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A similar method, but at higher X-ray energy, was employed at BM23 to probe the rare element yttrium in fluorapatite. This mineral is used as geochemical proxies to reconstruct conditions such as temperature, acidity, redox state and the composition of crystallising fluids associated with sedimentary or hydrothermal-magmatic geological settings (page 132).

The journey proposed by X-rays is not limited to the Earth. They allow us to wander to the edge of the solar system. B. Journaux et al. take us beneath the surface of icy moons such as those of Jupiter and Saturn. Thanks to ID15B s intense X-ray beam, they were able to reveal the structures of novel NaCl hydrates in these icy worlds (page 134).

Making our world safer

Let us return to the surface of the Earth, where humanity faces considerable challenges. The warming of our planet, linked to the increase in the level of CO2 in the atmosphere, is undoubtedly the most important. In this context, the development of effective CO2 sorbents is vital for achieving net-zero CO2 emission targets. At beamline ID31, M. Rekhtina et al. provide fundamental insights into the mechanism of CO2 capture of MgO-based sorbents promoted with molten nitrates, using in-situ, time- resolved powder XRD (page 136).

Another important issue concerns nuclear power plants, which provide our societies with a source of carbon-free energy, but which present significant risks due to the inevitable production of highly radioactive waste. In this chapter, four articles are devoted to this theme. G.L. Murphy et al. have studied the complex chemistry of Cr-doped UO2, an advanced nuclear fuel, using high-energy-resolution fluorescence-detection X-ray absorption near-edge structure (HERFD-XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy at beamline BM20 (page 138).

Still on BM20, two studies are devoted to the improvement of radioactive waste storage. More specifically, A. Poulain et al. were interested in the mobility of 79Se, the fission product of 235U and a long-lived radioisotope, a crucial parameter for assessing the safety of radioactive nuclear waste storage systems (page 140). Another study, by S. Hilpmann et al., focused on the reduction of U(VI) by microorganisms present in clay formations that potentially can be used as host rocks for deep geological repositories storing high-level radioactive waste, with bentonite as backfill material (page 142). Finally, C. Le Gall et al. investigated the behaviour of fission products during the start of a severe nuclear accident by probing model materials that simulate irradiated nuclear fuels using combined scanning electron microscopy and high-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XAS) at BM16 (page 144).

M. MEZOUAR