Application of Radionuclides Migration Software(MNS) in Radionuclides Diffusion Modeling

The prediction of adsorption, diffusion and migration behavior of critical radionuclides in the related media of deep ground disposal of high level radioactive waste(HLW) is one of the cores for the safety evaluation of the HLW disposal repository. The availability of tools for modeling radionuclide diffusion in China remains limited. To satisfy the need of the prediction of radionuclides in long time and large scale, the disposal chemistry team at China Institute of Atomic Energy developed a radionuclide migration software, MNS, which could implement 2D or 3D system modeling by GID software and provide the concentration distribution of radionuclides in desired time and space point. In this research, the basic information of MNS software is introduced, including the calculation mechanism and calculation process, the calculation mechanism is based mainly on the basic equation of water migration, radionuclides adsorption and diffusion. The workflow consists of pre-processing and post-processing stages. Pre-processing involves the construction of spatial models and parameter assignment, such as porosity, mineral density, radionuclide diffusion and migration parameters, and water flow velocity. The post-processing mainly consists of the obtain of the concentration of radionuclides and the distribution of water flow in any desired space point or time. Meanwhile, the results calculated by analytical software, COMSOL and MNS by inputting the same diffusion and adsorption data of critical radionuclides in the same space model were compared. The calculation results are almost the same, verifying preliminarily that this software is reliable. Besides, based on the developed repository design(mainly depend on the mature design of Sweden and Korea) and existing adsorption and diffusion data, the software is used to model the diffusion processes of ⁷⁹Se, ⁹⁹Tc and ¹²⁹I in Gaomiaozi bentonite(the first choice of the buffer materials in deep ground disposal of HLW) and Beishan granite(Beishan is the first selection area for the disposal repository of HLW in China, also the place where the Beishan Underground Research Laboratory is constructed) under the atmospheric conditions on the scale of millions of years. The space model is the ture size of the repository design based on the results, the inhibition property of Gaomiaozi bentonite and Beishan granite to those radionuclides is preliminarily evaluated. The modeling results indicate that bentonite could hinder the diffusion of ⁷⁹Se, ⁹⁹Tc and ¹²⁹I in ten thousands of years and the dense Beishan granite can effectively block the diffusion of ⁷⁹Se, ⁹⁹Tc and ¹²⁹I in hundreds of thousands of years, both Gaomiaozi bentonite and the dense Beishan granite can hinder the diffusion of ⁷⁹Se better.