Vasily Velichkin
Deputy Director of the Scientific Research Institute of Ore Deposit Geography, Petrography and Geochemistry under the RF Academy of Science, a corresponding member of the RF AC.

R

ussia has built up significant volumes of radioactive wastes. The most effective and traditional method to dispose of them is to bury the wastes underground. However, there are other alternative radioactive wastes disposal methods widely discussed now, namely: to transmutate long-lived radio nuclides, to launch rockets charged with radioactive wastes into space, to put radioactive wastes encapsulated inside infusible containers downhole for the wastes to find their way into the Earth mantle in the form of melts formed under the influence of heat generated by highly radioactive wastes, to dump containers with radioactive wastes into oceanic troughs or to bury them in the dead ice of Arctic or Antarctic. To date none of the methods has been practically supported.
Yet, it is highly radioactive wastes, which are fraught with radiobiological danger. In Russia the main generators of such wastes are radiochemical plants, with Maiak, a radiochemical company located in the Chilyabinsk area, the Ural region, ranking the first among them.
According to standards and the practice well established in Russia, at the first stage of disposal of the highly radioactive wastes are transferred into other radioactive forms safe enough for further disposal (generally they are exposed to so called "conditioning"). When transferred into safer forms, radioactive wastes are subject to burial in compliance with safety rules. The most effective radioactive wastes disposal methods are those for disposal of short-lived radio nuclides of low and average level of radioactivity which must be securely isolated only during the first hundreds of years. Liquid wastes with such a level of radioactivity are exposed to concentration by boiling, cementing, bituminisation, and, sometimes, to glazing. Solid inflammable nonferrous wastes are compacted or sometimes cemented. Solid ferrous wastes are re-melted. Solid flammable wastes are burnt, with their ashes being cemented and bituminized. The resultant matrix with radioactive wastes bound inside is generally kept in ground burials. As a rule such ground burials are made of concrete, in argillaceous of other rocks, which feature good isolating properties. Thus, Radon, a group of specialized radioactive wastes disposal companies, implements the most effective technologies of safe disposal and isolation of low-to-average radioactivity wastes.
Radon group also practices underground storage of liquid radioactive wastes of low and average radioactivity. The liquid wastes are injected downhole into sandstone formations, which occur deep inside sedimentary rocks. Russia has practiced underground storage of radioactive wastes since the mid 1960s. Thousands of tons of radioactive wastes have been disposed of in such a way. There are three radioactive underground burials currently in use, namely: in Dimitrovgrad, in Tomsk-7 and Krassnoiarsk -26 (such a combination of words and numbers would be used to denote top secret towns in Russia). The accumulated experience shows that with a strict observation of wastes disposal process requirements, geological, hydrodynamic and hydro chemical properties of such underground burials ensure a safe isolation of liquid radioactive wastes during the whole period of their radio toxicity.
There are many more problems with disposal of highly radioactive wastes. The most radiotoxic components of such wastes are 90Sr and 137Cs heat generating isotopes, half-life period of which does not exceed 30 years. Other components are long-lived actinides like U, Pt, Np, Am, Cm, which keep their radio-biological toxicity for many thousands of years. Naturally, it is pretty difficult to forecast the state of burials for such a long period of time. Therefore, radioactive wastes disposal experts have developed a multi-barrier strategy of safety. The strategy implies to immobilize radionuclides to be buried by means of glass–like and crystal mineral matrix to keep the immobilized radionuclides in corrosion proof containers filled with impermeable stuff of high sorption capacity. According to IAEA (International Atomic Energy Agency) standards, highly radioactive liquid wastes shall be transferred into solidified forms which are supposed to act as matrix to securely bind radionuclides inside. Isolation properties of a matrix are determined by its ability to dissolve in water as well as by its ability to resist high temperatures, radiation and mechanical loading.
