Integral transform analysis of radionuclide transport in variably saturated media using a physical non-equilibrium model: application to solid waste leaching at a uranium mining installation

Abstract: The Generalized Integral Transform Technique (GITT) was employed to simulate transient one-dimensional flow in variably saturated porous media, as well as radioactive waste transport within different layers (a solid waste pile, nearby soil, and a granular aquifer) towards the edge of a uranium mining installation under institutional control. Computational codes, written using the Mathematica software system, were implemented and tested for solution of the coupled advection-dispersion equations for an arbitrary number of daughter products within a radioactive chain migrating in saturated and unsaturated soil layers. The computer simulations were verified in great detail against results obtained using the built-in routine NDSolve of the Mathematica platform and the HYDRUS-1D software system. The present work reports the main results for 238U chain radionuclide transport using data extracted from a safety assessment of solid waste repositories at a uranium mining and milling installation in Caetité, state of Bahia, Brazil, operated by INB (Indústrias Nucleares do Brasil). Concentration evolutions of the various radionuclides obtained with the simulations were analyzed for five different cases to explore variations in the infiltration and recharge rates, the effect of assuming physical equilibrium or non-equilibrium transport conditions, and of different initial concentrations of some of the radionuclides.

摘要：本研究采用广义积分变换技术（Generalized Integral Transform Technique, GITT），模拟了变饱和多孔介质中的瞬态一维流动，以及监管管控下铀矿设施周边三类介质层（固体废物堆、邻近土壤及颗粒含水层）内放射性废物向设施边界的运移过程。本研究基于Mathematica软件开发了专用计算程序，可求解饱和与非饱和土层中放射性衰变链内任意数量子体产物运移的耦合对流弥散方程，并完成了程序的验证与测试。通过与Mathematica平台内置求解程序NDSolve及HYDRUS-1D软件的计算结果进行细致比对，本模拟的可靠性得到了充分验证。本研究依托巴西巴伊亚州卡埃蒂特市某由INB（Indústrias Nucleares do Brasil，巴西核工业公司）运营的铀矿冶设施固体废物处置库的安全评估数据集，报道了铀-238（238U）衰变链放射性核素运移的核心模拟结果。针对五种不同工况，本研究分析了模拟得到的各类放射性核素的浓度演化过程，以探究入渗与补给速率变化、物理平衡/非平衡运移条件的影响，以及部分放射性核素初始浓度差异的作用效果。