Institute of Radiological Protection and Nuclear Safety (IRSN), PSE-ENV, SRTE, LR2T, CE Cadarache, 13115, Saint Paul les Durance Cedex, France.
J Environ Radioact. 2021 Jun;232:106567. doi: 10.1016/j.jenvrad.2021.106567. Epub 2021 Mar 6.
Sorption hypotheses and models are required for the prediction of Cs migration in soils contaminated after nuclear reactor accidents and nuclear weapons tests. In assessment models, the K (distribution coefficient) hypothesis for sorption, which assumes that sorption is instantaneous, linear and reversible, has often been coupled with the convection-diffusion equation (CDE) to model Cs migration. However, it fails to describe Cs migration velocities which often decrease with time. Alternative equilibrium-kinetic (EK) hypotheses of Cs sorption/desorption have been suggested by laboratory experiments but have not been fully validated in field conditions. This work addressed the influence and magnitude of non-equilibrium Cs sorption in field conditions by reinterpreting, with an inverse approach, series of Cs profiles measured in mineral soils of forest plots located in Fukushima Prefecture (2013-2018). Our results show that the inclusion of non-equilibrium sorption significantly improves, compared to the equilibrium hypothesis, the realism of simulated Cs profiles. Fitted sorption parameters suggest a fast sorption kinetic (half-time of 1-7 h) and a pseudo-irreversible desorption rate (half-time of 3.2 × 10-3.4 × 10 years), whereas equilibrium sorption (4.0 × 10 L kg on average) only affects a negligible portion of Cs inventory. By June 2011, such EK parameters fitted on our plots realistically reproduced profiles measured in the same forest study site (Takahashi et al., 2015). Predictive modeling of Cs profiles in soil suggests a strong persistence of the surface Cs contamination by 2030, with exponential profiles consistent with those reported after the Chernobyl accident. This study demonstrates that hypotheses and parameters of Cs sorption can be partially inferred from in situ measurements. However, further experiments in controlled conditions are required to better estimate the sorption parameters and to identify the processes behind non-equilibrium sorption.
在预测核反应堆事故和核武器试验后污染土壤中 Cs 的迁移时，需要吸附假说和模型。在评估模型中，常将吸附的 K（分配系数）假说与对流-扩散方程（CDE）相结合，以模拟 Cs 的迁移。该假说假设吸附是瞬时的、线性的和可逆的，但它无法描述 Cs 迁移速度，后者通常随时间降低。已经通过实验室实验提出了替代的平衡-动力学（EK）Cs 吸附/解吸假说，但尚未在野外条件下得到充分验证。本研究通过反演方法，重新解释了 2013-2018 年在福岛县森林样地的矿物土壤中测量的一系列 Cs 剖面，研究了野外条件下非平衡 Cs 吸附的影响和程度。研究结果表明，与平衡假说相比，非平衡吸附的纳入显著提高了模拟 Cs 剖面的真实性。拟合的吸附参数表明存在快速的吸附动力学（半衰期为 1-7 小时）和拟不可逆的解吸速率（半衰期为 3.2×10-3.4×10 年），而平衡吸附（平均为 4.0×10 L kg）仅影响 Cs 库存的一小部分。到 2011 年 6 月，在我们的样地上拟合的这种 EK 参数真实地再现了在同一森林研究点测量的剖面（Takahashi 等人，2015）。土壤中 Cs 剖面的预测模型表明，到 2030 年，表面 Cs 污染仍将强烈存在，指数剖面与切尔诺贝利事故后报告的剖面一致。本研究表明，可以从原位测量中部分推断出 Cs 吸附的假说和参数。然而，需要在控制条件下进行进一步的实验，以更好地估计吸附参数并确定非平衡吸附背后的过程。
