Reassessing the solid/liquid phase transitions of
U1-y-zPuyAmzO2±x mixed oxides

Pauline Fouquet Metivier, Davide Robba, Luka Vlahovic, Dragos Staicu,
Christine Gueneau, Philippe Martin

► To cite this version:

Pauline Fouquet Metivier, Davide Robba, Luka Vlahovic, Dragos Staicu, Christine Gueneau, et al. Reassessing the solid/liquid phase transitions of U1-y-zPuyAmzO2±x mixed oxides. Journal of Nuclear Materials, 2025, 619, pp.156300. 10.1016/j.jnucmat.2025.156300. cea-05419848

HAL Id: cea-05419848

https://cea.hal.science/cea-05419848v1

Submitted on 16 Dec 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

# Reassessing the solid/liquid phase transitions of U₁-y-zPu_yAm_zO₂±x mixed oxides

Pauline Fouquet-Métivier<sup>a,1,*</sup>, Davide Robba<sup>b</sup>, Luka Vlahovic<sup>b,†</sup>, Dragos Staicu<sup>b</sup>, Christine Guéneau<sup>c</sup>, Philippe M. Martin<sup>a,‡</sup>

## HIGHLIGHTS

* First determination of O/M ratio variation during melting for U₁-y-zPu_yAm_zO₂±x.

* No influence of up to 2 mol. % Am content on the Tsolidus of U₁-y-zPu_yAm_zO₂±x.

* Melting behavior of U₁-y-zPu_yAm_zO₂±x depends on the initial stoichiometry.

## ARTICLE INFO

### Keywords:

Melting transition

Fast reactor

MOX

Laser heating

O/M ratio

## ABSTRACT

For safety reasons, it is crucial to understand the melting behavior of U₁-y-zPu_yAm_zO₂±x (0.2 ≤ y ≤ 0.4 and 0 ≤ z ≤ 0.06) mixed oxide fuels for Sodium Fast Reactors. In this study, melting temperature measurements were conducted on seven U₁-y-zPu_yAm_zO₂±x samples (with 0.235 ≤ y ≤ 0.44 and 0.005 ≤ z ≤ 0.02) using a laser heating technique. Thanks to the addition of oxygen gauges to the setup, the variation of the oxygen partial pressure inside the chamber during the experiment, and thus the O/M ratio (M = U+Pu+Am), was recorded for the first time for U₁-y-zPu_yAm_zO₂±x. This study discusses the influence of the Pu and Am content, as well as the O/M ratio on the melting behavior of U₁-y-zPu_yAm_zO₂±x. A decrease in the solidus temperature was observed with increasing Pu content, whereas no significant influence of the Am content on the solidus temperature was noted. Regarding the influence of the O/M ratio, the hypotheses used to calculate its variation are discussed. It was demonstrated that only the molten pool reacted with the surrounding atmosphere. Two distinct melting behaviors were observed depending on the initial O/M ratio of the fuels. For initially hypo-stoichiometric fuels, oxidation of the samples was observed during melting, as demonstrated by thermodynamic calculations. On the contrary, for initially stoichiometric or slightly hyper-stoichiometric fuels, reduction was occurred during the melting.

## 1. Introduction

Currently, uranium-plutonium mixed oxides U₁-yPu_yO₂±x are the reference fuels for Sodium Fast Reactors. These reference fuels must meet specific requirements. Among these requirements, the Oxygen/Metal ratio (O/M ratio, where M = U+Pu+Am) must range between 1.94 and 2.00 to ensure adequate thermal conductivity (and prevent fuel melting) and to minimize cladding corrosion respectively. Additionally, the plutonium content (y) must be between 0.20 and 0.40. Since

plutonium can originate from a multi-recycling of irradiated MOX fuel, small quantities of <sup>241</sup>Am (up to 5 mol. %), formed by β decay of <sup>241</sup>Pu, could be present. To study the influence of the fuel composition and to properly assess the safety of the nuclear reactor, an accurate knowledge of the fuel's melting behaviour under normal and accidental conditions is mandatory.

Various studies have been performed over the last few decades to determine the solidus temperature of uranium-plutonium mixed oxides U₁-yPu_yAm_zO₂±x [1-7], using two main techniques. Originally, a

<footnote><sup>a</sup> CEA, DES, ISEC, DMRC, SASP, Univ Montpellier, Marcoule, France</footnote>

<footnote><sup>b</sup> European Commission, Joint Research Centre, Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany</footnote>

<footnote><sup>c</sup> Université Paris-Saclay, CEA, Service de recherche en Corrosion et Comportement des Matériaux, 91191, Gif-sur-Yvette, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: pauline.fouquetmetivier@cea.fr (P. Fouquet-Métivier).</footnote>

<footnote n="1">¹ Present address: CEA, DES, IRESNE, DEC, Cadarache F-13,108 Saint-Paul-Lez-Durance, France.</footnote>
thermal arrest technique with Re or W crucibles was used [2,3,6,7]. However, an interaction between the sample and the W crucible was found for $y > 0.50$, causing an underestimation of the solidus temperature [3,8]. To avoid this effect, a laser heating setup was developed at the JRC-Karlsruhe. This technique has the advantage of being a self-crucible method with quasi-containerless conditions and has the ability to perform short laser pulses, which prevent high vaporization rates [1,4,5].

Aside from the differences related to the use of different techniques, the plutonium content was found to affect the solidus temperature of $U_{1-y}Pu_yO_{2.00}$ and the melting point was found to decrease nearly linearly with the Pu content. When compared to more recent data measured by laser heating, a decrease in the solidus temperature was observed from $T = 3147 \pm 20$ K (the melting point of $UO_{2.00}$ [9]) down to a minimum located for $0.50 < y < 0.80$, when increasing the plutonium content [1, 4]. For $y > 0.80$, the solidus temperature increases again to $T = 3050 \pm 59$ K (the melting point of $PuO_{2.00}$ [4]).

Furthermore, as with any impurities, the presence of small quantities of americium ($<5$ mol. %) can influence the melting behaviour. However, studies on americium-bearing oxides are rare in the literature. Only one study reports the melting point of $AmO_2$, at 2383 K, but it lacks of experimental details and raises questions about the exact oxygen stoichiometry of the oxide, as $AmO_2$ begins to be reduced for $T > 1100$ K [10]. Additionally, only one study reports on the melting behaviour of various $U_{1-y}Pu_yAm_zO_{2\pm x}$ mixed oxides [3,8]. Kato et al. observed a 3 K decrease of the melting temperature with the addition of 1 mol. % Am using the thermal arrest technique; however, this result lies within the experimental uncertainties.

Another parameter that influences the melting behaviour of uranium-plutonium mixed oxides is the O/M ratio. In the study by Kato et al. [8], an increase in the melting temperature was reported when the O/M ratio decreased from 2.00 up to a maximum located around O/M = 1.90 [11]. Although post-melting characterizations were performed in two studies and did not show any significant variation in the O/M ratio before and after melting [1,4], in most cases, the final O/M ratio was not determined. Thermodynamic calculations show that, at thermodynamic equilibrium, the sample composition should change at a very high temperature [11]. Additionally, no in-situ measurements were performed, so the exact O/M ratio at which the melting occurs remains unknown. As previously studied by several authors [4,5,11], the atmosphere affects the melting temperature. Uranium-rich samples tend to oxidize in air, while a decrease in the O/M ratio is expected for Pu-rich samples under argon.

Thus, based on these considerations, further work is needed to describe the melting behaviour of $U_{1-y}Pu_yAm_zO_{2\pm x}$ mixed oxides, taking into account the simultaneous influence of plutonium and americium content and the O/M ratio.

To this end, new measurement campaigns using a laser-heating technique available at the JRC-Karlsruhe (Germany) [12,13] were performed in this work to determine the melting behaviours of various $U_{1-y}Pu_yAm_zO_{2\pm x}$ mixed oxides (with $0.235 \le y \le 0.44$ and $0.005 \le z \le 0.02$). In addition to determining the solidus temperatures, the variation of the O/M ratio of the sample during the melting was assessed for the first time for $U_{1-y}Pu_yAm_zO_{2\pm x}$ mixed oxides. With the implementation of oxygen gauges at the entrance and exit of the experimental chamber, the variation of the in situ oxygen partial pressure could be recorded [14].

