the designed processing capacity of the existing storage site. To reduce the volume of water being transferred into storage, plans call for using an ENERGO-70,-45–type reverse osmosis unit. Using such units makes it possible to obtain purified water meeting household consumption or technical use standards for reuse, as well as concentrated solutions (which make up 35 percent of the purified water), which will be transferred to the tailings repository and then pumped through boreholes into the collector layer. Design work is under way on a technological system for rehabilitating the water-bearing layer according to option 1. A similar system for cleaning near-surface groundwater layers may be considered for the Mayak enterprise. To bury the water pumped out, which is con-
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Cleaning Up Sites Contaminated with Radioactive Materials: International Workshop Proceedings taminated with radioactive substances, deep underground collector layers in the Teche-Brodskaya structure (1,000-1,500 m deep) may be used. This structure is located very close to the enterprise and lies partially within the boundaries of its protected sanitary zone. According to the results of geological prospecting work done in 1960-1965, the Teche-Brodskaya structure was deemed to be unsuitable for the burial of liquid wastes. However, an analysis in recent years of the geological conditions and results of experimental work, taking into account the results of studies of existing liquid radioactive waste storage sites at the Mining-Chemical Complex (Krasnoyarsk Krai), the Siberian Chemical Complex (Tomsk Oblast), and the Scientific Research Institute of Nuclear Reactors (Ulyanovsk Oblast), indicated that the burial of liquid radioactive wastes in the collector layer of the Teche-Brodskaya structure was entirely feasible. To reduce the volume of water that would be transferred into storage, reverse osmosis desalination units (like the ENERGO-70,-45, for example) and sorption units to extract radionuclides could be used. If this is done, the salt solutions from the reverse osmosis units and the regenerates from the sorption units, which contain radionuclides, would be sent to storage. To develop the design for the deep repository (storage site) for radioactive water pumped out during rehabilitation of near-surface underground layers, geophysical studies (seismological prospecting) must be carried out to select locations for the drilling of boreholes and deep (up to 2,000 m) exploratory-operational wells in the Argayash Fault zone, very close to reservoirs 10 and 11 of the Mayak enterprise. Studies of the boreholes must also be conducted. If the results are positive, the collector layer between the depths of 1,000 and 1,500 m could also be used to store the liquid radioactive wastes that are currently being transferred into open surface repositories as the reservoirs of the Techa Cascade are being decommissioned and treated.