In order to synthesize chemical filters for the selective removal of volatile fluorides, commercial magnesium fluoride MgF2 with high specific surface area (HSA) was investigated. The amount of -OH ...groups substituting fluorine is not negligible, partly due to the high surface area, but also due to the synthesis route. These hydroxyl groups induce a Lewis basicity on the surface of metal fluorides. The amount of these Lewis basic sites has been tailored using fluorination with F2 gas. The sorption of VOF3, used as model gas, onto these fluorides was investigated. The versatility of surface chemistry as a function of a number of Lewis basic sites opens the way to filter selectivity mixture of volatile fluorides depending on their Lewis acidity. HSA MgF2 acts as a stable matrix towards the gas to be purified, and the selectivity may be achieved by a higher Lewis acidity of the gaseous impurity.
A new strontium uranyl oxyfluoride, (UO2)4F13Sr3(H2O)8(NO3)·H2O, was synthesized under hydrothermal conditions. The single-crystal X-ray structure was determined. This compound crystallizes in the ...triclinic space group P1̅ (No. 2), with unit cell parameters a = 10.7925(16) Å, b = 10.9183(16) Å, c = 13.231(2) Å, α = 92.570(8)°, β = 109.147(8)°, γ = 92.778(8)°, V = 1468.1(4) Å3, and Z = 2. The structure is built from uranyl-containing ( UO 2 ) 4 F 13 ∞ 1 chains of tetrameric units of corner-sharing UO2F5 pentagonal bipyramids. These chains are linked through trimeric strontium units to form strontium–uranyl oxyfluoride layers further assembled by nitrate groups. The interlayer space is occupied by free water molecules. This compound was characterized by spectroscopic methods, especially 19F NMR highlighting the many different fluoride sites. Structural relationships with other uranyl oxyfluorides were investigated through the different F/O ratios, the structural building unit, and the structural arrangement.
The present article reviews a selection of results obtained in the AREVA/CNRS/UCA joint research laboratory. It focuses on interfaces formed by uranium hexafluoride (UF6) with chemical filter ...(purification), carbon (UF6 storage), and metallic substrate (corrosion). As a matter of fact, along the nuclear fuel cycle, metallic surfaces of the fluorination reactors, cooling systems (for the liquefaction of UF6), and storage containers are in contact with UF6, either in the gas or in the liquid phase. For the removal of volatile impurities before the enrichment, surface of chemical filters with a high specific surface area must be enhanced for both selectivity and efficiency. To store depleted UF6 (238U), graphite intercalation compounds are proposed and preliminary results are presented.
Cet article présente une sélection de résultats obtenus au laboratoire commun de recherche AREVA/CNRS/UCA « Chimie du fluor ». Il se focalise sur les différentes interfaces formées par l'hexafluorure d'uranium UF6 avec les filtres chimiques (purification), le graphite (stockage de UF6) et les substrats métalliques (corrosion). En effet, au cours du cycle du combustible nucléaire, les surfaces métalliques des réacteurs de fluoration, les cristallisoirs (pour la liquéfaction de UF6) et les conteneurs sont en contact avec UF6 sous ses formes gazeuse et liquide. Pour la séparation des impuretés volatiles avant l'enrichissement de UF6, la surface des filtres chimiques doit être optimisée pour atteindre la sélectivité et l'efficience. Enfin, pour stocker UF6, en particulier celui riche en 238U sous-produit de l'enrichissement, des composés d'intercalation du graphite sont proposés et des résultats préliminaires sont donnés.
A new strontium uranyl oxyfluoride, (UO
)
F
Sr
(H
O)
(NO
)·H
O, was synthesized under hydrothermal conditions. The single-crystal X-ray structure was determined. This compound crystallizes in the ...triclinic space group P1̅ (No. 2), with unit cell parameters a = 10.7925(16) Å, b = 10.9183(16) Å, c = 13.231(2) Å, α = 92.570(8)°, β = 109.147(8)°, γ = 92.778(8)°, V = 1468.1(4) Å
, and Z = 2. The structure is built from uranyl-containing Formula: see text chains of tetrameric units of corner-sharing UO
F
pentagonal bipyramids. These chains are linked through trimeric strontium units to form strontium-uranyl oxyfluoride layers further assembled by nitrate groups. The interlayer space is occupied by free water molecules. This compound was characterized by spectroscopic methods, especially
F NMR highlighting the many different fluoride sites. Structural relationships with other uranyl oxyfluorides were investigated through the different F/O ratios, the structural building unit, and the structural arrangement.
L’hexafluorure d’uranium, de formule chimique UF 6 , est un composé clé du cycle du combustible car il est utilisé pour l’étape d’enrichissement isotopique. Toutefois, des impuretés présentes et ...mélangées à l’UF 6 sont à éliminer afin de garantir un uranium de pureté nucléaire. Dans ce manuscrit, les réactions chimiques entre les polluants et les filtres chimiques retenus pour la purification sont étudiées. Par ailleurs, l’étude de la réactivité en milieu liquide est réalisée afin de se rapprocher des conditions industrielles. Une voie de recyclage des filtres chimiques est également investiguée afin de réutiliser les adsorbants sur plusieurs cycles de purification. L’ensemble des performances (taux de sorption des polluants, décontamination lors du recyclage) sont évaluées avant la mise au point du pilote à l’échelle industrielle.
Uranium hexafluoride, which chemical formula is UF 6 , is a key compound of nuclear fuel cycle due to its use during isotopic enrichment process. Nethertheless, pollutants melted with UF 6 have to be removed in order to ensure a nuclear purity fuel. In this manuscript, the reactions occurring between pollutants and chemical filters selected for this application are studied. Then, the reactivity in liquid UF 6 is also examined with the aim of being close to the industrial process. The regeneration of adsorbents is investigated so that chemical filter can be used for several purification cycles. The performances (sorption rate of pollutants, purifying during recycling step) are evaluated before simulations at industrial scale.
PDF