In underground repository environments, various types of engineered barriers are installed to hinder the mobility of radionuclides. Cement admixtures, especially used to improve workability for concrete, are composed of fairly high organic molecules and have a dispersing effect through bonding with the C-S-H of the concrete. Previous studies have shown that complex-forming organics like EDTA, NTA, and ISA have a significant effect on the mobility of radionuclides, but the studies on the behavior and stability of combined complexes in hydrated cement are lacking. So, we selected a commonly used polycarboxylic-ester (PCE) type cement admixture and stable Co as a surrogate of Co-60 to perform desorption experiments from hydrated cement containing the admixture. Radioactive Co is known to be a common contaminant in nuclear fission and medical facilities and considered to exist as a relatively stable phase in repositories. In addition, the evaluation of cobalt can be a standard of safety issue for other radionuclides with the presence of cement admixture in repository. In this study, cement samples were prepared at water/cement ratio of 0.55 and cured for 28 days at 23-25°C and at least 80% of humidity with varying cement admixtures of 0.0, 0.1, and 2.0wt%. To evaluate the stability of cobalt in the weathered cement, a 0.001 M HCl solution was used to simulate cement weathering conditions on a hot plate at 60°C for 1 day using a solid/liquid ratio of 1:100. Degree of weathering was confirmed using XRD analysis. The adsorption experiments were performed by adding 0.0042 mmol of cobalt (CoCl2, Sigma-Aldrich, anhydrous ≥ 98.0%) to the weathered cement for 3 days using a platform shaker at 200 rpm, and the supernatant was separated using a syringe filter (<0.20 um) before ICP-MS analysis to determine the amount of Co adsorption. Cobalt desorption was tested for the Co-adsorbed cement using 0.019 mmol of calcium (Ca(NO3)2·4H2O, Sigma-Aldrich, 99%) for 3 hours to 14 days. The results showed that adsorbed cobalt with and without cement admixture was stably bound to cement, and did not increase any noticeable Co release by 2.0wt% PCE admixture. However, additional experiments using varying contents of PCE and other admixtures should be conducted to provide a standard for assessing the safety of cement admixtures in repositories.

Radioactive nickel (Ni59 and Ni63) is a major radionuclide that needs to be determined for quantifying the total radioactivity in radioactive waste disposal repository. Also, radioactive waste containing organic wastes, such as cotton and tissue can be decomposed to produce the Isosaccharinic acid (ISA) in a disposal facility. The presence of ISA in the disposal facility could increase the mobility of radionuclides. Therefore, it is necessary to confirm the mobility of Ni with the presence of ISA in the repository. This study investigated the effect of ISA on the sorption and the solubility of Ni in synthesized groundwater. The sorption test was conducted in different time intervals with Ni and ISA. Nickel nitrate hexahydrate and Ca(ISA)2 were used after purchase. Granite was used as the solid medium to simulate the major rock type of the repository. Ni and ISA solution with the medium were mixed using a platform shaker for 6 days. After 6 days, the solid parts were separated by centrifugation and additional syringe filters, and the supernatant was analyzed for Ni and ISA concentration using ICP-MS and IC, respectively. The solubility experiments were conducted at different temperatures (20, 40, and 80°C). Nickel hydroxide was used as the solubility limiting solid phase. To balance the ionic strength and confirm the effect of ISA on Ni solubility, 0.01 M of CaCl2 solution was prepared in a sample without ISA, and 0.01 M of Ca(ISA)2 solution was prepared in a sample with ISA. In solubility tests, the solution was also analyzed by ICP-MS and IC for Ni and ISA, respectively. The concentration of Ni was found to increase with ISA compared to Ni concentration without ISA. The concentration of ISA was not changed during the solubility test periods. For solubility tests, the concentration of Ni also increased according to the increase in temperature. The solid phase was characterized by XRD, FT-IR, and SEM-EDS. Based on the results of this study, the presence and effect of ISA on radioactive Ni mobility should be carefully investigated to secure the long-term safety assessment for the low and intermediate-level waste repository.