## 2. Materials and methods

### 2.1. Samples

Five batches of uranium-plutonium mixed oxide $U_{1-y}Pu_yO_{2\pm x}$, manufactured between 1996 and 1999, were used in this study: four manufactured by the powder metallurgy process ($y = 0.24, 0.29, 0.31, 0.40$) and one by the sol-gel route ($y = 0.45$). Due to the presence of $^{241}$Pu in

all starting $PuO_2$ powders and considering the age of the samples, a few molar percent of $^{241}$Am was produced in the samples during their storage. In this study, the samples are referred to as MOXy, where y is the initial Pu content (e.g., MOX31).

The chemical compositions, O/M ratios and densities of the samples at the time of this work are reported in Table 1.

Details on the sintering conditions and chemical compositions manufactured by powder metallurgy can be found in [15] for MOX24, MOX29 and MOX31 and in [16] for MOX40, designated as MOX39.4-B sample. Details on the manufacturing of the MOX45 sample using a sol-gel route can be found in [17].

To study the influence of the O/M ratio on the melting behaviour, two pellets of MOX29 and MOX40 samples were annealed under the following conditions: 8 hours at 1173 K, with heating and cooling rates of 6 K.min$^{-1}$, under Ar/H$_2$ 4 % + 24 000 ppm H$_2$O. These conditions were chosen based on thermodynamic calculations to ensure an O/M ratio close to 2.00. The annealed samples are referred to as MOX29-OM and MOX40-OM.

Only FCC-structured phases were observed by X-Ray Diffraction, and their lattice parameters were determined by analysing the diffraction patterns of all samples. Due to α-decay, the FCC lattice swelled between the manufacturing of the samples and the date of the experiments. Thus, the obtained lattice parameters were corrected to account for this self-irradiation, using the equation of Kato et al. [18]. Using this corrected values, the O/M ratios of each phase and the global O/M ratios were determined using the relation of Duriez et al. [19]. Table 2 summarizes the number of phases found, their lattice parameters and the O/M ratios of the samples, except for the MOX45. The O/M ratio of the latter was determined within the framework of the ESNII+ project deliverable 7.34, where the sample is referred to as 14KB0040 [20].

Further details on the phase determination for the MOX24, MOX29 and MOX31 samples can be found in [15]. For the MOX40-OM sample, one fluorite phase (Fm$\bar{3}$m, space group 225) was observed, corresponding to the $U_{1-y}Pu_yAm_zO_{2\pm x}$ phase. On the contrary, two fluorite phases were obtained for the MOX29-OM sample. One phase was found to correspond to the $U_{1-y}Pu_yAm_zO_{2\pm x}$ matrix phase at a proportion of 92%, while the other phase corresponds to $UO_2$ agglomerates, based on the obtained lattice parameter. Fouquet-Métivier et al. previously observed these two phases in aged samples, along with $UO_2$-rich agglomerates measuring a few tens of microns [15].

To perform the melting experiments, disks of approximately 1 mm thickness were cut out of the full pellets.

<table><caption>**Table 1**

Chemical compositions and densities of the samples at the time of this study.</caption><thead><tr><th>Samples</th><th>Chemical composition determined by ICP-MS and TIMS</th><th>O/M ratios determined by XRD (± 0.005)</th><th>Density determined by immersing method in bromobenzene (g.cm<sup>-3</sup>) (± 0.06)</th></tr></thead><tbody><tr><td>MOX24</td><td>y = 0.235 ± 0.01<br/>z = 0.005 ± 0.0005</td><td>1.985 [15]</td><td>10.29</td></tr><tr><td>MOX29</td><td>y = 0.28 ± 0.01<br/>z = 0.01 ± 0.001</td><td>1.984 [15]</td><td>10.37</td></tr><tr><td>MOX29-OM</td><td>y = 0.28 ± 0.01<br/>z = 0.01 ± 0.001</td><td>1.998</td><td>10.37</td></tr><tr><td>MOX31</td><td>y = 0.286 ± 0.01<br/>z = 0.024 ± 0.002</td><td>2.004 [15]</td><td>10.75</td></tr><tr><td>MOX40</td><td>y = 0.39 ± 0.01<br/>z = 0.01 ± 0.001</td><td>1.976 [16]</td><td>10.50</td></tr><tr><td>MOX40-OM</td><td>y = 0.39 ± 0.01<br/>z = 0.01 ± 0.001</td><td>1.997</td><td>10.50</td></tr><tr><td>MOX45</td><td>y = 0.44 ± 0.01<br/>z = 0.01 ± 0.001</td><td>2.00</td><td>9.60</td></tr></tbody></table>
<table><caption>**Table 2**
Phases, lattice parameters and O/M ratios determined by XRD.</caption><thead><tr><th>Samples</th><th>Phases</th><th>Proportion</th><th>Raw lattice parameter (± 0.001) (Å)</th><th>Corrected lattice parameter (Kato et al. [18]) (± 0.001) (Å)</th><th>O/M ratio (± 0.005)</th><th>Global O/M ratio (± 0.005)</th></tr></thead><tbody><tr><td rowspan="2">MOX24 [15]</td><td>Matrix</td><td>> 95 %</td><td>5.473</td><td>5.457</td><td>1.984</td><td>1.985</td></tr><tr><td>U<sub>1-y</sub>Pu<sub>y</sub>Am<sub>z</sub>O<sub>2±x</sub></td><td>&lt; 5 %</td><td>5.483</td><td>-</td><td>2.000</td><td>-</td></tr><tr><td rowspan="2">MOX29 [15]</td><td>Matrix</td><td>92 %</td><td>5.469</td><td>5.453</td><td>1.982</td><td>1.984</td></tr><tr><td>U<sub>1-y</sub>Pu<sub>y</sub>Am<sub>z</sub>O<sub>2±x</sub></td><td>8 %</td><td>5.482</td><td>-</td><td>2.000</td><td>-</td></tr><tr><td rowspan="2">MOX31 [15]</td><td>Matrix</td><td>95 %</td><td>5.463</td><td>5.447</td><td>2.005</td><td>2.004</td></tr><tr><td>U<sub>1-y</sub>Pu<sub>y</sub>Am<sub>z</sub>O<sub>2±x</sub></td><td>5 %</td><td>5.475</td><td>-</td><td>2.000</td><td>-</td></tr><tr><td>MOX40 [16]</td><td>Matrix</td><td>100 %</td><td>5.456</td><td>5.448</td><td>-</td><td>1.976</td></tr><tr><td rowspan="2">MOX29-OM</td><td>Matrix</td><td>84 %</td><td>5.449</td><td>-</td><td>1.998</td><td>1.998</td></tr><tr><td>U<sub>1-y</sub>Pu<sub>y</sub>Am<sub>z</sub>O<sub>2±x</sub></td><td>16 %</td><td>-</td><td>-</td><td>2.000</td><td>-</td></tr><tr><td>MOX40-OM</td><td>Matrix</td><td>100 %</td><td>5.442</td><td>-</td><td>-</td><td>1.997</td></tr></tbody></table>

## 2.2. Experimental procedure

### 2.2.1. Melting temperature

The laser heating setup developed at the Joint Research Centre Karlsruhe (Germany) in the last decades was used. This experimental set-up is only briefly described here and further details can be found in [1,4,9,13,21].

The disks were held in a vertical position by means of graphite screws in a graphite sample holder, which was then placed in an airtight autoclave. A continuous flow of argon (5 L·h⁻¹), with an overpressure of 0.3 MPa to minimize vaporization, was used to flush the chamber.

