Chromium Doped UO2-Based Ceramics: Synthesis and Characterization of Model Materials for Modern Nuclear Fuels

Cr-doped UO2 as a modern nuclear fuel type has been demonstrated to increase the in-reactor fuel performance compared to conventional nuclear fuels. Little is known about the long-term stability of spent Cr-doped UO2 nuclear fuels in a deep geological disposal facility. The investigation of suitable model materials in a step wise bottom-up approach can provide insights into the corrosion behavior of spent Cr-doped nuclear fuels. Here, we present new wet chemical approaches providing the basis for such model systems, namely co-precipitation and wet coating. Both were successfully tested and optimized, based on detailed analyses of all synthesis steps and parameters: Cr-doping method, thermal treatment, reduction of U3O8 to UO2, green body production, and pellet sintering. Both methods enable the production of suitable model systems with a similar microstructure and density as a reference sample from AREVA. In comparison with results from the classical powder route, similar trends upon grain size and lattice parameter were determined. The results of this investigation highlight the significance of subtly different synthesis routes on the properties of Cr-doped UO2 ceramics. They enable a reproducible tailor-made well-defined microstructure, a homogeneous doping, for example, with lanthanides or alpha sources, the introduction of metallic particles, and a dust-free preparation.


Introduction
The introduction of modern nuclear fuels such as those including Cr-, Al-, and Sidoped types [1][2][3] has led to a significant improvement in the performance of conventional light water reactors (LWR) over traditional non-doped forms. Specifically, modern fuels provide a reduction in fuel swelling and amelioration in heat transfer, leading to higher burn up and power output [4][5][6][7]. The physical origin behind these enhanced properties, in the case of the Cr-doped fuels, is due to the induced coarse-grained microstructure with a narrow grain size distribution. This increases the retention of gaseous fission products inside the fuel during operation due to improved diffusion behavior compared to conventional LWR fuels [8]. During the production of Cr-doped fuels, the UO 2 matrix can only incorporate Cr in trace amounts of up to approximately 1500 ppm [9,10]. Concurrently, some of the Cr precipitates as a separate phase typically located at the grain boundaries of the UO 2 matrix [8,11]. The introduction of modern, Cr-doped, nuclear fuels at present, is still a relatively recent addition to current reactor fleets. Subsequently, key questions remain regarding their properties and behavior, making them a topical focus of recent fuel investigations.
Similar to conventional UO 2 -based LWR fuels, spent modern fuels are likely to be disposed of in deep geological repositories, which are considered the most appropriate sintering, providing the potential for single effect studies, even when only trace amounts of additional elements are used. Here, we present two new, wet chemical approaches for the preparation of Cr-doped UO 2 based model systems for single effect dissolution studies related to modern fuels. A parameter variation study was carried out including optimization of powder synthesis, green body compaction, and variation of the oxygen potential. Detailed investigations were carried out on the effects of the individual parameters on the microstructure, density, grain growth of the ceramics, and the structural uptake of Cr into UO 2 as well as the formation of separate Cr-rich precipitates. The effect of adding chromium nitrate at different stages of the synthesis route to UO 2 was tested and compared to results of the common powder syntheses. Finally, the results were compared to pure UO 2 pellets, produced within the framework of this work, used as reference for the single effect studies and to an industrially produced, chromium doped UO 2 pellet made by AREVA used as the reference material for fresh nuclear fuel.

Preparation of a Cr-Doped UO 2 Based Ceramic
The preparation was adapted from the general procedure of [28] for the wet chemical preparation of ammonium di-uranate ((NH 4 ) 2 U 2 O 7 , ADU) from uranyl nitrate solution. The main steps: (1) powder preparation; (2) Cr-doping; and (3) preparation of ceramic pellets and their optimization, are described in detail below. Two different methods for the chromium doping of the pellets were applied: (1) a co-precipitation method (CPM) and (2) a wet coating method (WCM) as adapted from [29][30][31].

