Effect of ionizing radiation on solvent extraction systems for the separation of minor actinides

With the use of nuclear power, inevitably highly radiotoxic waste is produced. The irradiated nuclear fuel contains many different radionuclides which contribute to the radiotoxicity and heat production. The most determining elements for these undesired properties are the minor actinides and plutonium, which are the result of a combination of neutron absorption and radioactive decay processes. Historically, uranium and plutonium can be recovered from the used nuclear fuel and made into new nuclear fuel elements (Mixed Oxide (MOX) fuel). Nowadays, research is focusing on increasing resource efficiency and reducing the amount of nuclear waste to optimize the capacity of the highly expensive underground repositories. Minor actinides and plutonium are aimed to be burned in fast neutron gen IV nuclear reactors. However, to be able to succeed in this closure of the fuel cycle, suitable separation processes are essential. In the past, many hydrometallurgical solvent extraction processes were evaluated to achieve this goal. Grouped ActiNide Extraction (GANEX) processes aim at extracting the actinides together and avoiding the existence of pure plutonium streams, which is important to reduce concerns towards proliferation. A recent development is the EURO-GANEX process in which first the bulk amount of uranium is extracted and in a second step the remaining actinides are separated from the fission and corrosion products. This second separation is currently achieved by a co-extraction of actinides and lanthanides, followed by a selective stripping of the actinides (plutonium, neptunium, americium, curium and traces of uranium). Previously, a mixture of two extractants, N,N,N’,N’-tetraoctyl diglycolamide (TODGA) and N,N’-dimethyl-N,N’-dioctyl-2(2-hexyloxyethyl)-malonamide (DMDOHEMA), was used. A new diglycolamide extractant, 2,2'-oxybis(N,N-di-n-decylpropanamide) (mTDDGA) was proposed to replace the current two extractants with one. This means a huge simplification of the process, which on its turn reduces synthesis costs and the complexity of solvent regeneration. The challenges for using this single extractant are mainly situated in the plutonium loading capacity and fission product extraction. Early reported results are very promising and strongly motivate further in-depth study, which was conducted in this research. The focus of this doctoral thesis is on the radiolytic stability of mTDDGA, since ionizing radiation in nuclear fuel solutions causes degradation of the extractant. In the first phase of the research, the irradiation procedure was validated. This involved an assessment of different dosimeters in a low (0.5 kGy h-1) and high dose rate (13.6 kGy h-1) 60Co gamma irradiation field. The results confirmed that Perspex dosimeters offer a reliable robust dosimetric method. It was demonstrated that in conjunction with High Performance Liquid Chromatography coupled with Mass Spectrometry (HPLC-MS), dose constants of TODGA could be accurately determined. The results of this experimental work confirmed the applicability of the whole chain of methods for the determination of dose constants of new diglycolamides, such as mTDDGA. Computational work based on Density Functional Theory (DFT) was conducted to complement experiments. This involved method validation, for which a combination of B88 and Perdew-Burke-Ernzerhof (PBE) exchange-correlation was found to give good results for the calculation of the bond dissociation energy (BDE). These calculations supported previously observed decrease in stability of a single methylated diglycolamide. Analysis of the radical Fukui function showed that PBE functionals are suitable for mapping the most reactive positions. Here, it is found that by including a solvation entropy model, the computational results align better with the experimental results found in literature. HPLC-MS analysis of irradiated of 0.05 mol L-1 mTDDGA in n-dodecane showed that the degradation of mTDDGA is similar to the degradation of TODGA, with degradation products from breaking the ether bond being the most prominent. However, mTDDGA was proven to be more stable towards gamma irradiation, and some previously unreported degradation products were detected such as n-dodecane adducts and double and triple de-alkylation products. In the following chapter, extraction properties of un-irradiated mTDDGA, irradiated mTDDGA and its degradation products are studied. The (R,S)-mTDDGA diastereomer shows much better extracting properties than the (S,S) diastereomer, which is in agreement with the Me2TODGA analogues. For an irradiated (up to 454 kGy) solvent of 0.5 mol L-1 of (R,S)-mTDDGA in Exxsol D80, distribution ratios for actinides and lanthanides remain above 100 (except for neptunium). This irradiated solvent was also still capable to extract 36 g L-1 of plutonium without any third phase formation. These results strongly support further developments towards implementation of mTDDGA in the EURO-GANEX process. The most abundant degradation compounds were the ones from breaking the ether bond and de-alkylation. Solvent extraction experiments showed that, 2-hydroxydi-n-decylamide, the main degradation compound, mainly extracts plutonium. The de-alkylation product extracts lanthanides and actinides, but tends to form precipitates at high nitric acid concentrations.