Sorption behavior of Np(IV), Np(V) and Am(III) in the disturbed zone between engineered and natural barriers

In order to be more confident of the performance assessment of high-level radioactive waste disposal, radionuclide transport must be investigated in more detail in the disturbed host rock region adjacent to the engineered barriers where disturbance has been introduced during the construction and waste-emplacement period. Geochemical, hydrological, and rock-mechanical properties should be quite different from those of undisturbed host rock. We have to elucidate the effect of bentonite intrusion into intersecting fractures from the standpoint of radio nuclide confinement. In the present work, sorption distribution ratios (Kd's) of Np and Am are measured experimentally for various values of redox potential (Eh) in a simulated rock fracture filled with bentonite. The Kd of Am is approximately 6.5 x 103 ml/g and found to be insensitive to the redox potential. Under anaerobic conditions, the Kd of Np is approximately 6 x 104 ml/g. Under aerobic conditions, Kd is as small as 30 to 100 ml/g. This is the first report to measure the sorption behavior of Np and Am in a simulated rock fracture filled with bentonite (namely, in a disturbed zone) under pH, Eh and ionic strength control. We can make use of these Kd data for numerically evaluating the mass transfer from bentonite filed fractures into the water-flowing fracture network. (author). 7 refs., 2 figs