A 4.5 kW Nd:YAG continuous-wave laser operating at 1064 nm was used to emit a short pulse (500 ms) of 250 W, which was focused onto a 3-mm-diameter circular beam on the sample surface. The radiance temperature $T_λ$ at the sample's surface was measured by a fast two-channel pyrometer, one of the channels operating near 650 nm, with a bandwidth of 27 nm and a nominal measurement spot size of 0.5 mm in diameter. This temperature corresponds to the one measured by a pyrometer calibrated in terms of blackbody radiance (emissivity equal to unity). An emissivity of 0.83 ± 0.05 was chosen, as experimentally determined for $UO_2$ and $PuO_2$ respectively by Manara et al. and De Bruycker et al. [9,12]. This value is consistent with the emissivity measured in previous studies on $U_{1-y}Pu_yO_2$ mixed oxides by De Bruycker et al., Böhler et al. and Strach et al. [1,4,5].

After each laser shot, the sample was allowed to cool naturally before the next pulse. The heating and cooling rates were adapted to the needs of each experiment. The conditions of the shots for each sample are reported in Appendix A. Additionally, when cracks were observed at the surface of the disks before melting, pre- and post-heating steps at 1700 K were performed to minimize thermal shocks and ensure the thermomechanical stability of the pellet. In some cases, power ramps performed before or after the shots were added to improve the quality of the thermograms and allow an easier determination of the thermal arrest. Between four and nine shots were successively performed on the same face of the sample.

The uncertainties in the measured temperatures were estimated according to the propagation error law [13], considering the uncertainties associated with the pyrometer calibration, the emissivity and the transmittance of the optical system. With a coverage factor $k = 2$, the cumulative uncertainty was estimated to be ± 30 K at 3000 K for $U_{1-y}Pu_yAm_zO_{2±x}$, i.e. ± 1 % of the recorded temperature.

### 2.2.2. Oxygen partial pressure

Two SETNAG oxygen gauges (one GEN'AIR and one JOK'AIR), located at the inlet and outlet of the chamber, enabled monitoring of the oxygen partial pressures every 3 s during the experiments [14].

The quantity of oxygen absorbed or released by the sample during each shot was calculated by integrating the variation in oxygen partial pressure. In this study, it was assumed that only the sample could react with the residual oxygen present in the chamber. A decrease in the oxygen partial pressure corresponds to the oxygen uptake by the sample, and its subsequent oxidation during melting. Conversely, an increase in the oxygen partial pressure corresponds to the release of oxygen from the sample, meaning its reduction during melting.

The oxygen partial pressure measured at each time step was then converted to a mass flux of dioxygen according to the following formula (1):

$$ \dot{m}_{O_2}(t) = \frac{p_{O_2}(t) M_{O_2} \dot{V}}{RT} \qquad (1) $$

Where $m_{O_2}(t)$ is the mass flux of dioxygen (in g·s⁻¹), $p_{O_2}(t)$ is the oxygen partial pressure measured by the setup (in atm), $M_{O_2}$ is the molar mass of dioxygen (equal to 32 g·mol⁻¹), $\dot{V}$ is the volumetric flux (equal to 5 L·h⁻¹), $R$ is the gas constant (taken as 0.0825 L·atm·K⁻¹·mol⁻¹) and $T$ is the temperature of the gas (300 K).

Eq. (1) was integrated using the trapezoidal rule between the initial time and the time at which the atmosphere restabilized to its initial value. A residual oxygen content of a few vpm ($p_{O_2} \approx 10^{-6}$ atm) was measured in the argon used in this study.

This result was then corrected from the baseline to determine the mass of dioxygen absorbed or released by the sample. The obtained dioxygen mass was then converted into the number of moles of dioxygen. Knowing the initial oxygen stoichiometry of each sample, the O/M ratio was deduced after each shot.

In this study, the uncertainty of the resulting O/M ratio was assumed to be ± 0.005.

During the experiments, a time shift in the measurement of the $p_{O_2}$ was observed compared to the moment at which the laser shot was performed. Indeed, the latter lasted 500 ms, while the oxygen partial pressure was measured every 3 s at the exit of the chamber. In addition, waiting steps ranging from 30 min to 2h30 were performed for the oxygen partial pressure recorded at the exit of the experimental chamber to restabilize to its residual value. This resulted in much longer recording of $p_{O_2}$ compared to the shots.

## 3. Results

The determination of the solidus temperatures and the assessment of the O/M ratio is here detailed for the MOX29 sample. The same methodology was applied to all the samples, and the results are described below. The solidus temperatures and O/M ratios determined in this work for all the samples can be found in Appendix B.
## 3.1. MOX29

### 3.1.1. Solidus temperatures

Eight successive shots were performed under three different conditions (see Appendix A). Shots 1 and 2 involved both a pre-heating (30 s at 1700 K) and post-heating (15 s at 1700 K) steps, while shots 3 to 6 only involved a pre-heating step. Finally, shots 7 and 8 did not involve any pre- or post-heating step.

Fig. 1 shows the thermogram recorded by the pyrometer in purple and the laser power profile in black for shot 7. Note that the pyrometer's minimum limit of detection is 1500 K.

As already observed by various authors [4,22] on $U_{1-y}Pu_yO_{2\pm x}$ samples, only one thermal arrest is visible, even though non-congruent melting is expected for these mixed oxides. Böhler et al. [4] explained this behaviour thanks to Phase-Field simulations, considering the cooling rate of the experiment, the thermal conductivity of the samples, the diffusivity of the species, etc. Indeed, these authors stated that in order to distinguish a liquidus and a solidus, the molecular diffusion of the actinides needs to be faster than the cooling rate of the melting experiment, which is not the case, thus leading to the determination of only one thermal arrest in most cases. Additionally, they attributed this inflection point to the sample solidification and thus to the solidus temperature.

Fig. 2 compares the eight thermograms recorded for MOX29 in the range of the solidus temperature. Overall, similar heating/cooling curve profiles are observed. The slight offsets in time and in temperature visible in the first two curves (blue and brown) are assumed to be related to the post-heating step. A slight time offset is observed between the shots 3 to 6 and the shots 7 and 8, indicating a slight delay in the thermogram due to the pre-heating step. However, this step does not seem to affect the solidus temperature value. The solidus temperatures are reported in Fig. 3.

Throughout the shots, a slight decrease in the solidus temperature is noticed, ranging from 3060 ± 30 K for shot 1 to 3035 ± 30 K for shot 8. A stabilization around 3037 K is observed from shot 5 to 8. The first two shots, which were performed with pre- and post-heating steps, have slightly higher solidus temperatures (3060 and 3064 K respectively) than the other shots (~ 3040 K), though they remain within the experimental uncertainties. Looking at shots 3 to 6, which were performed with only a pre-heating step, no significant difference is observed among them or with the shots 7 and 8 (performed with no pre- or post-heating step). Thus, only the post-heating appears to have an influence on the solidus temperature, while the pre-heating step has no effect. However, the small decrease in the melting temperature lies

within an uncertainty of ± 30 K; thus, no clear conclusion can be drawn about this trend.

As Fig. 3 shows, the measurements are reproducible and the average solidus temperature obtained for the eight shots is 3046 ± 30 K (dashed line in Fig. 3).

### 3.1.2. Estimation of the O/M ratio along the shots

Fig. 4 plots the oxygen partial pressure measured at the exit of the experimental chamber for MOX29 throughout the shots. The dashed red line corresponds to the oxygen content in the Ar gas used ($pO_2 = 3.5 \times 10^{-6} \text{ atm}$).

A decrease in $pO_2$ at the exit of the setup is observed after each shot. For shot 1, the oxygen partial pressure decreases to $1 \times 10^{-17} \text{ atm}$, reaching a value of $10^{-15} \text{ atm}$ for the shots 4 to 6. For shots 7 and 8, only a very slight depletion in oxygen was observed. As described in section 2.2.1, the quantity of oxygen absorbed by the MOX29 corresponds to 10 to 15 µg per shot for the shots 1 to 6 (all performed with pre-heating), while this quantity was only around 1 µg for the shots 7 and 8 (performed without pre-heating).

