<sup> 99 </ Sup> Tc On Simulated Geological Conditions Of Adsorption, Diffusion, Dispersion And Aqueous Chemistry Behavior Research

It is a concerned problem how to dispose of high level radioactive wastes produced by the development of nuclear energy on a large scale. The deep geological disposal of the high level radioactive wastes is expected to be a safer disposal method in most countries. Technetium-99, a fission product, is one primary element of the high level radioactive wastes and has a half life of 210,000years. 99Tc is released into groundwater surrounding a high level radioactive waste repository to cause environmental pollutions, as Tc(V, which is not expected to be adsorbed significantly on the ordinary rocks and materials, because of its negative charge.It is important to study migration behaviors of 99Tc in preselected backfill materials of disposal repository and granite samples from Beishan for the determination and the safety assessment of disposal repository sites of high level radioactive wastes. The main contents of research in this paper are as follows:(1) The adsorption behaviors of Tc in Fe, Fe2O3, Fe3O4, Na-bentonite and Ca-bentonite were studied with the batch methods under aerobic and anoxic conditions. The effects of pH values and CO32", Fe2+, Fe3+ concentrations in the solution and the mass percents of additives on adsorption distribution coefficients were studied. The valences of Tc in the solution after the adsorption equilibriums were obtained by solvent extraction and pH-Eh figures of Tc. The adsorption isotherms of 99Tc in Fe, Fe2O3, Fe3O4, Na-bentonite and Ca-bentonite are all in fairly agree with the Freundlich's equation. The adsorption distribution coefficients for Tc in actual groundwater are less than that in simulated groundwater for the same kind of adsorption materials. Under anoxic conditions the distribution coefficients of Tc in adsorption materials are little without reducing agents, but the distribution coefficients increase very drastically with reducing agents added.(2) The diffusion of 99Tc in granite was studied under aerobic and anoxic conditions. The adsorption distribution coefficient Kd value for weakly adsorbed radionuclide (Kd<1 mL/g) was obtained with the diffusion experiments for the first time. The experimental results show that the diffusion curves yielded by C, / C0 against time are accordant withtheoretical breakthrough curves. The apparent diffusion coefficient of 99Tc in granite samples under anoxic conditions is less than that under aerobic conditions. The existence of CO32" accelerates the migration of 99Tc. The apparent diffusion coefficient of 99Tc in granites decreases with the increase of reductive components of granite, whereas theQOadsorption distribution coefficient of Tc in granites increases with the increase of reductive components of granite. The diffusion experimental results have some actual referential values for the determination of preselected sites and the safety assessment of disposal repository.(3) Experimental crushed granite column breakthrough curves, using 99Tc as spike tracer and 3H as invariant tracer, were analyzed by different linear regression techniques. Dispersity of crushed granite and retardation factor of 99Tc(V on the crushed granite were determined simultaneously by one linear regression technique. Dispersity of crushed granite was also obtained with 3H as invariant tracer by the other linear regression technique. The dispersities found by spike source and invariant source methods were compared. Experimental results show that the dispersity found by spike source method is close to that found by invariant source method. It indicates that dispersity is only the characteristic of dispersion medium.(4) The prediction of migration degree in geological medium indicates that in the process of the long-term geological disposal the migration rate of 99Tc is very small if the diffusion in fissureless granite is only considered. The migration rate of "Tc is large if the diffusion, velocity of groundwater and hydraulic dispersion of 99Tc are considered. Relative to velocity of groundwater, hydraulic dispersion coefficient and retardation fac...