Estimate of radionuclide yield from containers with wastes of different forms

Abstract

An important dement of the engineered-barriers system used for burying radioactive wastes is the container in which the wastes are placed in solid or solidified form. The construction of the containers and the materials from which they are made can differ substantially. When choosing and developing a new construction, container types must be compared with one another according to their capability to retain radionuclides. For underground storage or burial of wastes radionuclides escape primarily by diffusion into the interstitial moisture through the container wall. Many factors influence the escape of radionuclides. The main ones are the character and rate of leaching from the solid matrix or solid wastes, the diffusion and sorption properties of the container wails and the medium surrounding the container, and the construction of the container, specifically, the shape of the container.It is most convenient to compare different container types or container constructions, taking account of the large number of variable factors, on the basis of a mathematical model describing the escape of radionuclides. As a measure or criterion characterizing the retaining properties of a container, it is convenient to choose the quantity of radionuclides escaping from the container surface per unit time into the surrounding medium (the flux from the entire surfaceor the escape rate). This quantity, as indicated in [1], can serve as a direct characteristic of an engineered barrier, such as a container. The amount of radionuclides entering the external medium and the amount remaining inside the container can be determined by summing the amount of radionuclides escaping from the container in a definite time interval.In the present paper a numerical-analytical method of calculating approximately the rate of escape of radionuclides from containers with different shapes -sphere, cylinder, parallelepiped -is proposed and investigated. It is assumed that a container with solid or solidified waste is placed in a medium whose properties are different from those of the container walls. The proposed computational method makes it possible to find the rate of escape of radionuclides when the diffusion properties of the container walls and of the surrounding medium differ substantially. The difference in the effective diffusion coefficients can reach a factor of 105. Ordinarily, the finite-difference methods employed in such cases are unstable near the location where different media abut one another. This instability results in the appearance of oscillating solutions and makes it impossible to calculate with acceptable accuracy the rate of escape of radionuclides.To confirm that the accuracy of the calculation of the rate of escape of radionuclides is adequate, the proposed method is compared with the grid method for cubic and cylindrical containers.The proposed method is used to investigate the influence of the container shape on the escape rate of radionuclides. The retaining properties of concrete containers with the most common shapes and s...