The diameter of the molten pool was measured for each sample using the pictures of the surface taken before and after the shots (Fig. 5). The molten pool corresponds to the bright surface visible in Fig. 5b. An average value of $3 \pm 0.3 \text{ mm}$ was determined for all the samples. This value also corresponds to the size of the laser spot.

To assess the variation in the O/M ratio of the samples throughout the successive shots, the following hypotheses were formulated. The accuracy of these hypotheses will be discussed in Section 4.1.

* The oxygen is exchanged only by the molten pool.

* The molten pool of the samples exhibits a thickness of 150 µm, as measured by Epifano et al. [23] via SEM analyses of $U_{1-z}Am_zO_{2\pm x}$ molten samples obtained under similar conditions.

* As reported in [24] and [15], the samples are considered chemically homogeneous throughout the entire molten zone as only few uranium and plutonium agglomerates measuring ~10 and ~1 µm, respectively, are present in the matrix.

* A constant composition was assumed throughout the shots, meaning that no preferential vaporization of U, Pu or Am occurred.

* The O/M ratios obtained previously by XRD and given in Table 2 were used as initial values.

Due to a technical issue, the variation in $pO_2$ corresponding to shot 2 could not be recorded. Since shots 1 and 2 were performed under the same conditions (pre-heating + shot + post-heating), the same variation

<figure><img src="image_4.png" /><figcaption>Fig. 1. Thermogram (purple curve) recorded in a laser melting measurement of MOX29 in argon for the shot n°7, (a) with the laser power profile (black curve) and (b) zoom on the $T_{\text{solidus}}$ range.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Comparison of the eight thermograms recorded for MOX29 in the area of the solidus temperature.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Solidus temperature of MOX29 recorded for each shot in Ar. The dashed line corresponds to the average solidus temperature.</figcaption></figure>

in $pO_2$, and thus the same quantity of oxygen absorbed by the sample, were assumed for both shots. This hypothesis was validated by examining shots 3 to 6, which were performed in the same conditions and resulted in the same quantity of oxygen absorbed (approximately 10 µg).

The number of moles of $U_{1-y}Pu_yO_{2\pm x}$ in the molten pool was calculated to be $4.08 \times 10^{-5}$ mol. Based on the initial O/M ratio of 1.984 for MOX29, the corresponding number of moles of oxygen in the molten pool was determined to be $8.07 \times 10^{-5}$ mol. and used as initial value for the calculations. Fig. 6a shows the O/M ratios calculated after each shot based on the mass of oxygen absorbed by the molten pool of the sample. A gradual increase in the O/M ratio is observed throughout the shots, which corresponds to the progressive oxidation of the sample. The O/M ratio stabilizes at a final value of 2.009 for the two last shots (7 and 8). Fig. 6b plots the variation of the solidus temperature as a function of the O/M ratio after each shot in the MOX29. A decrease in the melting temperature is observed as the O/M ratio increases; however, this decrease lies within the experimental uncertainties. A plateau is reached for $2.005 < \text{O/M} < 2.010$ at $T = 3035–3040$ K.

## 3.2. Other samples

The same methodology was applied to the other samples to determine the variation in their solidus temperatures and O/M ratios during melting. The variations in the oxygen partial pressure measured for each sample are available in Appendix C. Three types of behaviours were observed:

* The $pO_2$ variations measured for the MOX24 and MOX40 samples are similar to those recorded for the MOX29 sample, which corresponds to the oxidation of the samples. Unfortunately, a technical issue during the recording of $pO_2$ for the first shot of MOX40 altered the curve's shape, though it was still possible to quantify the variation.

* With a systematic increase of $pO_2$, the MOX45 and MOX29-OM samples were reduced during the shots.

* For the MOX31 and MOX40-OM samples, an oxidation of the samples is observed during the first shot, followed by a slight reduction during subsequent shots.

Fig. 7 plots the resulting variations in solidus temperatures as a function of the O/M ratio for all samples. For clarity, the uncertainties are not represented ($\pm$ 30 K for the solidus temperature and $\pm$ 0.005 for the O/M ratio).

A slight increase in the solidus temperature was observed for the MOX24 and MOX40 when the O/M ratio increased, however within the experimental uncertainties, as previously observed for MOX29. Conversely, an increase in the solidus temperature was observed when decreasing the O/M ratio for the MOX31, MOX45, MOX29-OM and MOX40-OM samples. This is in agreement with the work of Kato et al. [3].

Looking at samples with the same composition but different initial O/M ratios, namely MOX29/MOX29-OM and MOX40/MOX40-OM, opposite behaviours are observed. Indeed, the samples with an initial O/M ratio of approximately 1.98 were oxidized, while the annealed samples with an O/M ratio close to 2.00 were reduced during the melting.

## 3.3. Comparison with literature data

Fig. 8 compares the solidus temperatures measured in this study to literature data with similar plutonium and americium contents. It should be noted that, in most previous studies, the O/M ratio was assumed to remain constant after the measurements, even though it was never measured during the measurements.
<figure><img src="image_7.png" /><figcaption>Fig. 4. Variation of the oxygen partial pressure measured at the exit of the experimental chamber throughout the eight shots for MOX29. The dashed line corresponds to the residual $pO_2$.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. MOX29 sample (a) before shot 1 and (b) after shot 1.</figcaption></figure>

For $y \approx 0.24$, our results are in good agreement with the data obtained by laser heating under argon by Böhler et al. [4], assuming an O/M ratio close to 2.00. Under the same conditions, Strach et al. [5] obtained a value 30 K lower. However, this difference falls within the experimental uncertainty range. For $y = 0.28–0.30$, our results for the MOX29, MOX29-OM and MOX31 samples are in good agreement with Kato et al. [3]. Our solidus temperatures for the MOX40 and MOX40-OM samples are in very good agreement with the data from Böhler et al. [4] and Kato et al. [3], except for our experimental value at O/M = 1.997, where a slightly lower temperature was measured. Nevertheless, these small deviations remain within the experimental uncertainties. For $y \approx 0.45$, only one measurement was reported by Strach et al. [5], which was 20–30 K lower than in this study.

## 4. Discussion

In addition to these differences in oxidation/reduction behaviours explained here, this study also observed the peculiar behaviour of the first shot for the MOX31 and MOX40-OM samples. Indeed, when examining their $pO_2$ variation as a function of the shots (see Appendix

C), oxygen uptake is observed during the first shot, followed by oxygen release for the subsequent shots. This observation calls into question the reliability of the first shot. One possible explanation for this particularity could be residual heterogeneity in the molten pool due to the presence of small U and/or Pu/Am agglomerates, as well as cracks and pores. Therefore, it could be recommended to not use the first shots when examining the variation of the O/M ratio as a function of the solidus temperature, in order to ensure a completely homogenized surface of the molten pool.

### 4.1. Influence of the hypotheses for the estimation of the O/M ratio

The assumptions made in this study to quantify the variation of the O/M ratio during the melting can be questioned. To address this, the example of the MOX29 sample will be used.

The first hypothesis relates to the dimensions of the sample zone that reacts with the atmosphere. In this study, it was assumed that only the molten pool absorbed or released oxygen. One of the main uncertainties is the thickness of the molten pool and thus of the zone reacting with oxygen. Indeed, no experimental determination could be performed in
<figure><img src="image_9.png" /><figcaption>Fig. 6. (a) Variation of the O/M ratio of MOX29 obtained after each shot and (b) Variation of the solidus temperature of MOX29 as a function of the O/M ratio. The dashed line corresponds to the initial O/M ratio.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 7. Variation of the solidus temperature as a function of the O/M ratio for the MOX24, MOX29-OM, MOX31, MOX40, MOX20-OM and MOX45 samples. The dashed lines correspond to the initial O/M ratio of the samples.</figcaption></figure>

this study. As previously mentioned, Epifano et al. measured it by SEM on U₁-zAmzO₂±x samples [23] to be between 150 and 200 µm. However, an intermediate partly-molten zone was evidenced beyond that point with round grains beginning to connect, so uncertainties remain. To address the influence of this parameter, calculations of the O/M ratio variation were performed for three different cases for the MOX29

sample:

* Case 1: Uptake of oxygen by the entire sample (5 mm in diameter and 1.7 mm thick).