Generally, such matrixes are synthesized from glass-like aluminophosphate materials at relatively low temperatures to minimize consumption of energy required to transfer radionuclides into the gaseous phase. Analysis of behavior of aluminophosphate glass exposed to underground water reveals that at 90° C taken as a test temperature the dissolution rate of aluminophosphate glass is 2 to 5 times more than that of borosilicate glass. Thus, there is an apparent necessity to replace aluminophosphate glass with more reliable matrix-type materials.

To day Russia carries out comprehensive research works in this field. In the course of experiments CaO-TiO2-REE2O3-ZrO2-MnO chemical system has been outlined as the most promising one for matrix synthesis. Other substances like garnet-britholite compounds based on ceramic crystal, sphene based glass ceramics etc. have also been tested. An optimum formulation of a binding matrix should be chosen depending on the composition of wastes materials in each particular case.
Foreign experts consider design of radioactive wastes containers the main engineering problem of burials` isolation systems. Stainless steel, copper, titan, zirconium based alloys and corrosion resistant pig iron are recommended as basic materials for manufacturing such containers. In Russia, Maiak radioactive wastes disposal company pours alumnophosphate glass using regular steel containers with wall thickness of 3 mm. Three containers with solidified radioactive glass inside are packed in one pen case of stainless steel. After a respective exposure the pan cases are forwarded for underground storage.
To fill space and allowances between the containers and inner walls of the pen case it is recommended to use impermeable materials of high sorption capacity like zeolites and concrete stones. Experiments have revealed that these substances keep their high sorption capacity under high temperature and pressure, which are expected to exist during the service life of such burials.
In principle it is also possible to store highly radioactive wastes underground in geological formations.
This idea is based on the fact that natural deposits of uranium, thorium, sulphide polymetallic ores, oil, gas and other natural resources have existed completely isolated from the ecosphere for many millions of years keeping their primary mineral and chemical compositions and structures. Uranium deposits of Streltsovsk ore field in Transbaikalia, Eastern Siberia, are a very close analogue to an underground storage of highly radioactive wastes. An extremely long half-life period of transuranium elements calls for a secure isolation of their burials for a period comparable to a geological age. Naturally, nobody can practically guarantee that the burials will remain safe for such a long term. The only way to get information on how such a burial will behave in the future is to use computer simulation. For this purpose it would be advisable to simulate a basic scenario of events, which could be predicted based on the initial data. This scenario could serve a framework within which one could develop mathematical models of individual barriers to predict behavior of the burial` s safety system as a whole. Such a simulation could reveal key factors able to affect the safety system that would allow to correct boundary conditions for keeping highly radioactive wastes and estimate radioactive consequences of such a burial. Based on parameters of the basic simulation, an alternative scenario of further events could be developed to include additional factors simulating a perturbation assumed by the alternative scenario. As a rule the most unfavorable scenario is taken as a basic one.
Comprehensive geological and geophysical surveys are carried out on selecting burial sites for underground storage of highly radioactive wastes in the Chelyabinsk area on the territory of Maiak radioactive wastes disposal company and on the territory of the Mine Chemical Plant on the south of the Krasnodar area.
Maiak was set up in 1948. Until 1986 Maiak had produced weapon plutonium. Today the company is involved in disposal of used nuclear fuel. There is a 2 km deep vulcanite rock mass on the territory of Maiak, which is most suitable for isolating highly radioactive wastes. Vulcanite features all the properties required for burying radioactive materials. For detailed surveys two sections have been outlined on the location as potential sites for creating an underground laboratories to be further transferred into radioactive wastes burials. Unfortunately, the sites are too small to accommodate the whole complex of engineering facilities required to construct a mine burial here. Thus, it looks more realistic to use for this purpose drilled wells of large diameter. Based on hydrodynamic conditions taken for simulation, analysis of peculiarities of the geological structure within the buffer area, which is formed mostly of volcanogenic rocks, shows that the optimum interval for the supposed burial is at a depth from 500 down to 1000 m.

