Effects of Smectite Illitization on Transport of Actinides Through Engineered Barriers of HLW Repositories
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EFFECTS OF SMECTITE ILLITIZATION ON TRANSPORT OF ACTINIDES THROUGH ENGINEERED BARRIERS OF HLW REPOSITORIES JOONHONG AHN*, S. NAGASAKI**, S. TANAKA**, AND A. SUZUKI** *Department of Nuclear Engineering, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa, 259-12, Japan "**Department of Quantum Engineering and Systems Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113, Japan ABSTRACT Effect of smectite illitization on mass release rate from the bentonite-filled buffer of the engineered barriers is analyzed based on experimental results of sorption of americium and neptunium onto illite-smectite mixtures, and on mathematical models for smectite illitization by K+ diffusion, water flow through the engineered barriers, waste glass dissolution, and transport of americium and neptunium including solubility sharing with two major americium isotopes and moving boundary effect of the neptunium precipitation region. Numerical results for mass release rates of americium and neptunium as well as time-dependent pore velocity and glass dissolution rate are shown. Illitization effect is negligible with parameter values assumed here. INTRODUCTION This paper presents effects of smectite illitization on transport of americium and neptunium through the bentonite-filled buffer region of engineered barriers of a high-level radioactive waste repository. Due to elevated temperature and presence of potassium ion in groundwater, smectite, the major mineral in the buffer, changes into illite. As the illite content in the buffer increases, hydraulic conductivity of the buffer will increase and sorption capacity of the buffer will decrease. Increase in hydraulic conductivity enhances water flow in the engineered barriers, resulting in faster waste glass dissolution and faster radionuclide transport. Furthermore, gradual decrease in sorption distribution coefficients with increasing illite content may result in increase in mass release rates of radionuclides at the outer boundary of the engineered barriers. Experimentally obtaining the sorption distribution coefficients of americium and neptunium for various illite-smectite mixtures, we propose the quantitative relation between sorption distribution coefficients and the illite content in the buffer. With the experimental result and the
reported reaction rate constant for smectite illitization [I], sorption distribution coefficients can
be written as a function of time. With time-dependent hydraulic conductivity and sorption distribution coefficients, mathematical models are established for (1) groundwater flow in the engineered barriers, (2) waste glass dissolution, and (3) mass transport through the buffer. Analytical solutions are obtained for the pore velocity of groundwater in the buffer and the glass dissolution rate as a function of the illite content, and so of time. Mass release rates of Am and Np at the outer boundary of the buffer are obtained numerically, and effects of smectite illitization on the performance of the engineered barriers are shown quantitatively
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