* Case 2: Uptake of oxygen by the molten pool and its periphery (6% of the total sample volume in both cases), corresponding to:
<figure><img src="image_11.png" /><figcaption>Fig. 8. Comparison of the solidus temperatures obtained in this work with the existing literature [3–5].</figcaption></figure>

○ a region with $\Phi = 4.7$ mm (molten pool + 30 % periphery) and $e = 150$ µm thickness,

○ or a region with $\Phi = 3.6$ mm (molten pool + 10 % periphery) and $e = 250$ µm.

• Case 3: Uptake only by the molten pool (2 % of the sample, $\Phi = 3$ mm and $e = 150$ µm), corresponding to the chosen hypothesis in this work.

In case 1, the O/M ratio shows almost no variation during the melting experiments, with initial and final values of 1.984 and 1.985 respectively ($\Delta(\text{O/M})=0.001$). Consequently, no variation in melting behaviour would have been observed, which is inconsistent with the observed variation in solidus temperature. Case 2, which has an absorbing zone consisting of a molten pool and a 30 % or 10 % periphery, is more consistent with the variation in the solidus temperatures. The initial O/M ratio is 1.984, and the final value is 1.994 ($\Delta(\text{O/M})=0.010$). In the third case scenario, the initial hypothesis leads to an O/M ratio variation from 1.984 to 2.009 ($\Delta(\text{O/M})=0.025$). As expected, the largest variation in the O/M ratio occurs in the smallest reactive zone.

Thus, the size of the reactive zone in contact with the atmosphere significantly influences the final O/M ratio of the sample. However, it remains difficult to choose between the cases 2 and 3. The underlying question is how much of the sample volume is heated during a laser-

heating melting experiment. To answer this question, Welland et al. coupled simulations and experiments using the same laser heating technique employed in this work on UO₂ to investigate this phenomenon [25]. The authors calculated the temperature gradient along the radius and the depth of the sample during melting. They demonstrated that, for a 290 ms pulse at 810 W (i.e., an energy of 235 J), the temperature at a radius distance of 4 mm and a depth of 500 µm from the centre is only 1600 K if the centre of the laser spot is heated at around 3000 K. Although the laser pulse in our study is longer (500 ms), it is performed at a significantly lower power (250 W), resulting in lower shot energy (125 J). Since the temperature gradient depends on the laser power, pulse duration, and the pulse energy, lower temperatures than those determined by Welland et al. are expected in the immediate vicinity of the molten pool in our study. Considering the temperature gradients that occur during the melting within the sample, it is impossible to define the precise boundaries of the region absorbing oxygen. Previously, it was assumed that only the molten pool reacted with the oxygen present in the atmosphere. Indeed, with a laser pulse duration of 500 ms, oxygen diffusion within the solid sample is insignificant (calculated to be a few nanometres at the melting temperature, according to [26,27]).

Additionally, it was assumed that the uranium, plutonium, and americium contents were homogeneous within the pellet and remained constant throughout the experiments. There are two post-melting characterizations studies available in the literature on U₁,yPu,yO₂±x:
one by De Bruycker et al. and one Böhler et al. [1,4], and one study on $U_{1-z}Am_zO_{2\pm x}$ by Epifano et al. [23]. For $U_{1-y}Pu_yO_{2\pm x}$, De Bruycker et al. measured the uranium and plutonium contents and Böhler et al. measured them by XRD. Both studies concluded that no change in the chemical composition occurred during the melting. In contrast, Epifano et al. found that the uranium and americium contents increased by 2–3 % after melting in samples containing 20, 30 and 50 % of Am, as determined through XAS experiments. As the authors stated, caution should be exercised when considering these results, as a reversible segregation phenomenon occurring at high temperatures cannot be ruled out. In this work, only visual inspections of the sapphire window of the experimental chamber and the graphite sample holder were performed between each shot to detect potential vaporization of the sample. Although no black powder was found, slight condensation traces were observed on the graphite sample holder for some shots. Unfortunately, as the samples were breaking into pieces when being removed from the experimental chamber, they could not be weighted after the experiments to confirm or not the calculated variations of the O/M ratios. However, note that even if the sample was weighted, as no post-melting characterizations were performed, it would not have been possible to determine if the possible weight change would have been caused by the variation of O/M ratio or possible changes in the U, Pu and Am contents. Thus, based on these works of De Bruycker et al. and Böhler et al. on $U_{1-y}Pu_yO_{2\pm x}$ and as no post-melting characterizations were performed in this work, to simplify the hypotheses, it was assumed here that no changes in the uranium, plutonium and/or americium content occurred during the melting experiments.

## 4.2. Effect of the Pu and Am contents

Fig. 9 plots the variation of the measured solidus temperature with the plutonium content. The data were selected from samples with the same americium content (approximately 1 %, i.e. MOX24, MOX29-OM, MOX40-OM and MOX45) and an O/M ratio of about 1.988.

The solidus temperature decreases linearly as the plutonium content increases. This trend aligns well with the experimental data of Kato et al., Böhler et al. and Strach et al. [3–5] and the thermodynamic calculations of Fouquet-Métivier et al. [11]. This is consistent with the melting point of $UO_{2.00}$ (3147 ± 20 K [9]), which is higher than the one of $PuO_{2.00}$ (3050 ± 60 K [4]). However, De Bruycker et al. [21] and Böhler et al. [4] reported that a minimum in the solidus temperature is

<figure><img src="image_12.png" /><figcaption>Fig. 9. Variation of the solidus temperature as a function of the plutonium content ($z = 0.01$ and O/M = 1.988).</figcaption></figure>

expected for $0.50 < y < 0.80$. Therefore, the linear decrease in the temperature with the plutonium content can only be confirmed up to $y = 0.45$. Further measurements on higher plutonium contents are needed to precisely locate this minimum.

Fig. 10 shows the influence of the americium content (up to 2 %) on the solidus temperatures. Data for a similar Pu content ($y \approx 0.28–0.30$) and O/M ratio (2.00) were selected. The solidus temperatures of MOX29 and MOX31 are compared to the value measured by Kato et al. [3].

A slight increase in the solidus temperature is observed, with $3030 \pm 30$ K for $z = 0.006$, $3047 \pm 30$ K for $z = 0.01$ and $3058 \pm 30$ K for $z = 0.02$. This observation disagrees with the work of Kato et al., in which they observed a 4 K decrease per mol. % of $AmO_2$ in the sample. Furthermore, Prieur et al. measured the solidus temperature of Am-MOX ($U_{0.74}Pu_{0.22}Am_{0.035}Np_{0.005}O_{2-x}$) and Np-MOX ($U_{0.74}Pu_{0.22}Am_{0.02}Np_{0.02}O_{2-x}$), [28] and compared them to the results of $U_{1-y}Pu_yO_{2\pm x}$, which had a similar Pu content, measured by Böhler et al. [4]. The authors observed a decrease in the temperature of a few tens of K in the case of the $U_{1-y}Pu_yAm_zO_{2\pm x}$. In these studies, the authors assumed that there was no change in the samples’ chemical composition or O/M ratio during the melting. Furthermore, care should be taken when comparing Prieur et al. and Böhler et al. because the samples have different plutonium content and O/M ratios.

The observed increase in temperature when increasing the americium content is inconsistent with the trend of the melting temperatures of pure end members (3147 ± 20 K [9] for $UO_{2.00}$, 3050 ± 60 K [4] for $PuO_{2.00}$ and 2386 K [10] for $AmO_{2.00}$) and previous studies on $U_{1-z}Am_zO_{2\pm x}$ samples [22,29]. In the study by Epifano et al. on $U_{1-z}Am_zO_{2\pm x}$ mixed oxides, a decrease in the solidus temperature was observed as the Am content increased. Post-melting characterizations were also carried out on these samples, and no change in the O/M ratio during the melting was determined. However, care should be taken when considering the melting point of $AmO_{2.00}$ as only one experimental study exists, where the sample likely does not correspond to stoichiometric $AmO_{2.00}$, but rather a significantly reduced $AmO_{2-x}$ oxide, for which the O/M ratio is unknown.

