Modelling for the long-term mechanical and hydraulic behavior of betonite- and cement-based materials considering chemical transitions

An analysis system for the long-term mechanical behavior of barrier materials (MACBECE: Mechanical Analysis system considering Chemical transitions of BEntonite-based and CEment-based materials) was developed in order to improve the reliability of the evaluation of the hydraulic field which is one of the important environmental conditions in the safety assessment of the TRU waste disposal in Japan. TRU wastes are low level radioactive wastes that include long-lived nuclides. The MACBECE is the system which calculates the deformation of barrier materials using their chemical property changes as inputs, and subsequently their hydraulic conductivity taking both their chemical property changes and deformation into consideration.MACBECE can evaluate the mechanical/hydraulic alterations on two types of barrier materials, cement-based and bentonite-based materials. These materials, in the long-term, may be altered chemically by the reaction between them, which leads to changes in hydraulic characteristics and mechanical properties. To evaluate the long-term mechanical behavior due to the chemical transitions, the nonlinear elastic model and the elasto-viscoplastic model were applied respectively on each material, based on the data from various laboratory tests. And also, the proposed models based on the data from permeability tests were applied to evaluate the alteration of hydraulic conductivity. In this paper, the model for the long-term behavior of bentonite-based materials adopted in MACBECE is mainly described.Additionally, by using MACBECE, the long-term deformation and the transition of hydraulic field in the near-field of the round-type disposal cavities were preliminary evaluated, assuming chemical evolutions as the input data. The analysis results indicate that the influence of chemical changes of barrier materials controls the hydraulic conductivity and the value of buffer materials remains under 10−11 m/s.