Study On The Adsorption And Diffusion Of Nuclide ⁷⁹Se（Ⅳ） And ⁷⁹Se（Ⅵ） In Tamusu Clay Rock

The rapid development of nuclear energy has brought about a large number of radioactive wastes that need urgent treatment.How to safely isolate high-level waste from the environment for a long time is a key issue related to the sustainable development of China’s nuclear industry.The Tamusu clay rock is located in Bayingobi basin,Inner Mongolia.It was selected as the host rock of the pre-selected site of the geological repository for high-level radioactive waste.It is widely distributed and has the advantages of low permeability,strong plasticity,chemical sealing,strong ion exchange,and adsorption capacity,pyrite and organic carbon in clay minerals have weak reducibility.Tamusu clay rock has become one of the important host rock types of pre-selected sites of high-level radioactive waste geological disposal repository in China.The migration rate of radionuclides is a key indicator for the safety assessment of nuclear waste repository.It is difficult for cationic radionuclides with strong adsorption to migrate from the repository.Soluble anionic nuclides with high migration rate have the characteristics of weak adsorption and fast migration rate,which are sensitive to redox environment.⁷⁹Se is mainly produced by the fission of ²³⁵U in the nuclear reactor.It has a long half-life and a high fission yield.It mainly exists in the form of HSe O₃^-/Se O₃^2-and HSe O₄^-/Se O₄^2-in the environmental aqueous solution.It is easily soluble in water,has strong mobility,high bioavailability,and has certain toxicity.It is one of the long-lived radionuclides concerned in the nuclear fuel reprocessing process.Anionic nuclides usually have poor adsorptivity.Their migration behavior in clay medium is mainly diffusion.Their migration behavior is closely related to such factors as wall rock barrier size,clay mineral composition,groundwater characteristics,including p H,ionic strength and humic acid content.The diffusion coefficient and distribution coefficient of nuclides in clay rock are key parameters for the safety assessment of pre-selected host rock.In this thesis,Se（Ⅳ）and Se（Ⅵ）are selected to replace ⁷⁹Se as the representative of weakly adsorptive nuclides.The adsorption characteristics and migration behavior of Se（Ⅳ）and Se（Ⅵ）in Tamusu clay rock under different conditions are studied by using the static adsorption method and through-diffusion method.The regional migration parameters are obtained.The diffusion rule and mechanism of Se（Ⅳ）and Se（Ⅵ）in clay rock are preliminarily discussed.The main research contents of the thesis are detailed below.（1）At first,batch adsorption tests of Se（Ⅳ）and Se（Ⅵ）in Tamusu clay rock were carried out.The single and simultaneous adsorption properties of powder samples under different conditions were studied.It was found that the adsorption of Se（Ⅳ）and Se（Ⅵ）in clay rock reaches equilibrium in about 48 hours,which conformed to the pseudo-second-order kinetic model.The diffusion model was mainly liquid-membrane diffusion.With the increase of the initial concentration（C₀）of Se（Ⅳ）and Se（Ⅵ）,the q_t value increased and the K_dvalue decreased,which conformed to the Freundlich model.The adsorption of Se（Ⅳ）and Se（Ⅵ）in clay rock was heterogeneous and multi-layer.When p H=4,q_t,and K_d were the maximum,and then decreased with the increase of p H value.The adsorption of Se（Ⅳ）was an endothermic process,and Se（Ⅵ）was an exothermic process.With the increase of ionic strength of solution,q_t and K_d of Se（Ⅳ）and Se（Ⅵ）were decreased.When there were iron and copper ions of different valence states in the solution,the three largest adsorption amounts of Se（Ⅵ）and Se（Ⅳ）were affected by univalent copper,zero-valent copper,and zero-valent iron.It was about three times more than the adsorption amount of Se（Ⅵ）and Se（Ⅳ）in pure clay.High valence selenium might be partially reduced to low valence selenium,which greatly improved its adsorption coefficient.Considering the safety of the disposal repository,this is a favorable factor.（2）The self-made stainless steel diffusion cell suitable for clay core diffusion was used to study the diffusion behavior of Se（Ⅳ）in the original core of Tamusu clay rock by the through-diffusion method.The diffusion parameters of Se（Ⅳ）under different conditions（different p H,ionic strength,and when humic acid is added）were obtained.The diffusion rule and mechanism were preliminarily analyzed.The D_e value of Se（Ⅳ）diffusion in clay rock increased with the increase of p H,and K_d decreased with the increase of p H value.With the increase of ionic strength,the D_e increased and K_d decreased.When humic acid was added to the system,the D_e value of Se（Ⅳ）in clay rock increased significantly both in contact experiment and equilibrium experiment,indicating that the complex of"humic acid+Se（Ⅳ）"was formed.The humic acid changed the surface characteristics of clay,broadened the ion diffusion channel,and increased the diffusion rate of Se（Ⅳ）in clay rock.（3）The polypropylene diffusion cell suitable for clay powder sample diffusion was designed,and the diffusion behavior of HTO（tritiated water）,Se（Ⅳ）,and Se（Ⅵ）in Tamusu clay rock with different compacted column lengths were studied by through-diffusion method.The total porosity of clay was obtained by diffusion of HTO.The diffusion coefficients,rock capacity factor,effective porosity,and other diffusion parameters were obtained by fitting calculation.The mechanism of nuclide diffusion was preliminarily discussed.The D_e value of HTO increased with the increase of the compacted column lengths,which was affected by the change of geometrical factor and tortuosity.The D_e values of Se（Ⅳ）and Se（Ⅵ）decreased with the increase of the compacted column lengths,mainly due to the existence of the anion repulsion effect.The D_e values of Se（Ⅳ）and Se（Ⅵ）are highly correlated with the lengths of the compacted column,and the diffusion behavior of Se（Ⅳ）and Se（Ⅵ）in different column lengths of Tamusu clay can be estimated through the parameters.（4）The polypropylene diffusion cell which is not affected by the matrix composition of stainless steel was selected to carry out the separate/joint diffusion experiment of Se（Ⅳ）and Se（Ⅵ）in the mixed samples of clay rock and different valence iron or copper（Fe,Fe S,Fe₂O₃,Cu,Cu₂O,Cu O）.The three factors with the best retarding effect of different valence iron and copper factors on Se（Ⅳ）and Se（Ⅵ）in Tamusu clay rock were“clay+Fe”>“clay+Cu₂O”>“clay+Cu”.Calcalite,ankerite,pyrite,and other minerals with strong adsorbability in Tamusu clay rock have a high content,which can effectively retard nuclide migration as the host rock of high-level radioactive waste repository.Under the reduction condition,it might occur that sulfur and iron combine to form pyrite.Selenium entered the lattice of pyrite with sulfur,resulting in an increase in pyrite content and enhancing its retarding effect.During co-diffusion of Se（Ⅳ）and Se（Ⅵ）in clay might be partially reduced to low valence selenium,and formed oxides of Fe（III）and Fe（II）with trace iron minerals in Tamusu clay rock.Finally,the comparison shows the D_e value of Se（Ⅳ）and Se（Ⅵ）diffusion in the original core is 2 orders of magnitude smaller than that of the broken powder sample because the porosity of the original core is smaller.Compared with other dense granites,it can be concluded that under the same conditions,the D_e value of Se（Ⅳ）and Se（Ⅵ）diffusion in Tamusu clay rock is 1～2 orders of magnitude smaller than that in granite,and the K_d value is larger.Therefore,it is believed that Tamusu clay rock is an ideal candidate for host rock to block the migration of selenium isotopes.The purpose of this study is to provide a theoretical reference for the safety assessment of pre-selected host rock.