Additionally, one must consider the various sources of uncertainties, primarily the temperatures (± 30 K in this study and ± 20 K in the study of Kato et al.), and the O/M ratio (± 0.005), as well as the plutonium content (± 0.1 %). Considering all the temperature uncertainties, one can conclude that for an americium content of up to 2 % Am has no significant effect on the melting temperature because the slight

<figure><img src="image_13.png" /><figcaption>Fig. 10. Variation of the solidus temperature as a function of the americium content (Pu content = 0.28–0.30 and O/M = 2.00).</figcaption></figure>
variations in solidus temperatures remain within the experimental uncertainties. Further experiments on samples with a larger range of americium content are needed to conclude on the influence of americium on the solidus temperatures of $U_{1-y}zPu_yAm_zO_{2\pm x}$ samples.

### 4.3. Effect of the O/M ratio

In our study, the variation of the O/M ratio along the shots was quantified during the experiments. This study shows that when a gas is flowing through the experimental chamber, changes in the O/M ratio cause an evolution of the solidus temperatures. Only one study in the literature addresses the influence of the O/M ratio on the solidus temperature (Kato et al. by thermal arrest on hypo-stoichiometric $U_{1-y}zPu_yAm_zO_{2\pm x}$ [3]). Starting with an O/M ratio of 2.00, the authors observed an increase in the solidus temperature as the O/M ratio decreased up to 1.9. However, this was not confirmed by *in situ* measurements, and it was not possible to measure the O/M variation because the used crucibles were closed. Our work confirms this increase in solidus temperature when decreasing the O/M ratio, except for MOX24 and MOX40 as previously discussed. These variations lie within the experimental uncertainties.

In addition to the influence of the O/M ratio variation on the solidus temperature, the initial O/M ratio of the sample before the melting also influences the oxidation/reduction behaviour of the sample during the experiments. Indeed, the samples that were initially hypo-stoichiometric (MOX24 at 1.985, MOX29 at 1.984 and MOX40 at 1.976) were oxidized during the experiments. Conversely, samples that were already stoichiometric or hyper-stoichiometric (MOX31 at 2.004 and MOX45 at 2.000) were reduced. The influence of the initial O/M ratio is demonstrated by the laser melting experiments performed on MOX29 and MOX40 samples with an initial O/M ratio of approximately 1.98, and on MOX29-OM and MOX40-OM samples with an initial O/M ratio close to 2.00. Fig. 11 compares the solidus temperature as a function of the O/M ratio. The arrows on the plots indicate the oxidizing/reducing behaviour of the mixed oxides.

The difference in the oxidation/reduction behaviour of the samples as a function of their initial O/M ratio can be explained by thermodynamic calculations on the U-Pu-Am-O system. To this aim, the CALPHAD method was used [30] and the calculations were performed with the Thermo-Calc software (version 2024a) [31]. The following models were used in this work: Kurata for U-Pu [32], Guéneau et al. for U-O [33],

Epifano et al. for Am-O [34], Guéneau et al. modified for the liquid by Fouquet-Métivier et al. for Pu-O and U-Pu-O [11,33], Epifano for U-Am-O [22,35], and Fouquet-Métivier for Pu-Am-O [24].

The oxygen potentials of the samples were calculated as a function of the temperature for their initial O/M ratio and were compared to the oxygen potential of the gas (Ar with $pO_2 = 3 \times 10^{-6}$ atm) in Fig. 12. In these calculations, the gas is assumed to govern the oxygen potential of the system (Sample + Gas).

The oxidation or reduction behaviour that occurs during the melting is conditioned by the temperature $T_{ox/red}$, which is the temperature at which the oxygen potential of the gas is equal to the one of the sample. Above this temperature, a reduction of the sample is predicted. Conversely, below the $T_{ox/red}$ temperature, an oxidation of the sample is expected, as the oxygen potential of the gas is predominant. As Fouquet-Métivier et al. demonstrated on the solidification paths in [11], the equilibrium O/M ratio is assumed to be fixed for $2000 < T < 2350$ K (temperature range represented by the yellow rectangle in Fig. 12). Two cases can be distinguished:

* For initially hypo-stoichiometric samples (MOX24, MOX29 and MOX40), the oxygen potential of the samples is lower than the one of the gas within the temperature range of interest. Thus, the samples will be oxidized for $T < 2350$ K, whereas above 2350 K, the samples could be reduced.

* For initially near- or hyper-stoichiometric samples (MOX31, MOX45, MOX40-OM and MOX29-OM), the oxygen potentials of the samples are higher than the one of the gas for $2000 < T < 2350$ K. This allows the samples to be reduced during the melting.

Considering that the O/M ratio is fixed for $2000 < T < 2350$ K, comparing the oxygen potentials of the samples and the melting atmosphere explains the different oxidation/reduction behaviours observed during the tests, depending on the initial O/M ratio of the samples.

### 5. Conclusion

This study investigated the melting behaviour of several $U_{1-y}zPu_yAm_zO_{2\pm x}$ samples using a laser heating technique. Thanks to the recent addition of oxygen gauges at the entrance and exit of the experimental chamber, the variation in oxygen partial pressure inside the chamber, related to changes in the oxygen stoichiometry of the samples,

<figure><img src="image_14.png" /><figcaption>Fig. 11. Variation of the solidus temperature as a function of the O/M ratio for (a) the MOX29 and MOX29-OM samples and (b) MOX40 and MOX40-OM samples.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Oxygen potentials as a function of temperature of the MOX samples, compared to the oxygen potential of the gas (Ar with $pO_2 = 3.10^{-6}$ atm). The yellow rectangle represents the temperature range where the equilibrium O/M ratio would be reached during cooling.</figcaption></figure>

was recorded during the experiments. For the first time, the variation of the O/M ratio after each shot was quantified based on several hypotheses.

A good agreement between this work and the available experimental data in the literature was found for the solidus temperatures. The accuracy of the hypotheses formulated to calculate the O/M ratio was discussed. It was assumed that only the molten pool absorbed or released oxygen. Consistent with previous studies, an increase in the Pu content was found to decrease the solidus temperature. However, a very small increase in the solidus temperature was observed here when increasing the americium content. This finding is in disagreement with the studies of Kato et al. [8] and Prieur et al. [28], but remains within the experimental uncertainties. It can be concluded that the effect of low Am contents is insignificant. Finally, the effect of the initial O/M ratio on the melting behaviour of the samples was studied. Hypo-stoichiometric samples tend to oxidize, while hyper- or stoichiometric samples are reduced. Thermodynamic calculations on the equilibrium between the samples and the gas phase explained their behaviour, assuming that the O/M ratio is fixed within the temperature range of 2000 - 2350 K during cooling.

This work sheds light on the melting behaviour of $U_{1-y}Pu_yAm_zO_{2-x}$ mixed oxides by studying the simultaneous influence of the plutonium and americium content, as well as of the oxygen stoichiometry. These new sets of solidus temperatures and O/M ratios are essential data for the thermodynamic assessment of the U-Pu-Am-O system. However, there are still some uncertainties. Further experiments are needed to precisely determine the size of the molten zone and the exact quantity of gas that reacts during the experiments. Additionally, the O/M ratio calculated here could not be verified experimentally. Thus, post-melting characterizations, such as XAS or XRD given the limited amount of material available in the molten pool (10–20 mg), are needed to confirm the various hypotheses made to calculate the changes in the O/M ratio.

