Decontamination of uranium-containing radioactive waste from stainless steel surfaces using NaOH-based molten salts.

Herein, we report a new strategy for the rapid removal of uranium-containing contaminants from metal surfaces, and it relies on decontaminants made of NaOH-based molten salts. The addition of Na2CO3 and NaCl to NaOH exhibited superior decontamination performance, with a decontamination rate of 93.8% within 12 min, outdoing the performance of the single NaOH molten salt. The experimental results demonstrated that the synergistic effects between CO32- and Cl- promoted the corrosion efficiency of the molten salt on the substrate, which accelerated the decontamination rate. Additionally, benefiting from the optimization of the experimental conditions by the response surface method (RSM), the decontamination efficiency was improved to 94.9%. Notably, it also showed remarkable results in the decontamination of specimens containing different uranium oxides at low and high levels of radioactivity. This technology is promising for broadening the path in rapid decontamination of radioactive contaminants on metal surfaces.

Effect of copper ions on transformation of organic sulfur in cationic exchange resins in Li2CO3-Na2CO3-K2CO3 molten-salt system.

Cationic exchange resins (CERs) were applied for purification and clarifying process of radioactive wastewater in nuclear industry, which was a kind of sulfur-containing organic material. Molten-salt oxidation (MSO) method can be applied for the treatment of spent CERs and the absorption of acid gas (such as SO2). The experiments about the molten salt destruction of the original resin and Cu ions doped resin were conducted. The transformation of organic sulfur in Cu ions doped resin was investigated. Compared with the original resin, the content of tail gas (such as CH4, C2H4, H2S and SO2) released from the decomposition of Cu ions doped resin was relatively high at 323-657 °C. Sulfur elements in the form of sulfates and copper sulfides were fixed in spent salt through XRD analysis. The XPS result revealed that the portion of functional sulfonic acid groups (-SO3H) in Cu ions doped resin was converted into sulfonyl bridges (-SO2-) at 325 °C. With the enhancement of temperature, sulfonyl bridges (-SO2-) were further decomposed to sulfoxides sulfur (-SO-) and organic sulfide sulfur. The destruction of thiophenic sulfur to H2S and CH4 was prompted by copper ions in copper sulfide. Sulfoxide were oxidized to the sulfone sulfur in molten salt. Sulfones sulfur consumed by reduction of Cu ions at 720 °C was more than it produced by oxidation of sulfoxide through XPS analysis, and the relative proportion of sulfone sulfur was 16.51%.

Effects of oxygen content on gaseous and solid products during molten salt oxidation of cation exchange resins.

Molten salt oxidation (MSO) is an advanced method for waste resins treatment; nevertheless, the research about gas product variations of resins under different stoichiometric air feed coefficient (α) is rare. The optimal working condition of hazardous waste disposal is obtained through thermodynamic equilibrium calculation, and the method to improve the treatment efficiency is found to guide the optimization of the actual experiment. In this paper, Fact Sage was used to calculate the oxidation products of cation exchange resins (CERs) at different temperatures and α, focusing on the similarities and differences through the contents of CO, CH4, CO2, and SO2 during the oxidation of CERs, the MSO of CERs, and the theoretical calculation. The results indicated that the gas products of the calculation and reality of the oxidation process of CERs are quite different, while the CO contents of CERs during MSO are close to the calculated values. The main reason for this consequence is that in the oxidation process of CERs, the S in the sulfonic acid group will form thermally stable C-S with the styrene-divinylbenzene skeleton. Moreover, the introduction of carbonate can promote the destruction of C-S and absorb SO2 as sulfate, weakening the influence of C-S on the oxidation products of CERs. The gas chromatograph results indicated that the SO2 content is reduced from 0.66% in the process of CERs oxidation to 0.28% in MSO of CERs. When 1.25 times stoichiometric air feed coefficient is fed, the sulfate content in the carbonate is the highest at 900 °C, which is 23.4%.

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.