Large diameter boreholes (LDB) for low and intermediate radioactive waste storage/disposal in clay deposits

Document available in extended abstract form only. The State Unitary Enterprise of Moscow MosSIA 'RADON' has been carrying out collecting, treatment, conditioning and storage/disposal of low and intermediate level radioactive wastes (LILW) produced by research, medical and industry enterprises in the Central Region of Russia since 1961. Typical near surface facilities were and still are widely used for long-term storage of conditioned low and intermediate level radioactive wastes (LILW). They are the vault type constructions made of monolithic reinforced concrete or from concrete blocks placed mostly below the ground level in previously excavated trenches in clayey rocks. The depth of trenches is usually from 3 to 6 m and the volume of such repositories varies from 200 up to 20 thousand m3. Operation practice and monitoring results has revealed their common disadvantage typical for 'RADON'-type facilities on the territory of the Russian Federation and some other countries. As a result of continental climate conditions with cyclic seasonal freezing and thawing of host rock and underground constructions, the permeability of grouting cement and engineering barriers is increasing in time more quickly then was supposed when designing and constructing such facilities due to cracks and cement destruction caused by these cycles. This leads to water infiltration and accumulation inside the vault, leaching of radionuclides and their migration out of the repository. In some cases radionuclide migration into the near field and radioactive contamination of the ground around the storage facility was detected. Decontamination of such ground results in generation of secondary wastes that requires additional space in existing repositories for its storage or disposal and corresponding growth of final costs of RAW isolation. Construction of new near surface repositories for the same purpose at the operating sites within the boundaries of lease area is problematic because of the shortage of free area after 30-40 years of operation or because of ecological and radiological safety reasons. To minimize these effects and increase ecological safety of near-surface RAW storage facilities as well as to use operating territory in more effective manner the new type of long term storage/disposal facility has been developed on the basis of Large Diameter Borehole (LDB), drilled in clayey sediments. In the case of LDB applications for LILW storage the waste are removed from the scope of human activity into a stable geological medium. Wastes are placed below the frost zone where damage to engineered barriers due to climatic factors is practically impossible. Construction of the repository by means of drilling prevents large disturbances of the host rock, as happens during excavation work for trenches. When using an LDB-type repositories, the usage factor (ratio of repository surface area, protected with natural barrier, to total repository surface area) of the geological barrier (host rocks) is about 1, as almost all the surface of the repository is protected with not disturbed host rock. This helps to draw the conclusion that, even in the case of total destruction of all engineered barriers, which is very unlikely, radionuclides will still be constrained in a small volume of the host rock and their release to the human activity sphere is virtually impossible. Two LDB-type repositories (LDB-1 and LDB-2) with the internal diameter of 1.5 m and the depth near 40 m have been constructed at the site of SUE MosSIA 'Radon'. In October 2003 LDB-2 for the first time has been loaded with low and intermediate level radioactive waste, conditioned in 36 standard steel drums. In May 2006 LDB-1 has been loaded with 90 similar drums. Later on both Large Diameter Boreholes were licensed for RAW storage. Since 2003 a complex of manual observations within the repository, in engineered barriers and in near field was carried out to assess the tightness of construction, its temperature and radiation regime, humidity of air inside the borehole and ground water level out side. In 2004 the system of geological-and-radio monitoring has been developed specifically for this type of facility for automated continuous control over parameters. Advanced ecological safety of LDB facility in comparison with the typical near surface vault or trench type repositories in Russia is reached, basically, due to impermeability of internal working space of the repository during all operation period. The metal column of LDB facility performs both bearing and waterproofing fictions important in multi-barrier protection system of the LDB long-term storage for radioactive waste. Fragments of the column and standard metal drum have been lowered into the LDB facility on depths of 3, 5, 15, 25 and 38 meters (LDB bottom) in August 15, 2002 and investigated after more then 74 months. On the basis of the analysis results it was found that the minimal time period for which the LDB column preserves its bearing ability can be assessed in 85 years during which the activity of short lived LILW can significantly decrease. The full-scale experiment organized to obtain the qualitative and quantitative characteristics of specific radioactive waste repository metal engineering barriers corrosion in natural conditions is considered in the article