### CRediT authorship contribution statement

**Pauline Fouquet-Métivier:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Conceptualization. **Davide Robba:** Writing – review & editing, Visualization, Validation, Resources, Methodology, Investigation, Conceptualization. **Luka Vlahovic:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Methodology, Investigation, Conceptualization. **Dragos Staicu:** Writing – review & editing, Validation, Supervision, Resources. **Christine Guéneau:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis. **Philippe M. Martin:** Writing – review & editing, Validation, Supervision, Methodology, Formal analysis, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

This work was supported by the Cross-cutting basic research Program (RTA Program) of the CEA Energy Division. The authors would like to thank the ESFR-SMART project (funded by the Euratom research and training programme 2014–2018 under grant agreement No 754501) for providing the $(U,Pu,Am)O_{2+x}$ samples. P. Fouquet-Métivier thanks the ENEN European H2020 project (grant agreement No 755576) and the INSPYRE project (Euratom research and training programme 2014–2018, grant agreement No 7543289) for funding mobilities at the JRC Karlsruhe.

## Appendix A – Conditions of the shots performed for the different MOX samples
<table><thead><tr><th>Sample</th><th>Shots n°</th><th>Pre-heating</th><th>Shot</th><th>Post-heating</th></tr></thead><tbody><tr><td rowspan="2">MOX24</td><td>1 to 4</td><td>10 s at 40 W</td><td rowspan="2">500 ms at 250 W</td><td>/</td></tr><tr><td>5 to 8</td><td>/</td><td>/</td></tr><tr><td rowspan="3">MOX29</td><td>1 & 2</td><td>30 s at 40 W</td><td rowspan="3"></td><td>15 s at 40 W</td></tr><tr><td>3 to 6</td><td>30 s at 40 W</td><td>/</td></tr><tr><td>7 & 8</td><td>/</td><td>/</td></tr><tr><td rowspan="2">MOX31</td><td>1 & 2</td><td>0 to 60 W in 1 s</td><td>/</td><td>/</td></tr><tr><td>3 to 6</td><td>0 to 60 W in 500 ms</td><td>/</td><td>/</td></tr><tr><td>MOX40</td><td>1 to 6</td><td>/</td><td>/</td><td>/</td></tr><tr><td>MOX45</td><td>1 to 5</td><td>/</td><td>/</td><td>/</td></tr><tr><td rowspan="6">MOX29-OM</td><td>1 to 7</td><td>/</td><td>/</td><td>/</td></tr><tr><td>8</td><td>/</td><td>/</td><td>250 to 40 W in 100 ms</td></tr><tr><td>9</td><td>/</td><td>/</td><td>200 to 40 W in 100 ms</td></tr><tr><td>1</td><td>/</td><td>/</td><td>/</td></tr><tr><td>2</td><td>/</td><td>/</td><td>250 to 40 W in 100 ms</td></tr><tr><td>3</td><td>/</td><td>/</td><td>250 to 40 W in 150 ms</td></tr><tr><td rowspan="5">MOX40-OM</td><td>4 to 6</td><td>/</td><td>/</td><td>250 to 40 W in 200 ms</td></tr><tr><td>7</td><td>/</td><td>/</td><td>/</td></tr><tr><td>8</td><td>/</td><td>/</td><td>250 to 100 W in 100 ms</td></tr><tr><td>9</td><td>/</td><td>/</td><td>/</td></tr></tbody></table>

## Appendix B – Solidus temperatures and O/M ratios recorded for each shot for MOX24, MOX29, MOX31, MOX40, MOX45, MOX29-OM and MOX40-OM

<table><thead><tr><th></th><th>Shot n°</th><th>Solidus temperature (K) ± 30 K</th><th>O/M ratio after shot (± 0.005)</th></tr></thead><tbody><tr><td rowspan="7">MOX24</td><td>1</td><td>3056</td><td>1.988</td></tr><tr><td>2</td><td>3057</td><td>1.991</td></tr><tr><td>3</td><td>3065</td><td>1.993</td></tr><tr><td>4</td><td>3065</td><td>1.996</td></tr><tr><td>5</td><td>3062</td><td>1.997</td></tr><tr><td>6</td><td>3062</td><td>1.997</td></tr><tr><td>7</td><td>3064</td><td>1.998</td></tr><tr><td rowspan="7">MOX29</td><td>8</td><td>3064</td><td>1.998</td></tr><tr><td>1</td><td>3060</td><td>1.988</td></tr><tr><td>2</td><td>3064</td><td>1.992</td></tr><tr><td>3</td><td>3050</td><td>1.998</td></tr><tr><td>4</td><td>3047</td><td>2.001</td></tr><tr><td>5</td><td>3037</td><td>2.005</td></tr><tr><td>6</td><td>3039</td><td>2.008</td></tr><tr><td rowspan="7">MOX31</td><td>7</td><td>3036</td><td>2.009</td></tr><tr><td>8</td><td>3035</td><td>2.009</td></tr><tr><td>1</td><td>3040</td><td>2.004</td></tr><tr><td>2</td><td>3056</td><td>2.001</td></tr><tr><td>3</td><td>3064</td><td>2.000</td></tr><tr><td>4</td><td>3058</td><td>1.999</td></tr><tr><td>5</td><td>3052</td><td>1.999</td></tr><tr><td rowspan="7">MOX40</td><td>6</td><td>3057</td><td>1.999</td></tr><tr><td>1</td><td>3020</td><td>1.984</td></tr><tr><td>2</td><td>3015</td><td>1.984</td></tr><tr><td>3</td><td>3030</td><td>1.985</td></tr><tr><td>4</td><td>3025</td><td>1.986</td></tr><tr><td>5</td><td>3031</td><td>1.987</td></tr><tr><td>1</td><td>2992</td><td>1.995</td></tr><tr><td rowspan="7">MOX45</td><td>2</td><td>2992</td><td>1.991</td></tr><tr><td>3</td><td>2990</td><td>1.989</td></tr><tr><td>4</td><td>3001</td><td>1.987</td></tr><tr><td>1</td><td>3019</td><td>1.997</td></tr><tr><td>2</td><td>3048</td><td>1.996</td></tr><tr><td>3</td><td>3060</td><td>1.995</td></tr><tr><td>4</td><td>3051</td><td>1.993</td></tr><tr><td rowspan="7">MOX29-OM</td><td>5</td><td>3044</td><td>1.992</td></tr><tr><td>6</td><td>3048</td><td>1.991</td></tr><tr><td>7</td><td>3052</td><td>1.990</td></tr><tr><td>8</td><td>3050</td><td>1.989</td></tr><tr><td>9</td><td>3056</td><td>1.988</td></tr><tr><td rowspan="7">MOX40-OM</td><td>1</td><td>2970</td><td>1.997</td></tr><tr><td>2</td><td>3000</td><td>1.995</td></tr><tr><td>3</td><td>3013</td><td>1.994</td></tr><tr><td>4</td><td>3017</td><td>1.992</td></tr><tr><td>5</td><td>3017</td><td>1.991</td></tr><tr><td>6</td><td>3025</td><td>1.990</td></tr><tr><td>7</td><td>3016</td><td>1.990</td></tr><tr><td rowspan="2">MOX40-OM</td><td>8</td><td>3021</td><td>1.989</td></tr><tr><td>9</td><td>3017</td><td>1.988</td></tr></tbody></table>
<figure><img src="image_16.png" /><figcaption>Appendix C – Variation of the oxygen partial pressure measured at each shot for MOX24, MOX29-OM, MOX31, MOX40, MOX40-OM and MOX45. The dashed line corresponds to the residual pO2</figcaption></figure>

## Data availability

The data supporting this article have been included in the Appendix B.

## References

[1] F. De Bruycker, et al., On the melting behaviour of uranium/plutonium mixed dioxides with high-Pu content: a laser heating study, J. Nucl. Mater. 419 (1) (2011) 186-193, https://doi.org/10.1016/j.jnucmat.2011.08.028.

[2] W.L. Lyon, W.E. Baily, The solid-liquid phase diagram for the UO₂-PuO₂ system, J. Nucl. Mater. 22 (3) (1967) 332-339, https://doi.org/10.1016/0022-3115(67)90051-7.

