Electron-counting rules were applied to understand the stability, structural preference, and physical properties of metal disilicides. Following predictions made by 14 electron counting rules, the ordered semiconductor ReGaSi, the first ternary phase in this system, was proposed and successfully synthesized. It crystallizes with a primitive tetragonal structure (space group P4/nmm) closely related to that of MoSi-type ReSi, but with Ga and Si orderly distributed in the unit cell. The band structure, density of states, and crystal orbital calculations confirm the electron count hypothesis to predict new stable compounds. Calculations, based on 14 electrons per ReGaSi units, show a small indirect band gap of ∼0.2 eV around Fermi level between full and empty electronic states. Additionally, first-principles calculations confirm the site preference of Ga and Si, which is observed through the structural refinement. Experimental magnetic measurements verified the predicted nonmagnetic properties of ReGaSi.
Highlights 15 • First study on the effect of aqueous ions on the degradation of waste form for I-129 16 • First summary on probable iodine release pathways in various aqueous environments 17 • Accelerated iodine release by enhanced ion-exchange, basicity or acidity, and ionic 18 strength 19 • Discovered secondary phase vanadinite Pb5(VO4)3Cl and hydroxylvanadinite 20 Pb5(VO4)3OH 21 • Low ionic content and neutral pH are vital to the disposal safety of nuclear waste 22 23 Abstract 24To ensure the safe disposal of nuclear waste, understanding the release process of radionuclides 25 retained in the nuclear waste forms is of vital importance. Iodoapatite Pb9.85(VO4)6I1.7, a potential 26 waste form for iodine-129, was selected as a model system for ceramic waste forms in this study 27 to understand the effect of aqueous species on iodine release. Semi-dynamic leaching tests were 28 conducted on bulk samples in cap-sealed Teflon vessels with 0.1 mol/L NaCl, Na2CO3, Na3PO4, 29 and Na2SO4 solutions under 90 °C, fixed sample surface area to solution volume ratio of 5/m, 30 and periodic replacement of leaching solutions. The reacted solutions were then analyzed by 31 Inductively Coupled Plasma-Mass Spectrometry and Inductively Coupled Plasma-Optical 32 Emission Spectrometry; the leached surfaces were characterized by X-ray diffraction, scanning 33 electron microscopy, and infrared spectroscopy. The result shows that, compared to deionized 34 water, the ion-rich solutions enhanced the iodine release as a result of the increased ionic 35 strength, reduced activity coefficient of dissolved species, and increased solution pH. Surface 36 reactions can lead to the formations of secondary phases by ion-exchange and precipitation. 37These findings suggest that an ion-rich environment in the geological repository can be 38 detrimental to the disposal safety of the nuclear waste form. 39 1 Introduction 40 Nuclear energy is emission-free. The deployment of nuclear energy is motivated by the 41 pressing demand to mitigate climate change. 1 Sustainable development of the nuclear energy 42 requires concrete plans to safely dispose radionuclides waste generated by nuclear fission. 2 43 Among those radionuclides, iodine-129 is particularly challenging to handle due to its long half-44 life (15.7 million years), high yield (0.7% yield per fission of uranium-235), 3 and weak 45 interactions with common materials in repository environments such as engineering barrier and 46 rock in geology formation. 4,5 Iodide (I -) is the most stable form of iodine in an environment with 47 pH and redox potential typically found in nature. 6-8 Under highly oxidizing conditions, iodide 48can be oxidized to iodine (I2) and/or iodate (IO3 -). All these iodine species are highly mobile in 49 nature given their high volatility and or high solubility. 9,10 Iodine, as an essential element for 50 human health, can accumulate in human bodies. 11 For a healthy adult, 30% of the total iodine, 51 approximately 15-20 mg, is concentrated in the thyroid gland. 12 Chro...
To ensure the safe disposal of nuclear waste, understanding the release process of radionuclides retained in the nuclear waste forms is of vital importance. Iodoapatite Pb9.85(VO4)6I1.7, a potential waste form for iodine-129, was selected as a model system for ceramic waste forms in this study to understand the effect of aqueous species on iodine release. Semi-dynamic leaching tests were conducted on bulk samples in cap-sealed Teflon vessels with 0.1 mol/L NaCl, Na2CO3, Na3PO4, and Na2SO4 solutions under 90 °C, fixed sample surface area to solution volume ratio of 5/m, and periodic replacement of leaching solutions. The reacted solutions were then analyzed by Inductively Coupled Plasma-Mass Spectrometry and Inductively Coupled Plasma-Optical Emission Spectrometry; the leached surfaces were characterized by X-ray diffraction, scanning electron microscopy, and infrared spectroscopy. The result shows that, compared to deionized water, the ion-rich solutions enhanced the iodine release as a result of the increased ionic strength, reduced activity coefficient of dissolved species, and increased solution pH. Surface reactions can lead to the formations of secondary phases by ion-exchange and precipitation. These findings suggest that an ion-rich environment in the geological repository can be detrimental to the disposal safety of the apatite waste form.
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