Hideo Kimura

Numerical Model of Radionuclide Migration in Geologic Media

A three-dimensional radionuclide migration model has been developed by using of the direct-simulation method. The phenomena taken into account are radioactive decay, convection and dispersion in the ground water and sorption and desorption in the geologic media. Decay chain is represented by particles character change using decay probability. Smoothing method is proposed to make an even distribution of particles and to obtain the exact radioactive inventory. Numerical calculations of 245Cm decay chain and 234U decay chain in the single layered geologic media have been carried out, and reasonable results have been obtained in comparison with INTRACOIN study. Nuclide migration of 237Np decay chain in the three-layered geologic media were examined and shown graphically.

Validation studies of tracer tests in a fracture zone at the Finnsjön research area

Abstract Tracer experiments were performed in a fracture zone, extending several hundred metres, in crystalline rock in Sweden. This paper describes modellings of tracer experiments (radially converging and dipole test) and their numerical results. We have applied a variable aperture channeling model to both tracer tests and evaluated steady-state channel flows in the fracture zone. Solute transport in the channel flows was simulated by a particle-tracking technique considering matrix diffusion. Calculated breakthrough curves and pressures were compared with experimental ones. The calculated breakthrough curve obtained by an equivalen porous medium model was also compared with data from the dipole experiment. Our models seem to explain the experimental results well, but some important assumptions are necessary for calibration of the breakthrough curves. Further experimental data related to the assumptions and geostatistics would be needed for the full validation of the flow and transport model. Study shows that the mean apertures of fractures calibrated with the tracer tests increase with increasing flow rates.

Moving Boundary Model for Leaching of Nuclear Waste Glass

The leaching experiments of Soxhlet type have been carried out[l, 2] and on the basis of the results we developed the mathematical leaching model which used one dimensional diffusion and could treat the growth of surface layers. The model adopted the following assumptions: 1) The velocity of the bulk glass-surface layers boundary depends on time alone. 2) Some of the diffusing substances are immobilized in the surface layers by an irreversible first-order reaction. 3) A fictitious film exists at the solution-surface layers interface. The fundamental equations were established based on these assumptions and the numerical solutions were obtained by the Crank-Nicholson implicit method. The values of the diffusion coefficient, the reaction rate and the film mass transfer coefficient were obtained by a trial-and-error method. The applicability of the model was confirmed by the fact that the leaching mechanisms deduced from the calculated results were consistent with those mechanisms deduced from the experimental results. The present study showed the proposed model was valid for calculation of the leach rates of waste glasses when surface layers grew during leaching, and the study also indicated which parameters should be measured experimentally to predict the leach rates.

Estimates of Parameter and Scenario Uncertainties in Shallow-Land Disposal of Uranium Wastes Using Deterministic and Probabilistic Safety Assessment Models

Safety and uncertainty analyses for the shallow-land disposal of uranium wastes were performed using the deterministic and probabilistic safety assessment models. The analyses for uranium accumulated with 4.5% enrichment show that the doses in residence scenario are of great importance in the safety assessment owing to the influence of daughters built up by uranium decay chain. The dose in residence scenario is sensitive to the release condition of radionuclides from the facilities over long-term period. The parameter uncertainties for the important pathways in residence scenario are estimated from the probabilistic analyses using the statistical methodology. The uncertainty analysis indicates that the influence of parameter uncertainty is the most remarkable in the estimation for the inhalation of radon gas with residence. The parameter importance in each exposure pathway is estimated from using the partial rank correlation coefficients (PRCCs) between variable parameters and the evaluated doses. The important parameters identified by the PRCCs are depth of intrusion, infiltration rate, thickness of covered soil, diffusion coefficient of radon in soil etc. for the inhalation exposure of radon.

The leaching behavior of a glass waste form - Part III: The Mathematical Leaching Model

The one-dimensional diffusion model of leaching was developed on the basis of the Soxhlet-type leaching experiment of waste glass. Emphasis was placed on proposing a model for the growth of surface layers and for an immobilized reaction inside these layers. The equations derived from the modeling were solved numerically and the resulting equations were implemented in a computer code named LEACH. The computed and measured leach rates of sodium, cesium, calcium, and strontium were in good agreement under the Soxhlet-type leaching condition. The computed results revealed that the growth of surface layers, including the immobilized reaction, plays an important role in the leach rates of elements, because the diffusion coefficients of surface layers were much different from those of the bulk glass, and because for calcium and strontium the immobilized reactions affected their leach rates. Therefore, in order to predict the leach rates of waste glasses by using the proposed model, the time dependence of the growth of surface layers should be measured experimentally.

Mathematical Modeling of Groundwater Flow and Radionuclide Transport in Heterogeneous Aquifer

Atomic Energy of Canada Limited carried out a series of tracer tests in the Twin Lake site, to study geologic heterogeneity on the aquifer dispersion properties. Moltyaner et al. [6] analyzed the heterogeneity in statistical terms as variability in the hydraulic conductivity. Their statistical results were used here to develop a geostatistical approach based on the correlation between observed hydraulic conductivity values. The geostatistical model assumes that the correlation strength of the values of hydraulic conductivities between any two locations depends on the distances between these locations, and is expressed as an exponential function. Numerically, a matrix decomposition method is used. The groundwater flow field is calculated by three-dimensional finite element method using the realized spatial distribution of hydraulic conductivity that is calculated by the geostatistical model. The flow simulation results are used as input into the transport computer code, which in turn is used to simulate the tracer breakthrough curves at different observation wells. A random walk method is used for the radionuclide transport simulation. The simulated tracer plumes of tracer tests explain favorably the experimental tracer plumes. The simulations showed that the correlation length of the geostatistical model is a key parameter that characterizes the heterogeneous flow field, and the value of 5m in the flow direction and 0.5m in the direction perpendicular to the flow is obtained through the geostatistical analysis. The heterogeneous flow field in this aquifer is adequately characterized by the statistical spatial distribution of hydraulic conductivity based on the geostatistical model.