Influence of Finite Absorption of Radionuclides on Radionuclide Migration in an Engineered Barrier System

This study, which develops a safety assessment code for radioactive waste disposal, consists of two-dimensional analyses of underground water infiltrated flow and near-field radionuclide migration, one-dimensional analyses of far-field migration, and the dose equivalent. The study takes into account the influence of a finite absorption amount of radionuclides in an engineered barrier system (EBS). The safety assessment code is applied to 14C migration calculations. The near-field cylindrical model consists of an equally mixed region of wasteforms and backfill, bentonite, and rock. Carbon-14 coexists with 3.1 × 10^6 times as much 12C in the wasteforms. The distribution coefficient, maximum absorption amount, and solubility of CO32- against the equally mixed region are assumed to be 2.0 m^3/kg, 3.06 mol/kg, and 544 mol/m3, respectively. Then, the release rate from the wasteforms (10^-4 to 10^-6/yr) and underground water detachment period from the wasteforms are examined to lower the dose equivalent by the intake of well water. The 14C concentration on the EBS boundary is 20 times as large in the case of EBS finite absorption as in the case of infinite absorption. So, the EBS finite absorption leads to absorption saturation and accelerated release of the radionuclides. The influence of the absorption saturation could not be prevented by lowering the release rate. A 3 × 10^4/yr detachment lowered the dose equivalent to 1/40 of the original case.