He jishu (Jun 2024)
Gamma radiation decomposition and the extraction performance of TODGA in kerosene medium
Abstract
BackgroundWith the development of nuclear energy, spent nuclear fuel reprocessing has generated high-level liquid waste (HLLW) containing large quantities of minor actinides (e.g., Np, Am, and Cm). N,N,N',N'-tetraoctyl-diglycolamide (TODGA), an amide ether extraction agent, has immense potential for the extraction and separation of actinides from HLLW. However, HLLW is characterized by high radioactivity, which can destroy the molecular structure of TODGA and diminish its extraction capacity. In addition, the type of organic medium selected to dilute TODGA has a significant impact on its extraction ability and radiolysis.PurposeThis study aims to evaluate the gamma radiolysis and extraction performance of TODGA in a kerosene medium.MethodsFirstly, several experimental reagent systems were prepared. Organic phase: 50 mmol∙L-1 TODGA/kerosene; aqueous phase: 0~4 mol∙L-1 HNO3; absorbed doses: 5~1 000 kGy by a 60Co gamma-irradiator (3.7×1015 Bq) at room temperature; absorbed dose rates: 0.6 kGy∙h-1 (5~15 kGy) and 5.8 kGy∙h-1 (50~1 000 kGy). Then, ultra-performance liquid chromatography (UPLC) and mass spectrometer were used to identify the components in the organic phase and assess the relative concentrations of TODGA and radiolysis products before and after irradiation. Lanthanides (substitutes for actinides) and alkaline earth metals were extracted using TODGA at different absorbed doses (5~100 kGy) and the concentration of metal ions for extraction was 100 μg∙mL-1 in 3 mol∙L-1 HNO3. Finally, after extraction and dilution, the aqueous phase was detected using an inductively coupled plasma-optical emission spectrometer to estimate the lanthanide and alkaline earth metal concentrations.ResultsThe experimental results indicate that the concentration of TODGA steadily increases with increasing concentrations of HNO3 at a radiation dose of 1 000 kGy. The radiolysis rate of TODGA in the acid-free solution is >80%. Compared to the acid-free solution, the degree of radiolysis can be inhibited at approximately 8% (adding 0.5~3 mol∙L-1 HNO3) and 18% (adding 4 mol∙L-1 HNO3). Moreover, with an increase in the absorbed doses, the concentration of TODGA decreases in the studied systems. The radiolysis rate of TODGA is estimated to be 10%~40% when the absorbed dose was less than 100 kGy. However, it increases dramatically and reaches over 70% at 1 000 kGy. Breaking the ether bond results in the radiolysis of TODGA, generating two types of radiolysis products (C18H37NO2 and C18H37NO). HNO3 alters the radiolysis path, and breaking of the octyl side chain occurs with the appearance of another radiolysis product (C28H56N2O3). TODGA exhibits extremely poor extraction affinity for Sr, and the extraction rate decreases from 25% to non-extraction at an absorbed dose of 100 kGy. However, TODGA achieves approximately 100% lanthanide extraction (Ce, Eu, and Dy).ConclusionsTODGA has low radiation resistance in a kerosene medium at an absorbed dose of 1 000 kGy. Its radiolysis can be inhibited by the addition of HNO3; a higher concentration of HNO3 leads to a stronger inhibitory effect. The radiolysis of TODGA is insignificant within a 100 kGy absorbed dose, particularly in the range of 50~100 kGy. The molecular structure of TODGA is susceptible to breaking of its ether bond owing to gamma rays or radicals produced by kerosene. These findings demonstrate that TODGA can maintain the ability to extract lanthanides after irradiation at an absorbed dose of up to 100 kGy. Thus, it can be inferred that TODGA has a good extraction capacity for actinides in kerosene under the studied conditions.
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