Impact of the cation distribution homogeneity on the americium oxidation state in the U0.54Pu0.45Am0.01O2−x mixed oxide

doi:10.1016/j.jnucmat.2014.09.014

Journal of Nuclear Materials

Volume 456, January 2015, Pages 115-119

https://doi.org/10.1016/j.jnucmat.2014.09.014 Get rights and content

Abstract

The impact of the cation distribution homogeneity of the U_0.54Pu_0.45Am_0.01O₂₋_x mixed oxide on the americium oxidation state was studied by coupling X-ray diffraction (XRD), electron probe micro analysis (EPMA) and X-ray absorption spectroscopy (XAS). Oxygen-hypostoichiometric Am-bearing uranium–plutonium mixed oxide pellets were fabricated by two different co-milling based processes in order to obtain different cation distribution homogeneities. The americium was generated from β⁻ decay of ²⁴¹Pu. The XRD analysis of the obtained compounds did not reveal any structural difference between the samples. EPMA, however, revealed a high homogeneity in the cation distribution for one sample, and substantial heterogeneity of the U–Pu (so Am) distribution for the other. The difference in cation distribution was linked to a difference in Am chemistry as investigated by XAS, with Am being present at mixed +III/+IV oxidation state in the heterogeneous compound, whereas only Am(IV) was observed in the homogeneous compound. Previously reported discrepancies on Am oxidation states can hence be explained by cation distribution homogeneity effects.

Introduction

In the prospect of future sodium-cooled fast neutron reactors (SFR), uranium–plutonium mixed oxide fuels incorporating high amounts of plutonium are currently considered. Because of their specific neutronic spectrum, SFR will be able to burn long-lived minor actinides (MAs) such as americium [1]. Homogeneous transmutation is one possible way to reach this target by introducing small amounts (2–5%) of MAs into the U₁₋_yPu_yO₂₋_x mixed oxide fuel. Even if some specifications are still to be discussed, general trends are already identified for such nuclear fuels. For instance, the Oxygen/Metal (O/M) ratio of the SFR’s fuel will have to range from 1.94 to 2.00. This oxygen stoichiometry dictates many (or most) of the fuel properties (thermal conductivity, melting point, diffusion phenomena, …), hence studying the O/M ratio of MA-bearing mixed oxides is relevant. The O/M ratio relies mainly on the oxidation state of the cations because metal vacancies are not expected in the uranium–plutonium mixed oxides. It is well known that, in hypostoichiometric mixed oxides U₁₋_yPu_yO₂₋_x, uranium is tetravalent whereas plutonium could exhibit either fully reduced Pu³⁺ or a mixed +III/+IV valence [2], [3], [4], [5], [6]. Americium has been reported to be trivalent in (U,Pu,Am)O₂₋_x [7] and U₁₋_yAm_yO₂₋_x [8], [9], to be of mixed valence (+III/+IV) in (U,Pu,Am)O₂₋_x [10] and (Pu,Am)O₂₋_x [11] and finally to follow the plutonium reduction behavior in (U,Pu,Am)O₂₋_x [12], [13]. The americium chemistry therefore appears to be still not clearly identified within uranium–plutonium mixed oxides. Furthermore, the consequence of the cation distribution homogeneity on the americium chemistry has, to our knowledge, never been studied. Therefore, the intention of our work presented here was to investigate how the americium oxidation state is impacted by the cation homogeneity of the fuel pellet by coupling X-ray diffraction (XRD), electron probe micro analysis (EPMA) and X-ray absorption spectroscopy (XAS).

Section snippets

Sample preparation

In this study, uranium dioxide and plutonium dioxide powders were used. The uranium dioxide powder was produced by a wet fabrication route based on the formation of ammonium diuranate (ADU) from uranyl nitrate precipitated with ammonia. The obtained particles were then atomized, dried and calcined, leading to spherical-shaped agglomerates of around 20 μm in diameter. Plutonium dioxide powder was produced by precipitation of a plutonium nitrate solution within oxalic acid to form plutonium

Results and discussion

In the prospect of ranking the samples in terms of U–Pu distribution homogeneity, XRD measurements were performed on manually crushed pellets obtained (1) after sintering at 2023 K for 24 h under Ar + 5% H₂ + ∼1500 vpm H₂O and slowly cooled at 50 K h⁻¹ [17], (2) after the reducing annealing at 2023 K for 4 h under Ar + 5% H₂ + ∼5 vpm H₂O cooled at 300 K h⁻¹ and (3) after the re-oxidizing thermal treatment at 1173 K for 16 h under Ar + 5% H₂ + ∼24,000 vpm H₂O. The diffraction patterns (and lattice parameters) obtained

