Safety assessment of a waste disposal site with solidified radioactive wastes packed in concrete containers

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...

Estimate of radionuclide yield from containers with wastes of different forms

Федоров¹,

Dorozhkin²,

Fedotova³

et al. 1998

Seepage of radionuclides from a solid-waste storage pit

Dorozhkin¹,

Федоров²,

Sorokin³

1997

The design of surface storage pits for low-level radioactive solid waste has changed substantially as the nuclear industry has developed. The simplest design was a trench with no engineering structures, which was f'dled with wastes, spread with dirt, and then covered by a layer of earth. Today's design is a reinforced-concrete container, which can have an absorptive bottom if required, a cover consisting of absorptive and drainage layers, and water-resistant clay side walls.In spite of the significant changes, pit designs have retained their main features: an upper drainage layer, a more or less homogeneous mass of solid wastes inside (e.g., contaminated soil, debris, etc.), and a location near the surface of earth that is not completely saturated with moisture.In order to analyze the state of today's storage pits and to plan a generation of new ones, it is necessary to evaluate how they affect the environment over the time that the wastes are potentially hazardous.The essence of the safety analysis of surface storage pits is the calculation of the concentration of radionuclides that escape from the pit after water enters. This concentration is used to calculate the contamination of the aquifer under the pit and to determine the individual effective dose to a person using water from the contaminated source.Here we describe a mathematical model and give a numerical algorithm for determining the concentration of radionuclides that reach the surface of an underlying aquifer from a radioactive-waste storage pit when precipitation leaks through it. It is assumed that the pit design has all the above-mentioned main features. Our approach to solving this problem is to determine alternately the percolation rate of water and the migration of radionuclides in the saturated-unsaturated region, which consists of an aeration zone and an underlying aquifer. As opposed to other models for calculating percolation in the aquifer ([1] and [2], for example), this model has time-varying moisture-transport coefficients that depend on the saturation of the medium, and thus can consider how changes in the amount of precipitation and the level of the ground water affect the escape of radionuclides from the pit.Based on this solution method, we can calculate the escape of specific radionuclides that occur in different chemical states. The state of a radionuclides is considered by choosing the appropriate retention coefficient. In general, it depends not only on the porosity of the medium and the absorptive properties of the soil and the nuclide, but also on the chemical form of the nuclide, pH, the ground water, etc. Therefore, in order to obtain the desired results, the calculations must use input data that correspond to the specific pit location and the specific chemical form of the migrating nuclide.The initial radionuclide concentrations in the interstitial water inside the pit are assumed to be known. The best method for estimating the initial concentration for a similar type of heterogeneous sources is still an experiment to determ...

Estimation of the Escape of Radionuclides from Heterogeneous Media with a Complicated Structure Containing Solid Wastes

Dorozhkin

Федоров

2005

A mathematical model for estimating the escape of radionuclides from storage sites for radioactive wastes with an irregular arrangement of sources of contamination, for example, sources of ionizing radiation, piled on one another, is described. The model is tested for a particular case where the problem can be solved analytically. An example of the regularization of a heterogeneous medium with a complicated structure is given. Regularization makes it possible to simplify the initial problem and calculate the flow of radionuclides out of the volume containing the irregularly arranged sources of contamination.When analyzing the safety of storage and burial of radioactive wastes, storage sites not only with a regular arrangement of sources of radioactive contamination (concrete and reinforced concrete containers, steel barrels, and others) but also an irregular arrangement are studied. These include storage sites containing radioactive garbage, soil contaminated with radioactive fallout, sources of ionizing radiation piled in containers, debris from metal structures with induced activity, and others. To simulate the propagation of radionuclides along such irregular heterogeneous media, the internal structure of the storage site must be simulated in each specific case. The simplest method of simulation is regularization of a heterogeneous medium.In the present paper, an example of such regularization is studied. It makes it possible to simplify the initial problem and to calculate the flow of radionuclides out of a volume containing irregularly arranged sources of contamination. The calculations are based on the mathematical model of [1], which makes it possible to find the distribution of the radionuclide concentration in a volume filled in a regular manner with sources of contamination in the form of a square parallelepiped, a straight circular cylinder and a sphere, as well as outside the volume with sources.The criteria for assessing the radiation safety of storage sites could be the maximum concentration of radionuclides inside and outside the storage site (for monofactor contamination), which is compared with a normative quantity -the intervention level when individual radionuclides are ingested for the population (IL water ) [2]. For simultaneous contamination with several radionuclides, the safety criterion could be the effective equivalent irradiation dose to an individual person ingesting contaminated water, calculated according to the specific concentration of radionuclides in the water.Mathematical Formulation of the Problem. A regular, uniform block-type medium with voids (channels) between the blocks is studied. The blocks in such a medium are sources of contamination. The voids between the blocks can be free or filled with a filler material -gravel, sand, bentonite. This medium can be filled with standing water or ground or infiltrating waters with filtration rate U 0 can pass through the medium. In the absence of water in the voids between the blocks the radionuclides can be transfered along the mediu...