Optimization of composite admixtures used in cementation formula for radioactive evaporator concentrates

Catalytic effect of cesium on the oxidation behavior of cation exchange resins in Li2CO3-Na2CO3-K2CO3 melt

After the treatment of liquid radioactive waste, there is a certain amount of Cs in the waste resin, and these Cs-doped resins are prone to volatilize during the thermal treatment process and cause radionuclide leakage. The molten salt oxidation (MSO) can effectively prevent the volatilization of toxic metal, especially the volatilization of Cs. Under nitrogen and air conditions, it is found that the oxidation behavior between Cs-doped and clean cation exchange resins (CERs) is quite different. In the presence of oxygen and molten carbonate salt, Cs₂CO₃ is generated by the destruction of functional groups in Cs-doped CERs. The Cs₂CO₃ in Na₂CO₃-K₂CO₃-Li₂CO₃ reacts with oxygen to form Li₂O₂, which reduces the content of S in residue from 26.33 to 13.38% in air conditions at 400 °C and promotes the generation of sulfate in the molten carbonate salt. The elements Cs and S in the Cs doped CERs spontaneously form thermally stable Cs₂SO₄ in the molten carbonate salt.

Co-pyrolysis of spent radioactive ion exchange resin and manganese dioxide: Decrease the decomposition temperatures of functional groups

With the development of the nuclear industry and clean energy, spent radioactive ion exchange resin has become a major concern that needs to be solved urgently. In this study, the mixed resin (sulfonic aid and quaternary ammonium polystyrene beads, 1:2, v/v) is co-pyrolyzed with manganese dioxide in a tube furnace, selecting argon as the reaction atmosphere. Manganese dioxide exhibits unique catalytic and oxidative activity, and a low mass remaining efficiency of 34.14% is obtained under low heating temperature of 300 ℃. The required decomposition temperatures of functional groups and benzene are decreased by approximately 100 ℃, and that of polymer chain is decreased by 130 ℃. The TGA analysis shows the decomposition temperature rule of functional groups and base polymer. The FT-IR spectra and XPS analysis reveal the bridging effects of manganese sulfonate and sulfide group. The SEM diagrams prove that the two processes including depolymerization and reunion could be found in co-pyrolysis. The XRD analysis indicates manganese dioxide undergoes the reduction path of MnO₂→Mn₃O₄→MnO, and MnS is formed with the decomposition of manganese sulfonate. The possible mechanism of solid-phase reaction is proposed to explain the promotion of manganese dioxide on co-pyrolysis.

Assessment of reaction between thorium and polyelectrolyte nano-thin film using Box–Behnken design

Sandwich type polyelectrolyte nano-thin films (PENTFs) were prepared by using polyallylamine hydrochloride and polyacrylic acid from layer-by-layer assembly process with spin coating system. Their nanostructures have been studied by scanning electron microscope, atomic force microscope, and attenuated total reflectance Fourier transform infrared spectroscopy. In order to understand the effects of the initial concentration of thorium, initial solution pH, temperature, and contact time on the reaction between thorium and PENTF, an experiment data set was designed according to Box–Behnken model. The analysis of variance calculations for regression model were carried out in 95% confidence level and were checked for fitting experimental data and predicted values. The correlation coefficient value ( R2) obtained as 94% showed that there was a correlation between the predicted and the observed values. The optimum pH, temperature, initial concentration of thorium, and interaction time in studied ranges were found as 2.81, 35℃, 160 mg·L−1, and 120 min, respectively. At these conditions thorium (IV) ions adsorption yield was obtained as 89 ± 2%. The Freundlich, Langmuir, and Dubinin–Radushkevich isotherms were used to investigate the characteristics of the process. These characteristics data imply that the Freundlich model fits better than the Langmuir model for the Th (IV) sorption onto PENTFs with KF and n values were found to be 20.6 mg·g−1 and 1.08 L·mg−1, respectively. The thermodynamic parameters were also computed as negative Δ H value suggest that adsorption of Th (IV) is exothermic nature. The calculated negative and positive values of Δ G indicate that the sorption process is favorable (energetically) while running below 40℃ and over this point the process status change to non-spontaneous, respectively.

Effect of mineral viscosity-enhancing admixtures on the solidification of evaporator concentrates

It is known that partial replacement of cement by viscosity-enhancing admixtures, also known as anti-washout admixtures, affects the quality of the waste form or concrete. To reduce the bleeding rate of the paste, the characteristics of various mineral viscosity-enhancing admixtures dispersed in saline solutions were investigated, including sedimentation and viscosity. The admixture candidates included fly ash, silica fume, bentonite, and palygorskite. The effect of these admixtures blended with a cement-based matrix on the bleeding rate of the solidification of evaporator concentrates was also examined in this paper. The experimental results show the palygorskite Type 400 is the best choice to improve the quality of waste form, due to its excellent suspension property in the saline solution. The bleeding rate of paste decreased as the dispersion volume of the admixture suspension increased. For consideration of the quality of waste forms and the concentrate loading, the optimization of the palygorskite/concentrate ratio of 15-17 wt% and solidification agent/concentrate ratio of 1.0-1.2 were adopted. With this recipe, the quality of waste forms resulting from the solidification of simulated and actual evaporator concentrates mainly containing chloride met the regulations' requirements.