Properties of solidified forms of high-level wastes as one of the barriers in the storage system

Abstract

According to the multibarrier concept adopted in this country for the burial of high-level wastes, the main barriers of the burial system which isolate the system from man are the matrix which contains the radionuclides and the geological formation in which the burial site is located. The packing of the wastes, the driving complex of the burial site, and so on are viewed as additional engineering barriers. The properties of each component of the system must meet certain requirements and, on the whole, they must be coordinated with one another so that the radionuclides are reliably isolated from the biosphere until they decay to a safe level.The first step in assessing the reliability of the burial site is to characterize the properties of the solidified highlevel wastes which are included in different matrices. Phosphate, borosilicate, and aluminosilicate glasslike and minerallike materials were considered as matrices. The composition of these materials is given in Table 1.To choose the optimal composition, the properties of the wastes determining the conditions of temporary storage and burial are investigated: the chemical stability of the materials in de-ionized water and water of the corresponding geological formations, the thermal stability, the mechanical strength, the radiation resistance, and the thermophysical characteristics (linear expansion coefficient, thermal conductivity, and heat capacity).The assessment of the properties of the solidified wastes starts with the hydrolytic stability, which determines the main possible escape of radionuclides from the matrix during storage. According to GOST-211-91, the chemical stability of the materials is determined by the rate of leaching of the radionuclides (Table 2).To assess the behavior of the solidified wastes during storage in geological formations, the escape of nuclides from solidified wastes in simulators of formation waters of different formations in the temperature range 20-150~ was studied. It was shown that the true behavior of the solidified wastes during geological storage will depend not only on the temperature but also on the rate of the diffusion of water, the ratio of the flow volume of the soil water to the contact surface area with the solidified material, the composition of the ground waters (salt content and pH), and other factors that must be taken into account for the specific conditions of a geological storage site. For example, at high temperatures (100~ and higher) separate components of the formation water (for example, calcium and iron) can interact with the products of leaching of separate components of the solidified wastes with formation of insoluble compounds at the surface of the solidified waste. The "secondary" layer of insoluble compounds that forms screens the surface of the materials and slows down to some degree the escape of leached nuclides in the contact waters. The thickness and composition of the secondary layer depend on the composition of the solidified material and the contact water (Table 3) and the interaction ...

Modelling aqueous corrosion of nuclear waste phosphate glass

“…If the solution is unsaturated (S<<1) then from equation (32) follows formula (7). In the opposite case (S>>1) of hydrolysis in saturated solutions the corrosion layer grows at a constant rate:…”

“…From [10,32] we can get data given below in Table 2 for caesium leaching from NAP glass in НОТonТsОН watОr at 20° . For other temperatures we can construct a similar table using the data where one can find the activation energy of the corrosion process.…”

“…The latter is controlling the transport of species between the glass and ground water. Modelling is then used to process available experimental data from [32].…”

Glassy Wasteforms for Nuclear Waste Immobilization

“…[17] Table VI gives typical data on the parameters of HLW borosilicate and phosphate glasses. [5,51] Accelerates alteration product formation.…”

Polyfunctional microspherical materials for long-term disposal of liquid radioactive wastes

“…Since it is impossible to assure the preservation of engineering barriers for a long time (hundreds of thousands of years) during the storage of high-level wastes, the first and last barriers take on special significance [2,3]. However, with due regard for possible changes in geological conditions at a repository after a long time has elapsed, the main load associated with the protection against migration of radionuclides into the environment is carried by the first barrier, for which the degree of security is determined by the isolating properties of the immobilizing matrix [4]. Moreover, the geochemical equilibrium between the matrix and incorporating rocks at the disposal site should provide the chemical durability of matrix materials and, hence, the ecologically safe long-term storage [5].…”