Conclusion

The americium chemistry in uranium–plutonium mixed oxides is discussed in the literature without being able to conclude on the trivalent or tetravalent behavior of Am. The present study points out a clear cation distribution homogeneity effect on the Am oxidation state which may explain the discrepancies between the different authors. Americium exhibits a mixed valence III/IV when the mixed oxide shows some U–Pu distribution heterogeneities. When (U,Pu,Am)O₂₋_x is homogeneous, however, americium

Acknowledgments

The authors are pleased to acknowledge Dr. J. Léchelle, Dr. T. Truphémus, Dr. S. Berzati, Dr. R. Bes, I. Felines, Y. Marc and JC. Richaud for their precious contributions to this work. The ACTINET-I3 project is also to be acknowledged for allowing us to perform the XAS experiments at the ESRF facility, France.

References (23)

K. Aizawa
Prog. Nucl. Energy
(2002)
C. Guéneau et al.
J. Nucl. Mater.
(2002)
C. Guéneau et al.
J. Nucl. Mater.
(2011)
M. Osaka et al.
J. Alloys Comp.
(2007)
K. Mayer et al.
J. Alloys Comp.
(1994)
D. Prieur et al.
J. Nucl. Mater.
(2013)
W. Bartscher et al.
J. Nucl. Mater.
(1983)
M. Kato et al.
J. Nucl. Mater.
(2009)
S. Nakamichi et al.
CALPHAD
(2011)
T.M. Besmann et al.
J. Nucl. Mater.
(1985)

R. Vauchy

Ceram. Int.

(2014)

Cited by (22)

