Cellulose-based wastes can be degraded into short-chain organic acids at the cementitious radioactive waste repository. Isosaccharinic acid (ISA), one of the main degradation products, can form the chelate complex with metals and radionuclides, and these complexes have a potential that can accelerate to move the radionuclides to far-field from the repository. This study characterized the amount of generated ISA from typical cellulosic materials in the repository. Two different degradation experiments were conducted under alkaline conditions (saturated with Ca(OH)2 at pH 12.4): i) cellulosic material mixture under an opened condition (partially aerobic), and ii) cellulosic material under an anaerobic condition in a nitrogen-purged glove box. In the first case, three different types of cellulosic materials–paper, cotton, and wood– were mixed at the same ratio, and the experiments were carried out at three different temperatures (20°C, 40°C, and 60°C). It revealed that both the cellulose degradation rate and generated ISA concentration were high at high reaction temperatures, and various soluble degradation products such as formic acid and lactic acid were generated. The cellulose degradation in this work seems to still stay at a peeling-off process. In the second study, each type of cellulosic material was applied in its own batch experiments, and the amount of generated ISA was in the order of paper > wood > cotton. The above two experiments are supposed to be a long-term study until the generated ISA reaches an equilibrium state.

Technetium (Tc) is a long-lived radioactive isotope, which exists as TcO4 - with high solubility under oxidative condition. The solubility of Tc is fundamental to assess the safety of radioactive waste repository in the case of a leakage of radioactive wastes. Cellulosic materials (paper, wood, cotton, etc.) contaminated by radionuclides are disposed of in low-level and intermediate-level radioactive waste repositories. Cellulose can be decomposed under anaerobic and alkaline conditions when cement pore water is saturated, and then isosaccharinic acid (ISA) is generated as a degradation product of cellulose. ISA forms complexations with radionuclides in solution and affects the solubilities of radionuclides. Therefore, the effect of ISA should be accurately evaluated to predict and assess the mobility of radionuclides in repository environments. In this study, batch tests were conducted to confirm the effect of ISA on the solubility of Rhenium(IV) Oxide. Herein, rhenium was used as a non-radioactive analog of Tc due to their similar chemical properties. Deionized water (DIW) and 0.1 M NaOH solution in pH 12.5 were used as background solutions, and ISA concentration was varied to 1~20 mM using Ca(ISA)2 and NaISA, respectively. The batch tests were conducted under both aerobic and anaerobic conditions. The whole batch tests under anaerobic conditions were performed in the glove box using oxygen purged DIW with a high purity nitrogen gas (99.9%) and low oxygen concentration (< 0.5 ppm). As a result, the rhenium concentration decreases as more ISA is dissolved in the solution, which shows the contrary effect of ISA on the solubility of other metals and radionuclides (e.g., Co, Th, Fe, Ni, etc.). It is assumed that the reducing capacity of ISA decreases the rhenium dissolution in the solution. Additional characterization of the oxidation state of rhenium oxide and the mechanism will be tested and presented.