2021-2023 / Archived Project

Project leader: René Bès   Personnel

X-ray absorption spectroscopy (XAS) and emission spectroscopy (XES) are non-destructive methods allowing the direct characterization of the electronic structure (degrees of oxidation) and the local environment (coordination geometry) of a given element in a sample of any kind (solid, liquid, gas). Over the past decades, XAS has demonstrated itself as an essential approach for the study of materials submitted to extreme conditions of temperature, pressure and irradiation. Good examples of such a success are related to the study of UO₂ and (U, Pu)O₂ nuclear fuels in connection with the development of new manufacturing processes, their behavior in reactor and their stability under storage conditions. Moreover, XAS has provided essential data for the thermodynamic modeling of phase diagrams, as shown during the study of MOX (U,Am)O_2-x samples.

Those studies were made possible by the development of beamlines dedicated to the analysis of naturally or irradiation induced radioactive samples at synchrotron radiation facilities such as the MARS beamline (SOLEIL, France), the CAT-ACT beamline (ANKA, Germany) and the ROBL beamline (ESRF, France) to name a few. The obvious advantage of synchrotron in terms of flux compared to laboratory x-ray tubes partly explains this success, but also the lack of development of a credible alternative in the laboratory. However, the severely time-limited access to synchrotrons de facto precludes a significant number of experiments. In addition, there is the almost systematic need to modify or even transform the sample due to the activity limits authorized on the beamlines. The usage of XAS on those materials has been thus quite limited despite its evident interest, excluding a large amount of potentially important scientific breakthrough to be at least consider.

Recent developments in optics, X-ray sources and detectors now allow the renewal of laboratory XAS instruments with performance complementing the synchrotrons. We have recently demonstrated that this approach is highly adequate for routine experiments on uranium compounds. Following those promising results, a laboratory XAS apparatus was developed during the GAMMA project (2018-2020) in order to study radioactive nuclear materials which is opening new horizons on nuclear material research. In addition, the on-going collaborative project HotXAS between CEA Marcoule (France) and the Helsinki Institute of Physics is aiming to install at the CEA Atalante hot laboratory by 2021 a laboratory-scale XAS device adapted to highly radioactive samples. Its design is based on the GAMMA prototype and it is optimized for XAS experiments of highly radioactive materials such as Am and Pu bearing nuclear fuels. In particular, a direct determination of the degrees of oxidation of actinides and of other cations possibly present in the samples (dopants, fission products) will be achieved without any modification of the sample being necessary. However, similarly to what happen at synchrotron, interferences between the emission lines of the different elements present in the samples can limit the sensitivity of quantitative measurements in fluorescence mode.

The recent instrumental developments carried out on synchrotron beamlines accepting radioactive samples have demonstrated that the implementation of an X-ray emission spectrometer instead of the usual detectors can circumvent this issue. Indeed, emission spectrometers achieve usually energy resolutions of less than 10 eV against approx. 200 eV for conventional fluorescence detectors. This solution has not yet been implemented at laboratory scale and requires an in-depth study to demonstrate its feasibility and efficiency against complex samples. In parallel to those unsurpassed energy resolutions, emission spectrometers also provide access to XES, which is fully complementary to standard absorption measurements. Moreover, XES is a promising alternative to XAS for the determination of the degree of oxidation of actinides while having count rates at laboratory similar to those obtained with synchrotrons. Finally, the coupling of XAS and XES can provide quantitative experimental data on the electronic structures of actinides, and represents a major contribution for the atomistic calculations performed on these systems. However, no data regarding XES on actinide exists in the literature yet, as this approach is only in its infancy at synchrotron facilities.

The XTREME project has two main objectives:

1. To demonstrate the feasibility and to design an emission spectrometer adapted to a laboratory scale XAS apparatus, in order to circumvent the fluorescence line interferences when studying actinide-bearing materials. This capability will be also useful for other kind of samples such as environmental samples or high entropy alloys for example.
2. To pioneer the XES alternative to XAS in actinide physico-chemistry within the development of an overview of the XES sensitivity to degree of oxidation and local environment.

The XTREME project is financially supporting a joint PhD thesis between CEA Marcoule and Helsinki Institute of Physics, starting in 2021. This PhD thesis aims to develop the methodology and instrumentation associated to new XAS and XES techniques, while optimizing them for the study of MOX (U, Pu)O2 fuels in the laboratory. The thesis will also integrate an important part dedicated to the development of both theoretical and experimental knowledge of actinides through the understanding of the links between the experimental observations, i.e. the spectral components, and the electronic and local structures of actinide