Studies Of Radionuclide Migration Characteristics In Granite Rock

This thesis studies the migration characteristics of radionuclides in granite slices and the natural fractured granite cylinders. The following experiments were designed and carried out: multi-nuclide (57Co34Cs) diffusion and dispersion experiments in granite rock slices and in the natural fractured granite cylinders. Similar experiments were also conducted for individual radionuclides, such as ?4Cs, 3H, 99Tc and 99Tc. The granite samples taken from Yangfang, Changping County, Beijing, were used for the experiments. The migration parameter values were obtained from the experiments mentioned above. The experimental results show that: (l)The retardation coefficients for 34Cs range from 100.5 to 130.6 in fracture domain, while from 4822 to 5135 in granite matrix domain. (2)The retardation coefficients for 99Tc and 99Tc are between 1.68 and 26.3 in fracture domain and between 761.4 and 1640 in granite matrix domain. (3)The retardation coefficients for 57Co range from 419.8 to 422.7 in fracture domain. (4)The dispersion coefficients for 134Cs and 57Co in granite slices range from 1.02x 1012 to 5.72x10? and from 4.82x10? to 8.54xl0, respectively. Meanwhile, the bigger the pressure, the bigger dispersion coefficient. (5)The dispersion coefficients for 99Tc are between 0.827x I 0~ and 4.95x 1 O~ in fracture domain. The diffusion coefficients lie between 2.65x10?and 4.68x1013 in granite matrix domain. Moreover, the smaller the water flow rates, the smaller these values. (6)By comparison of the retardation coefficients of the nuclides studied, ?4Cs is the most retarded radionuclide, while 99Tc and 99Tc the second, 57Co the third, 3H the fourth. (7)The flow rates and the pressure have an important influence on the nuclide migration, and the smaller the flow rates and the pressure, the more difficult the migration of nuclides along the fracture. (8)The matrix-diffusion has an important influence on the nuclide migration in the fracture, too. It is another mechanism of nuclide migration. On the basis of experiments above, both the two-dimensional diffusion model and advection-dispersion model were presented for numerical simulation of nuclides migration in natural single-fractured granite and solved by the finite-element method. Further more, two computer programs for two models were written. The calculated concentration profile in the granite rock and the break-through curves are in agreement with analytical solutions. That shows that the assumptions and simplification taken as before are reasonable, and the discretization formulation too. Meanwhile, the matrix diffusion is studied by using the numerical method mentioned above. The results shows that the matrix-diffusion effect have an obvious retardation to the nuclides migration, especially if the flow rates are smaller, and the porosity of matrix-block is bigger and time considered is very long, the matrix-diffusion effect isn't neglected.