Thermal conductivity measurement of uranium–plutonium mixed oxide doped with Nd/Sm as simulated fission products
2024, Journal of Nuclear Materials
The thermal conductivities of near-stoichiometric (U,Pu,Am)O₂ doped with Nd and/or Sm, which are major fission products (FPs) generated during irradiation of uranium–plutonium mixed oxides (MOX) fuels, as simulated fission products are evaluated from 1073 to 1673 K. The thermal conductivities are calculated from the thermal diffusivities that are measured using the laser flash method. To evaluate the thermal conductivity from a homogeneity viewpoint of Nd/Sm cations in MOX, the specimens with different homogeneities are prepared using ball-mill and fusion methods. A homogeneous Nd/Sm distribution decreases the thermal conductivity of MOX with increasing Nd/Sm content, whereas heterogeneous Nd/Sm has no influence. The effect of Nd/Sm on the thermal conductivity is studied using the phonon transport model $λ = {(A + B T)}^{- 1}$ . The dependences of the coefficients A and B on the Nd/Sm content ( $C_{N d}$ and $C_{S m}$ , respectively) are evaluated as: $A (m K / W) = 1.70 \times 10^{- 2} + 0.93 C_{N d} + 1.20 C_{S m}$ , $B (m / W) = 2.39 \times 10^{- 4}$ .
Uranium–plutonium–americium cation interdiffusion in polycrystalline (U,Pu,Am)O<inf>2±x</inf> mixed oxides
2024, Journal of Nuclear Materials
Diffusion couples made of dense polycrystalline U_1−αPu_α−βAm_βO_2±γ/U_1−yPu_y−zAm_zO_2±x oxides were annealed in various thermodynamic conditions (temperature, oxygen partial pressure), and for different durations. The associated actinide redistribution was quantified using Electron Probe Micro-Analysis (EPMA). Average diffusion profiles were obtained from elemental U, Pu, and Am X-ray maps and the resulting interdiffusion coefficients were calculated, then analyzed at the light of our model of point defect chemistry.
Lattice parameters of fluorite-structured uranium–americium mixed oxides
2023, Journal of Nuclear Materials
The room temperature lattice parameters of fluorite-structured U_1–yAm_yO₂ uranium–americium mixed oxides were confronted with our crystallographic hybrid model. The complex charge compensation mechanisms that take place in this solid solution were considered to shed light on the available experimental unit-cell values. The effects of α self-irradiation, deviation from stoichiometry, and neptunium content on the lattice constants were also estimated.
Oxygen potential of neodymium-doped U<inf>0.817</inf>Pu<inf>0.180</inf>Am<inf>0.003</inf>O<inf>2±</inf><inf>x</inf> uranium–plutonium–americium mixed oxides at 1573, 1773, and 1873K
2023, Journal of Nuclear Materials
Oxygen potentials of U_0.817Pu_0.180Am_0.003O_2±_x incorporating 10 and 20 mol% of neodymium (Nd/Metal) were investigated by thermogravimetry at 1573, 1773, and 1873 K. The presence of neodymium induced an increase in the oxygen potential of the U–Pu mixed oxide. The correlation between oxygen partial pressure pO₂ and deviation from stoichiometry x was analyzed, and a model of defect chemistry was proposed. Finally, the crystal structure of these mixed oxides was discussed at the light of the mechanisms of possible Nd(III)/U(V) charge compensation, and deviation from stoichiometry.
Impact of the cationic homogeneity on Th<inf>0.5</inf>U<inf>0.5</inf>O<inf>2</inf> densification and chemical durability
2019, Journal of Nuclear Materials
Citation Excerpt :
Nevertheless, one must note that for protocol I, a small shoulder corresponding to a limited fraction of uranium-enriched zones was observed. On the other hand, and as expected from the results already compiled in the literature [19,42,43], the dry chemistry routes led to heterogeneous samples, illustrated by the dramatically wide aspect of the distribution curve. Indeed, even if a small peak was noticed for compositions closed to Th0.85U0.15O2, the incorporation rate of uranium appeared to be very scattered up to pure UO2 spots.
In order to study the effects of cationic homogeneity on the life cycle of Th_1-xU_xO₂ ceramics, including sintering and reprocessing (dissolution) steps, five different ways of preparation were set up, going from the most homogenous oxalic co-precipitation to a mechanical mixture of the parent oxides. Dilatometric experiments evidenced a better sintering capability for the most homogenous compounds obtained through wet chemistry methods while dry chemistry routes led to poor density values (between 80 and 90 %TD). However, the introduction of an additional mechanical grinding step prior to the powders sintering systematically led to the homogenization of the systems. Improved homogeneity also provide a better chemical durability associated with the congruent release of thorium and uranium in solution during dissolution tests of Th_0.5U_0.5O₂ samples. However, heterogeneous samples led to incongruent behaviors that can be lowered by introducing a grinding step before the sintered samples preparation. Since the impact of the cationic homogeneity must be followed carefully during dissolution, in operando observations of evolving solid/solution interface by ESEM were performed. They allowed imaging the preferential dissolution of uranium-enriched zones and confirmed the significant impact over dissolution rate of the presence of chemical heterogeneities at the interface.
New insight in the uranium valence state determination in U<inf>y</inf>Nd<inf>1−y</inf>O<inf>2±x</inf>
2018, Journal of Nuclear Materials
The charge compensation mechanisms in U_yNd_1−yO_2±x and its consequence on the overall O stoichiometry (or O/M ratio where M = Nd + U) have been studied through the uranium valence state mixture evolution as a function of Nd content up to y = 0.62 by means of high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) at the U M₄-edge. Our results clearly demonstrate the formation of U⁵⁺ at low Nd content (y $<$ 0.15). Upon increasing the Nd content, oxygen vacancies and the formation of U⁶⁺ appear as competing mechanisms for intermediate Nd concentrations, leading to the co-existence of U⁴⁺/U⁵⁺/U⁶⁺ mixed valence and an overall hypostoichiometry (O/M $<$ 2.00). Finally, the formation of U⁶⁺ associated with strongly distorted U local environment is observed for high Nd concentrations (y = 0.62), leading to an overall hyperstoichiometry (O/M $<$ 2.00).

View all citing articles on Scopus

View full text

Impact of the cation distribution homogeneity on the americium oxidation state in the U0.54Pu0.45Am0.01O2−x mixed oxide

Abstract

Introduction

Section snippets

Sample preparation

Results and discussion

Conclusion

Acknowledgments

Prog. Nucl. Energy

J. Nucl. Mater.

J. Nucl. Mater.

J. Alloys Comp.

J. Alloys Comp.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

CALPHAD

J. Nucl. Mater.

Ceram. Int.

Impact of the cation distribution homogeneity on the americium oxidation state in the U_0.54Pu_0.45Am_0.01O₂₋_x mixed oxide