Americium-based oxides: Dense pellet fabrication from co-converted oxalates

Horlait, Denis; Lebreton, Florent; Gauthé, Aurélie; Caisso, Marie; Arab-Chapelet, Bénédicte; Picart, Sébastien; Delahaye, Thibaud

doi:10.1016/j.jnucmat.2013.09.028

Cited by 24 publications

(13 citation statements)

References 20 publications

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“…The radiotoxicity and heat load of ultimate nuclear waste would thus be decreased as well as the ecological footprint of deep geological repositories [2,3]. In this context, several studies have been dedicated to U 1Àx Am x O 2±d compounds, not only to investigate synthesis methods [4][5][6][7][8][9][10][11] and assess their behaviour under irradiation in reactors [4,7,12,13], but also to determine their structural and thermophysical properties, the latter remaining scarcely known [14][15][16][17][18][19][20]. Among them, no data regarding the U 1Àx Am x O 2±d melting behaviour has been reported, despite the importance of such information with respect to safety margins during irradiation, notably in accidental conditions.…”

Section: Introductionmentioning

confidence: 99%

Melting behaviour of americium-doped uranium dioxide

Prieur

Lebreton

Caisso

et al. 2016

The Journal of Chemical Thermodynamics

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a b s t r a c t (Uranium + americium) mixed oxides are considered as potential targets for americium transmutation in fast neutron reactors. Their thermophysical properties and notably their melting behaviour have not been assessed properly although required in order to evaluate the safety of these compounds under irradiation. In this study, we measured via laser heating, the melting points under inert atmosphere (Ar) of U 1Àx Am x O 2±d samples with x = 0.10, 0.15, 0.20. The obtained melting/solidification temperatures, measured here, indicate that under the current experimental conditions in the investigated AmO 2 content range, the solidus line of the (UO 2 + AmO 2 ) system follows with very good agreement the ideal solution behaviour. Accordingly, the observed liquidus formation temperature decreases from (3130 ± 20) K for pure UO 2 to (3051 ± 28) K for U 0.8 Am 0.2 O 2±d . The melted and quenched materials have been characterised by combining X-ray diffraction and X-ray absorption spectroscopy.

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Section: Introductionmentioning

confidence: 99%

Melting behaviour of americium-doped uranium dioxide

Prieur

Lebreton

Caisso

et al. 2016

The Journal of Chemical Thermodynamics

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“…nature of the precursor, milling step, etc. [4][5][6][7][8][9], the sintering steps are often performed in similar reducing conditions at high temperature (ca. 41600°C).…”

Section: Introductionmentioning

confidence: 99%

Comparative XRPD and XAS study of the impact of the synthesis process on the electronic and structural environments of uranium–americium mixed oxides

Prieur

Lebreton

Martin

et al. 2015

Journal of Solid State Chemistry

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“…The plateau is most likely the onset of necking between platelets. The large porosity present between platelets delays the onset of the two-step porosity typical of actinide (IV) oxides from oxalate [24,[32][33][34][35]. The two-step shape is caused by bimodal porosity in the pressed pellet, one within platelets and one between them.…”

Section: Resultsmentioning

confidence: 99%

“…Removing the oxalate structure entirely by decomposition in an autoclave has also been used to improve the sinterability and produce extra small grain size sintered thoria [31]. Despite the large variations in density that can be achieved, the sintering of oxalate based actinide (IV) oxides takes place in two stages [14,24,[32][33][34][35], which are determined by the bimodal inter-and intragranular agglomerate/platelet porosity in the green pellet. The two types of porosity present in such a green pellet are the porosity between the individual nanometric ThO2 grains formed in a platelet during oxalate decomposition and the porosity determined by the stacking of these platelets.…”

mentioning

confidence: 99%

Morphology dependent sintering path of nanocrystalline ThO2

Wangle

Beliš

Tyrpekl

et al. 2020

Journal of Nuclear Materials

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Thorium is most commonly precipitated as oxalate, because of the high efficiency regardless of precipitation conditions. Thorium oxalate readily decomposes into fine-grained ThO2 during heating, but keeps the macrostructure of the oxalate platelets. To assess the effects of precipitate macrostructure on the sintering behavior, different platelet morphologies were prepared and sintered. There are two factors which influence the sintering: the presence of holes within the platelets and the size of the platelets. Small platelets or precipitates with holes generally sinter to thorium dioxide pellets of a higher density and smaller grain size. I. Introduction Thorium dioxide is a candidate fuel for several types of nuclear reactors. Hania and Klaassen [1] extensively reviewed the preparation, properties and use of thorium oxide as a nuclear fuel. The production process, however, still needs to be further developed to meet high industrial efficiency and standards. ThO2 is a refractory ceramic with the highest oxide melting point reported for binary oxides at 3665±70 K [2] or 3624±86 K [3]. As such, the temperature needed to efficiently sinter ThO2 to closed porosity (desired densities of ≥ 95%) often exceed the 1700-1750 °C achievable in commercial highvolume continuous furnaces. One may expect to improve sinterability by introducing defects on the cation or anion lattice. At elevated temperatures (T > 2000 K), a narrow non-stoichiometry domain develops under highly reducing atmospheres [4, 5]. Electrical conductivity measurements indicated that deviation of stoichiometry starts already at 1673 K [6]. Alternatively, sintering-enhancement can be achieved by suitable dopants that induce anion or cation lattice defects. Previous reports have shown that the addition of divalent M 2+ (Ca, Mg) [7], trivalent M 3+ (Y, Al) [8-11], or pentavalent M 5+ (V, Nb, Ta) [12] cations to ThO2 can increase the sinterability significantly. In mixed oxides (Th1−yUy)O2+x and (Th1−yPuy)O2-x deviations from stoichiometry readily occur as the substituting cations, U and Pu are also in valence states other than (IV). [13, 14]. The formation of defects on anion or cation sites in the fluorite crystal structure of ThO2promotes the diffusion of the sintering-limiting cation. This diffusion rate is a function of the square of the deviation from stoichiometry (x) in fluorite UO2±x [15] and in fluorite ThO2±x. In ThO2 without additives, this deviation is determined by the presence of impurities and the oxygen potential in the sintering atmosphere. Thus, both heavily reducing and oxidizing atmosphere can be used to promote sintering above about 1400 °C. Additionally, the morphology of the ThO2 plays a significant role in the packing density and sintering behavior. Most commonly, ThO2 is prepared from oxalates, according to eq. (1).

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Americium-based oxides: Dense pellet fabrication from co-converted oxalates

Cited by 24 publications

References 20 publications

Melting behaviour of americium-doped uranium dioxide

Melting behaviour of americium-doped uranium dioxide

Comparative XRPD and XAS study of the impact of the synthesis process on the electronic and structural environments of uranium–americium mixed oxides

Morphology dependent sintering path of nanocrystalline ThO2

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