Preparation of Pure UO 2 -Powder
The precursor powders for all pellets presented in this work were produced by precipitation of ADU from an aqueous solution containing 2 M uranyl nitrate (UO 2 (NO 3 ) 2 , Merck, p.a.) using a 16.5 M ammonia solution (Merck). The reaction was carried out with an excess of ammonia of at least 300% to adjust the equilibrium to the side of the products, according to: 2 UO 2 (NO 3 ) 2 + 6 NH 3 + 3 H 2 O → (NH 4 ) 2 U 2 O 7 + 4 (NH 4 )NO 3 (1) The uranyl nitrate solution was stepwise pipetted into the ammonia solution, which was constantly mixed by a magnetic stirrer. After complete addition of the solution, the mixture was stirred for 2 h. This led to the formation of ADU as a yellow precipitate, which was then separated from the solution by centrifuging for 5 min and afterward decanting the solution. The precipitate was then washed three times with high purity water to remove excess (NH 4 )NO 3 and NH 3 . During the final washing step, water was replaced with ethanol and the powder was left to dry. ICP-MS measurements of the supernatant solution showed that about 99% of the uranium from the stock solution was precipitated as ADU.
The dried ADU powder was calcined under air for denitrification, dehydration, and transformation to U 3 O 8 using a box furnace (Carbolite CWF 13/5, Neuhausen, Germany). In order to optimize the influence of the temperature on the decomposition, a series of ADU-powders was prepared and calcined at temperatures of 450, 600, 700, 800, and 900 • C, respectively, for 5 h. The uncertainties of the temperature determination for this step of thermal treatment are ±5 • C, based on the manufacturer's information.
The resulting U 3 O 8 powder was reduced to stoichiometric UO 2 via a 5 h thermal treatment in a tube furnace (ENTECH ESTF 50-18-SP-VK, Ängelholm, Sweden) under a 4% H 2 -96% Ar mixture (HYTEC). The completeness of the reduction as well as the effect of temperature on the final density of the sintered pellets was investigated by performing the reduction at three different temperatures, namely 600, 800, and 900 • C. Uncertainties for the temperature determination of both steps are ±5 • C each, based on the manufacturer's information.
Materials with doping levels of 1000 and 2500 ppm Cr 2 O 3 were precipitated and thermally treated in the same way as the pure ADU at 600 • C for calcination and reduction, respectively.
For the WCM preparation, a pure UO 2 from the wet chemical route according to Section 2.1.1. was used after a complete thermal treatment at 600 • C for both steps. The UO 2 was wet coated with a chromium nitrate nonahydrate solution (0.02 M) in appropriate amounts corresponding to initial doping levels of 1000, 1500, and 2500 ppm Cr 2 O 3 . The uncertainty of the initial doping level was derived from the errors of the balance and pipetting devices as below 2%.
In order to distribute the Cr homogeneously on the grain surfaces, suprapure water was admixed until the complete powder was covered with liquid. After intensive stirring, this mixture was dried and subjected to the same thermal treatment as the original pure UO 2 . Earlier studies showed that about 20 to 40% of the chromium initially added was lost to the furnace atmosphere during sintering, so all data described here are related to the initial doping level [9].

Preparation of Ceramic Pellets
In order to obtain disk shaped green bodies 10 mm diameter, about 1 g of the calcined powders were compacted for 10 s with a tungsten carbide piston of an uniaxial press (Hahn & Kolb, MP12, Stuttgart, Germany), without any addition of a binder or lubricant. The optimum preparation of the green body was tested according to an approach of Bukaemskiy et al. (2009) and Babelot et al. (2017) [32,33], who in detail tested the compressibility and sinterability of oxide ceramic powders synthesized by different wet chemical methods. Within a log unit (compaction) pressure versus relative green density plot, they identified three linear regions, the low, intermediate, and high pressure region and an optimal pressure corresponding to the highest sintered density, located in the upper part of the intermediate-pressure region. In the present work, the compressibility and sinterability of the studied powders were tested in the compaction pressure region from 190 to 760 MPa to identify the optimum compaction pressure. Uncertainty of the pressure during compaction was estimated due to the setup of the press to be 0.1 MPa. After pressure release, the green body was carefully pressed out of the die-plate using the lower piston of the uniaxial press and placed in a corundum crucible for subsequent sintering. The density of an ideal UO 2 crystal as directly calculated from lattice parameters (10.958 g/cm 3 ), the mass of UO 2 of the green body, and its height were used for the calculation of the green density [34]. Due to the low doping levels, all green densities and sintered densities refer to an ideal pure UO 2 crystal.
For the sintering of the pellets, the crucibles with the green bodies were placed in the hot zone of a horizontal tube furnace (ENTECH ESTF 50-18-SP-VK, Ängelholm, Sweden). The pellets were sintered at 1700 ± 5 • C for 10 h. Heating rates were 4 • C/min, and cooling rates were 6 • C/min. During the sintering process, the furnace was flushed with 1600 mL/min of a 4% H 2 -96% Ar mixture, or with the less reducing mixture resulting from mixing 1567 mL/min of 4% H 2 -96% Ar and 33 mL/min of 1% H 2 -99% Ar. This leads to two different values of oxygen partial pressure during the sintering process. The required proportions of the gas mixtures were calculated beforehand to achieve the desired oxygen partial pressures. In addition, the oxygen partial pressure was monitored during the sintering process using a yttrium stabilized ZrO 2 oxygen sensor.
Oxygen partial pressure (pO 2 ) can also be expressed as relative partial molar Gibbs free energy of oxygen, or oxygen potential, P O2 : where G • is the standard free energy of O 2 ; P O2 is the (pO 2 /p • ); and p • is the standard pressure. The oxygen potential for the 4% H 2 -96% Ar mixture was −510 kJ/mol O 2 , the less reducing mixture was prepared by mixing certain amounts of 4% H 2 -96% Ar with 1% H 2 -99% Ar to achieve an oxygen potential of −420 kJ/mol O 2 . As high purity gases were used, the uncertainty of the oxygen potential was less than 1%.

