Mechanisms of dynamic seepage response of 3H and 238Pu in the surrounding of a disposal repository in northwest China

Abstract

During the disposal of low- and intermediate-level waste, damage to the fuel casing through oxidation and breakage can lead to the leaching and migration of radionuclides, posing significant risks to the surrounding environment. This study addresses these challenges by highlighting the limitations of relying solely on batch experiments and chemical reaction models, which may lead to overly conservative assessments of radionuclide migration. Instead, a comprehensive approach that integrates both physical nonequilibrium and chemical nonequilibrium adsorption mechanisms is warranted for a more accurate appraisal of the adsorption behavior of ²³⁸Pu within environmental media. This article studied the migration of ³H and ²³⁸Pu in different stratigraphic media through column experiments, and fitted their non-equilibrium reaction processes. The obvious asymmetry in the break-through-curves of ³H and ²³⁸Pu in strongly weathered granite is due to the large pore preferential flow or channels. The symmetry of the peak shape of the ³H curve in medium sand and gravel sand with more uniform structure is significantly better than that in strongly weathered granite, indicating that the reaction is less affected by physical non-equilibrium mechanisms. Remarkably, all four models-LEA, OSM, TSM, and TRM-effectively align with the experimental data. When considering chemical non-equilibrium adsorption, both TSM and TRM models denote a superior fit compared to the LEA and OSM models, underscoring the influence of chemical nonequilibrium adsorption. For strongly weathered granite media, the TRM two-zone model exhibits a higher degree of precision in fitting the experimental data and highlights the exacerbating effect of large pore preferential flow conductive channels on ²³⁸Pu mobility.