Resumen de Advanced nuclear fuel cycles: new methodologies of extraction systems applied to hydrometallurgical separation processes
The present thesis, entitled “Advanced nuclear fuel cycles: New methodologies of extraction systems applied to hydrometallurgical separation processes”, is structured in four main blocks. The first part concerns the background information and the consequent introduction to the topic under discussion, emphasising the thesis aim and the objectives to be fulfilled. The second part is dedicated to the methodology followed throughout the different enclosed studies, in particular concerning the different materials, instrumentation and facilities employed as well as several procedures used during this thesis. Then, the third part corresponds to the publication collection where different results are presented. Finally, the fourth block involves the overall conclusions drawn from these outcomes.
Nowadays, nuclear power produces 11% of the world’s electricity, and nuclear power plants produce no greenhouse gases or air pollutants during their operation. Nuclear energy’s potential is essential to achieving a deeply decarbonized energy future. However, nuclear energy’s future role is highly uncertain for several reasons: chiefly, escalating costs and the persistence of historical challenges such as spent fuel and radioactive waste management. The selection of the strategy for spent fuel management is a very complex decision with a lot of factors to be taken into account including economics, politics, environmental protection, resource conservation, and public perception. Research and development in nuclear fission technologies is currently advancing towards technological progress in all aspects with the innovative fourth generation of nuclear reactors (GEN IV). Potentially, these reactors maximise not only the inherent safety, the resource use and energy efficiency, but could also minimise the production of radioactive waste and the proliferation of nuclear weapons. For this reason, they are the basis of the considered advanced nuclear fuel cycles. The current trend in R&D for the development of advanced cycles is the Partitioning and Transmutation (P&T) strategy. It consists of recovering not only the uranium (U) and plutonium (Pu) from the spent nuclear fuel (SNF) as in the PUREX process, but also the minor actinides (MAs, mainly neptunium (Np), americium (Am), and curium (Cm)), for either reuse as part of new fuels or their individualised treatment, because they are the main responsible of the long-term radiotoxicity of the SNF. Partitioning is the chemical process allowing the recovery of MA from the spent fuel dissolution, and transmutation is the physical process that transforms these MA into short-life radionuclides in fast reactors (GEN IV reactors) or dedicated systems (Accelerator-Driven System (ADS)). Regarding partitioning, the most accepted strategy for this reprocessing is based on the separation by liquid-liquid extraction of the metals present in the dissolution of the SNF. For this purpose, considerable scientific and technical efforts have been devoted to developing reprocessing processes in recent years, being the i-SANEX (Innovative Selective Actinide Extraction) and EURO-GANEX (EURO-Grouped Actinide Extraction) the most promising hydrometallurgical processes to achieve the separation of MAs.
One of the constraining points for the development of these extraction processes from the point of view of security is their resistance to the highly radioactive field and acid concentration where they must be used. Because of these conditions, the ligands suffer hydrolytic and radiolytic degradations that can lead to changes in the composition of the extraction system producing degradation compounds, changes in the physicochemical and chemical properties and secondary waste increase, being all of them security issues to be considered. In spite of the wide variety of studies that have been carried out in this regard, an improvement in the stability studies of the ligands involved in the extraction processes is needed, taking into account the most realistic situations to avoid loss of efficiency, identify unexpected behaviour and control security problems.
Hence, this thesis presents several advances in the stability studies of the main molecules involved in the separation of actinides from lanthanides elements included in the mentioned processes: N,N,N’,N’-tetraoctyl diglycolamide (TODGA), N,N’-dimethyl-N,N’-dioctylhexyloxyethyl malonamide (DMDOHEMA), 2,6-bis(5,6-di-(sulfophenyl)-1,2,4-triazin-3-yl)-pyridine (SO3-Ph-BTP) and acetohydroxamic acid (AHA). In order to study the stability of these important molecules, during this thesis, a methodology able to determine their degradation against hydrolysis and radiolysis has been developed:
- The High-Performance Liquid Chromatography-Mass Spectrometry (HPLC-MS) technique has been developed to examine the degradation structural of not only the organic molecules employed during this thesis as TODGA and DMDOHEMA but also their degradation compounds.
-Raman spectroscopy technique has been set up to study the characterization of molecules such as AHA and SO3-Ph-BTP, showing that this technique can be regarded as a useful and versatile tool for analysing the composition of the aqueous phase in the EURO-GANEX process, even for the monitoring of the process in a future.
Specifically, the main studies performed and the general results obtained from them can be summarised as follows:
- Identification of the most relevant conditions to design irradiation models for simulating the degradation of the promising extraction systems due to a highly radioactive field and nitric acid concentration, where nuclear fuel is dissolved. The articles presented in this thesis (4 articles in Section 3 and 1 in Annexe) are related to this identification: 1) Identification and quantification of the AHA hydrolysis by Raman spectroscopy technique; 2) Application of the previous method for evaluating the implications of the AHA hydrolysis in the extraction system studies for actinides and lanthanides separation; 3) Evaluation of the combination of effects due to hydrolytic and radiolytic degradation on the aqueous solvent-based on SO3-Ph-BTP and AHA under EURO-GANEX process conditions and their implications in the separation of actinides and lanthanides; 4) Assessment of degradation of the solvent-based on TODGA by radiolysis using realistic conditions (higher oxygen amount and contact between phases) during the irradiation process (article in Annexe), and 5) Analysis of TODGA/BTP system focusing on extractant degradation in the aqueous phase (SO3-Ph-BTP) using the same realistic conditions used in the previous study.
- Design, development and assessment of dynamic irradiation studies to simulate realistic conditions from the point of view of their application in a pilot plant. One of the articles presented in Annexe of this thesis is involved in this section, where the design of the irradiation loop device and the evaluation of EURO-GANEX system using the irradiation loop are presented.
The results obtained in this thesis evidence that effects of irradiation over extractants depend on experimental design and conditions employed. Therefore, although basic stability studies remain relevant for a fundamental understanding of degradation pathways, to predict the long-term performance of extraction systems submitted to nuclear fuel radiation, it is essential to design reliable simulating strategies. For this reason, the suitability of the extractants, as well as the general performance of the solvents submitted to radiation, should always be evaluated as a full system irradiating the corresponding organic and aqueous phases together.
All conclusions obtained provide important keys to be taken into account during the design of different stability experiments based on the most promising separation processes (i-SANEX and EURO-GANEX).