It is also planned to make a radioactive wastes burial nearby the Mine Chemical Plant (MCP) on the Krasnodar region. There is a radiochemical plant operating here, which processes exposed nuclear fuel. Another plant of a similar profile is under construction not far away. With the new plant put into operation, the volume of radioactive wastes generated on MCP` s territory will significantly increase. Bearing it in mind, MCP has already started selecting sites suitable for underground burials immediately next to MCP. Geological surveys have revealed two 7 square km. sites for further detailed study. There is another pretty realistic alternative: to arrange on one of the sites a ground storage for exposed nuclear fuel, with the second site being used to keep solidified highly radioactive wastes underground in a deep granite formation.
Besides technical difficulties, arrangement of a long-term isolation of highly radioactive wastes is associated with significant investments. At the same time constant build- up of radioactive wastes calls for urgent decisions. Under the existing circumstances a conceptual approach has been offered to take into account different half-life times of different radionuclides and peculiarity of their behavior under different geological conditions. Just to remind, the main toxicants in the composition of highly radioactive wastes have half-life periods not longer than 30 years while radionuclides of the actinide series, content of which does not exceed several percentages, can exist for many thousands of years. The main volume of radioactive wastes, however, consists of Cs-Sr mid-lived radionuclides.
Though it is required that the resultant matrix should be kept in an underground burial securely isolated for many thousands of years, currently highly radioactive liquid wastes are exposed to glassing without any pretreatment. Proceeding with such a practice can result in building up extraordinary volumes of solidified radioactive wastes.
There is another more effective radioactive wastes disposal technology based on fractionating highly radioactive wastes into individual fractions containing Cs-Sr complexes and actinide series with their further solidification and separate burial following safety procedures.
For pre-fractionating radioactive wastes before burial, one can use results of experimental researches which have given scientific and technological basics for synthesizing long resistant high sorption crystal matrixes, including actinides, from mixtures of Zr, Ti, Al, Ca and Si oxides.
Thus, a concept of more effective and economically justified isolation of highly radioactive liquid wastes implies a number of technological steps to be performed one after another, namely:
- to separate from liquid wastes individual fractions containing Cs-Sr radioisotopes and actinide series respectively;
- to bind Cs-Sr fraction inside a glass type matrix which is more resistant than alumino-phosphate glass used nowadays;
- to solidify actinide fractions into long resistant high sorption crystal matrixes of a pyrochlore-zirconolite-muriacite composition;
- to bury the above matrix materials separately.
Right location of burials in suitable rock formations at a burial depth of 400 to 500 meters from the surface shall ensure a required safety level for burial of 137Cs-90Sr fraction and fractions similar thereto in terms of half-life periods of their nuclides for a burial period from 500 till 1000 years.
To ensure a reliable isolation, the actinide fraction can be buried in stable blocks of paleoplatforms and shields which have existed in the tectonic quiescent state for several hundred millions of years. It is recommended that such burials should be arranged at a depth of at least one kilometer from the surface to make it impossible for toxic substances to find their way into the subsurface zone and to prevent radioactive isotopes from entering into ecosphere. As actinide series make up a small percentage of radioactive wastes the problem of their disposal can be solve by constructing one or two all-Russia burial sites.
When implemented the technology of independent burial of fractionated highly radioactive wastes will give an effective solution to the problem of dispose of nuclear fuel at the final stage of its cycle.
REFERENCE
The Scientific Research Institute of Ore Deposit Geography, Petrography and Geochemistry under the RF Academy of Science (IGEM RAN) was set up in 1955 to incorporate three independent scientific institutes, namely: Geochemistry, Petrography and Mineralogy scientific research institutes.
IGEM RAN became a house for scientific schools established in these fields by Vladimir Vernadskai, Alexander Fersman and some other famous Russian academics. The institute carries out fundamental scientific research works in the fields of ore geology, mineralogy, petrology, crystallochemistry, radiogeology, radiogeoecology etc. Famous Russian scientist Nikolai Laverov, vice president of the RF Academy of, heads the institute. IGEM RAN cooperates with many foreign institutes in USA, England, Canada, France, Austria and Japan.