[3] M. Kato, K. Morimoto, H. Sugata, K. Konashi, M. Kashimura, T. Abe, Solidus and liquidus temperatures in the UO₂-PuO₂ system, J. Nucl. Mater. 373 (1) (2008) 237-245, https://doi.org/10.1016/j.jnucmat.2007.06.002.

[4] R. Böhler, et al., Recent advances in the study of the UO₂-PuO₂ phase diagram at high temperatures, J. Nucl. Mater. 448 (1-3) (2014) 330-339, https://doi.org/10.1016/j.jnucmat.2014.02.029.

[5] M. Strach, D. Manara, R.C. Belin, J. Rogez, Melting behavior of mixed U-Pu oxides under oxidizing conditions, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. At. 374 (2016) 125-128, https://doi.org/10.1016/j.nimb.2016.01.032.

[6] E.A. Aitken and S.K. Evans, *A Thermodynamic Data Program Involving Plutonia and Urania at High Temperatures*. Quarterly Report No. 4. 1968.

[7] K. Konno, T. Hirosawa, Melting temperature of mixed oxide fuels for fast reactors, J. Nucl. Sci. Technol. 39 (7) (2002) 771-777.

[8] M. Kato, Melting temperatures of oxide fuel for fast reactors, in: Int. Congr. Adv. Nucl. Power Plants 2009 ICAPP 2009 3, 2009, pp. 1940-1947.

[9] D. Manara, C. Ronchi, M. Sheindlin, M. Lewis, M. Brykin, Melting of stoichiometric and hyperstoichiometric uranium dioxide, J. Nucl. Mater. 342 (1) (2005) 148-163, https://doi.org/10.1016/j.jnucmat.2005.04.002.

[10] R.E. McHenry, Melting points of curium and americium oxides, Trans. Am. Nucl. Soc. 8 (1) (1965) 75.-.

[11] P. Fouquet-Métivier, et al., Investigation of the solid/liquid phase transitions in the U-Pu-O system, Calphad 80 (2023) 102523, https://doi.org/10.1016/j.calphad.2022.102523.

[12] F. De Bruycker, K. Boboridis, D. Manara, P. Pöml, M. Rini, R.J.M. Konings, Reassessing the melting temperature of PuO₂, Mater. Today 13 (11) (2010) 52-55, https://doi.org/10.1016/S1369-7021(10)70204-2.

[13] D. Manara, M. Sheindlin, W. Heinz, C. Ronchi, New techniques for high-temperature melting measurements in volatile refractory materials via laser surface heating, Rev. Sci. Instrum. 79 (11) (2008) 113901, https://doi.org/10.1063/1.3005994.

[14] L. Casini, et al., Melting behaviour of mixed plutonium and iron oxides: an experimental study, Nucl. Mater. Energy 41 (2024) 101761, https://doi.org/10.1016/j.nme.2024.101761.

[15] P. Fouquet-Métivier, et al., Insight into the cationic charge distribution in U₁-y₂Pu₃Am₂O₂-x mixed oxides, Inorg. Chem. 63 (43) (2024) 20482-20491, https://doi.org/10.1021/acs.inorgchem.4c03084.

[16] L. Medyk, et al., Determination of the plutonium content and O/M ratio of (U,Pu)O₂-x using Raman spectroscopy, J. Nucl. Mater. 541 (2020) 152439, https://doi.org/10.1016/j.jnucmat.2020.152439.

[17] J. Somers, et al., Status of the TRABANT irradiation experiments, in: Global'99: International Conference on Future Nuclear Systems-Nuclear Technology-Bridging the Millennia, Las Vegas, NV (United States), 1999.

[18] M. Kato, et al., Self-radiation damage in plutonium and uranium mixed dioxide, J. Nucl. Mater. 393 (1) (2009) 134-140, https://doi.org/10.1016/j.jnucmat.2009.05.020.

[19] C. Duriez, J.-P. Alessandri, T. Gervais, Y. Philipponneau, Thermal conductivity of hypostoichiometric low Pu content (U,Pu)O₂-x mixed oxide, J. Nucl. Mater. 277 (2-3) (2000), https://doi.org/10.1016/S0022-3115(99)00205-6.

[20] D. Staicu et al., "ESNI+ Deliverable D7-34 Characterisation measurement of properties fesh TRABANT fuel.pdf," D7.34, 2017.

[21] F. De Bruycker, K. Boboridis, P. Pöml, R. Eloirdi, R.J.M. Konings, D. Manara, The melting behaviour of plutonium dioxide: a laser-heating study, J. Nucl. Mater. 416 (1) (2011) 166-172, https://doi.org/10.1016/j.jnucmat.2010.11.030.

[22] E. Epifano, et al., Melting behaviour of uranium-amerium mixed oxides under different atmospheres, J. Chem. Thermodyn. (2019) 105896, https://doi.org/10.1016/j.jct.2019.105896.

[23] E. Epifano, et al., Melting behaviour of uranium-amerium mixed oxides under different atmospheres, J. Chem. Thermodyn. 140 (2020) 105896, https://doi.org/10.1016/j.jct.2019.105896.

[24] P. Fouquet-Métivier, Study of the Influence of Americium on Thermodynamic and Structural Properties of (U,Pu)O₂-x Mixed Oxides, Université Paris Saclay, 2022. PhD thesis.
[25] M.J. Welland, R. Böhler, L. Vlahovic, K. Boboridis, D. Manara, Co-development of experimental and simulation methods for the laser flash heating and melting technique: the thermoelastic effects of UO₂, Int. J. Therm. Sci. 132 (2018) 174–185, https://doi.org/10.1016/j.ijthermalsci.2018.05.035.

[26] K. Lassmann, The oxired model for redistribution of oxygen in nonstoichiometric uranium-plutonium oxides, J. Nucl. Mater. 150 (1) (1987) 10–16, https://doi.org/10.1016/0022-3115(87)90088-2.

[27] M. Kato, et al., Oxygen chemical diffusion in hypo-stoichiometric MOX, J. Nucl. Mater. 389 (3) (2009) 416–419, https://doi.org/10.1016/j.jnucmat.2009.02.018.

[28] D. Prieur, et al., Linear thermal expansion, thermal diffusivity and melting temperature of Am-MOX and Np-MOX, J. Alloys Compd. 637 (2015) 326–331, https://doi.org/10.1016/j.jallcom.2015.03.032.

[29] D. Prieur, et al., Melting behaviour of americium-doped uranium dioxide, J. Chem. Thermodyn. 97 (2016) 244–252, https://doi.org/10.1016/j.jct.2016.02.003. Jun.

[30] N. Saunders and A.P. Miodownik, "CALPHAD (Calculation of Phase Diagrams): a Comprehensive Guide, Volume 1 - 1st Edition." Available: https://www.elsevier.co

m/books/calphad-calculation-of-phase-diagrams-a-comprehensive-guide/saunders/978-0-08-042129-2.

[31] "Computational Materials Engineering," Thermo-Calc Software. Available: http://thermocalc.com/.

[32] M. Kurata, Thermodynamic assessment of the Pu-U, Pu-Zr, and Pu-U-Zr systems, Calphad 23 (3) (1999) 305–337, https://doi.org/10.1016/S0364-5916(00)00004-3.

[33] C. Guéneau, et al., Thermodynamic modelling of advanced oxide and carbide nuclear fuels: description of the U-Pu-O-C systems, J. Nucl. Mater. 419 (1–3) (2011) 145–167, https://doi.org/10.1016/j.jnucmat.2011.07.033.

[34] E. Epifano, et al., Insight into the Am-O phase equilibria: a thermodynamic study coupling high-temperature XRD and CALPHAD modeling, Inorg. Chem. 56 (13) (2017), https://doi.org/10.1021/acs.inorgchem.7b00572.

[35] E. Epifano, "Study of the U-Am-O ternary phase diagram," PhD thesis, Paris Saclay, 2017. Available: http://www.theses.fr/2017SACLX084.