Manish Chopra

Effect of uranyl ion concentration on structure and dynamics of aqueous uranyl solution: a molecular dynamics simulation study.

The effect of uranyl ion concentration on structure and dynamics of aqueous solutions of uranyl ions is investigated by molecular dynamics simulations. In order to get an idea about the effect of concentration of uranyl ions on local structural arrangements of water molecules around the uranyl ion, radial distribution functions of water molecules around the uranyl ion are analyzed for aqueous uranyl solutions of various concentrations. The concentration effect on translational dynamics has also been analyzed by calculating diffusion coefficients of uranyl ion, water, and nitrate ions in solution from their respective mean squared displacements. Mobility of water as well as uranyl ions has been found to decrease with increasing concentration of the uranyl ions. Orientational dynamics of water about different molecular axes of water have also been analyzed and decreasing orientational mobility of water with increasing uranyl concentration has been found. In order to get further insight into origin of slowing down of the translational mobility of water molecules with increasing uranyl ion concentration, two separate effects namely long-range effect of uranyl ions on the dynamics of water molecules beyond the solvation shell and short-range effect involving dynamics of solvation shell water have been analyzed. It is found that long-range effect is responsible for the slowing down of translational dynamics of water molecules in the presence of uranyl ions.

Migration of radionuclides from a high-level radioactive waste repository in deep geological formations

The paper presents the development of an analytical model to estimate the migration of radionuclides through a single fracture with right circular cylinder geometry from a high-level radioactive waste repository in deep geological structures. The processes considered are advection, dispersion, surface sorption and radioactive decay for transport in the fracture; diffusive loss to the host rock; radial diffusion, adsorption and radioactive decay for transport in the host rock. The model is applied to a typical case of high-level radioactive waste repository in deep geological rock formations. The concentrations showed an increasing trend with increasing dispersion in fracture or reducing dispersion in the host rock or reducing porosity. It is shown that the models which do not consider the dispersion of radionuclides within the fracture underestimate the radionuclide concentrations in the fracture water. Also, the effect of dispersion reduces with increase in fracture water velocity. The concentration of radionuclides in the fracture water in general increases with increase in fracture radius until a critical radius is reached. The model development is targeted for the performance assessment of high-level radioactive waste repositories.

Structure and dynamics of aqueous solution of uranyl ions

The present work describes a molecular dynamics simulation study of structure and dynamics of aqueous solution of uranyl ions in water. Structural properties of the system in terms of radial distribution functions and dynamical characteristics as obtained through velocity autocorrelation function and mean square displacements have been analyzed. The results for radial distribution functions show the oxygen of water to form the first solvation shell at 2.4 A around the uranium atom, whereas the hydrogen atoms of water are distributed around the uranium atom with the major peak at around 3.0 A. Analyses of transport behaviors of ions and water through MSD indicates that the diffusion of the uranyl ion is much less as compared to that of the water molecules. It is also observed that the dynamical behavior of water molecules gets modified due to the presence of uranyl ion. The effect of increase in concentration of uranyl ions on the structure and dynamics of water molecules is also studied.

Numerical Solution of Two-Dimensional Solute Transport System Using Operational Matrices

In this study, the numerical solution of the two-dimensional solute transport system in a homogeneous porous medium of finite length is obtained. The considered transport system has the terms accounting for advection, dispersion and first-order decay with first-type source boundary conditions. Initially, the aquifer is considered solute free, and a constant input concentration is considered at inlet boundary. The solution is describing the solute concentration in rectangular inflow region of the homogeneous porous media. The numerical solution is derived using a powerful method, viz. spectral collocation method. The numerical computation and graphical presentations exhibit that the method is effective and reliable during the solution of the physical model with complicated boundary conditions even in the presence of reaction term.

Source term evaluation model for high-level radioactive waste repository with decay chain build-up.

ABSTRACT A source term model based on two-component leach flux concept is developed for a high-level radioactive waste repository. The long-lived radionuclides associated with high-level waste may give rise to the build-up of activity because of radioactive decay chains. The ingrowths of progeny are incorporated in the model using Bateman decay chain build-up equations. The model is applied to different radionuclides present in the high-level radioactive waste, which form a part of decay chains (4n to 4n + 3 series), and the activity of the parent and daughter radionuclides leaching out of the waste matrix is estimated. Two cases are considered: one when only parent is present initially in the waste and another where daughters are also initially present in the waste matrix. The incorporation of in situ production of daughter radionuclides in the source is important to carry out realistic estimates. It is shown that the inclusion of decay chain build-up is essential to avoid underestimation of the radiological impact assessment of the repository. The model can be a useful tool for evaluating the source term of the radionuclide transport models used for the radiological impact assessment of high-level radioactive waste repositories.

Source Term Evaluation Model for Uranium Tailings Ponds

AbstractA multicompartmental source term model, which is designed to assess the radionuclide leaching and transport in a saturated or unsaturated porous medium such as a uranium tailings pond (or near-surface radioactive waste disposal facility) is developed. The subsurface environment is assumed to be composed of a series of compartments. In the case of a uranium tailings pond, these compartments represent the natural evolution of several layers of uranium mill wastes due to their disposal into a tailings pond over a few decades. The bottommost layer forms at the beginning of the disposal, and the topmost layer develops at the flag end of the disposal. The model incorporates the ingrowths of progeny from the parent radionuclide. Upon entry of radionuclides into a layer, they mix, sorb, decay, and are eventually removed by the downward movement of water, ultimately reaching the aquifer below the bottommost layer. Each compartment may have its own unique properties. The primary outputs of the model are the...

Radiological Impact Assessment of the Uranium Tailings Pond at Turamdih in India

Groundwater flow and contaminant transport modeling has been carried out for the uranium tailings pond at Turamdih in Singhbhum district of Jharkhand, India using the finite element based FEFLOW software with a view to assess its radiological impact on human and the environment. Hydrogeological investigations; including laboratory and field based studies; have been carried out to collect site-specific data on geological settings, cross sections and aquifer characteristics of the site. Results indicate that the concentrations of U-238 and its progenies and the corresponding annual effective dose rates to members of the public through groundwater drinking pathway are less than the corresponding standards even at a distance of 500 m from the boundary of the tailings pond. The radiological impact in groundwater at this distance is practically nil up to a period of 4000 years and trivial beyond this period.

Radiological impact assessment of opencast uranium mining for large reservoirs-uncertainty analysis using Wilk's method

Opencast mining may lead to natural erosion of ore material due to overland flow of water accumulated from rainfall. The overland flow may ultimately reach the nearby surface water body. This process may lead to the release of U-238 and its daughter products into the surface water body. The present study is carried out to assess the radiological impact of the erosion in terms of dose due to U-238 and its progenies in the surface water body and to quantify the uncertainty associated with the dose due to consumption of the reservoir water. The in-growth of progenies is also taken into account. The properties like settling velocity, distribution coefficients etc. are having inherent uncertainty associated with their values. The uncertainty of various parameters involved is propagated to the model output. Hence, the uncertainty analysis becomes important to build confidence in the results. In this paper, Wilks method is used to calculate a value greater than 95th percentile value for the dose rate through drinking water pathway with 95 percent confidence level (95/95 value). In present study, the 95/95 value for the annual effective dose to the public due to U-238 and its progenies through drinking water pathway is found to be 12.5 times lower as compared to the WHO guidelines for drinking water.