Physical Properties
The densities of the pellets were determined by the geometrical method for the green bodies. The density of the sintered pellets and the closed porosity were measured by the hydrostatic weighing method in water. The open porosity was estimated via a penetration-immersion method [35,36] with hot paraffin (150 • C) as the impregnation fluid. The measurements were performed with a Mettler Toledo density determination kit in combination with a Mettler Toledo AT261 (Giessen, Germany) precision balance. The theoretical densities of the materials (ρT) were calculated from the results of the Xray diffraction (XRD) measurements as described in [37]. All densities described later are relative densities, provided in percent of the theoretical density. Uncertainties of all determined densities are 2 sigma deviations and are provided in Table S1.

Crystal Structure
XRD analyses were performed with a D4 Endeavor diffractometer in Bragg-Brentano geometry (Bruker AXS GmbH, Karlsruhe, Germany) using Cu radiation (λ CuK α1 = 1.5405 Å) at a power setting of 40 kV and 40 mA. A linear silicon strip LynxEye detector (Bruker-AXS, Karlsruhe, Germany) was used to determine the crystal structure of the powder and pellet samples. The aperture of the motorized divergence slit was set to 0.3 mm and the receiving slit to 8.0 mm, respectively. XRD patterns were collected from the final pellets at ambient conditions in the 2Θ range from 10 to 120 • using a step size of 0.01 • /2Θ and a counting time of 2 s per step.
The structure of UO 2 in different pellets was refined by the Rietveld method as implemented in the GSAS2 software [38,39]. The peak shapes were modeled using a pseudo-Voigt function and the background was estimated using a 12-18 term shifted Chebyschev function. The scale factor, detector zero-point, and lattice parameters were refined together with the peak profile parameters.
Microstructural observations on powders and ceramic pellets were carried out with an environmental scanning electron microscope (SEM, Quanta 200F, FEI, Eindhoven, The Netherlands) in low vacuum mode at a pressure of 60 Pa. Energy-dispersive X-ray spectroscopy (EDS) employing an Apollo X detector system (EDAX, Weiterstadt, Germany) was used to determine the chemical composition of the synthesized solids and to assess their chemical homogeneity. EDS spot analyses as well as EDS elemental mappings, to determine the spatial distribution of Cr, were carried out at 20 kV and a working distance of 10 mm. Image analyses of EDS mappings were applied using the software package Fiji (ImageJ, Bethesda, USA). To assess possible variations in the average grain size across the pellets, SEM images were taken along the diameter of the pellets in three fields: edge, midway, and center. The grain size was determined using the intercept method ASTM-E112-13 as described in the ASTM standards [40]. Uncertainties of the grain sizes are 2 sigma deviations and listed in Table S1 in supplementary materials.

Results
In the following, first, the optimization of the important steps for the preparation of Cr-doped UO 2 ceramics is addressed. In the second part of the results section, a detailed parameter study of the consequences of Cr-doping upon the physical properties, crystal structure, and microstructure of the UO 2 based ceramics is presented. Two important aspects of the powder preparation route were examined: (1) temperature, and (2) the corresponding powder reactivity during the production of the green body of the UO 2 based ceramic including a variation of the pressure applied during green pellet preparation. Figure 1 shows the XRD patterns of ADU powders after thermal treatment at 450, 600, and 700 • C for 5 h. Inspection of the XRD pattern for the 450 • C sample indicated that the sample did not completely transform to U 3 O 8 . Phase analysis indicated that this sample contained a mixture of U 3 O 8 , ADU, and UO 3 . The XRD patterns of powders calcined at temperatures higher than or equal to 600 • C showed a complete transformation from ADU to U 3 O 8 .   The XRD patterns of the products after the second, reducing, thermal treatment are shown in Figure 2. U 3 O 8 powder, which was synthesized from ADU at 800 • C, was treated at 600, 800, and 900 • C for 5 h in a 4% H 2 -96% Ar mixture. The measurements showed that a complete transformation from U 3 O 8 to UO 2 occurred at all temperatures.    In the pressure range investigated in this work, a compaction pressure increase led to an increasing green and sintered density (see also Table S1 in the supplementary information). Reducing the temperature of the first thermal treatment from 800 • C to 600 • C led to a significant decrease in green density. For example, at a compacting pressure of 637 MPa, the values of the green densities were 63.1% for 800 • C and 59.8% for 600 • C, respectively (Figure 3a, Table S1 in supplementary materials). After the subsequent sintering, the sintered density was lower for the sample with the lower green density (Figure 3b). Increasing the second step of thermal treatment from 600 • C to 800 • C led to an increase in the green density ( Figure 3a), but to a decrease in the sintered density ( Figure 3b).
Chromium doping led to green densities similar to those of pure UO 2 treated at the same temperatures (600 • C/600 • C, Figure 3 a), but to much higher sintered densities (e.g., 97.24% for CPM and 98.55% for WCM at 637 MPa, Figure 3 b). Additionally, WCM Cr-doping results in a less pronounced pressure dependence compared to pure UO 2 and material doped by CPM.
In addition to the sintering temperature and the compaction pressure, the sintered density of pure UO 2 pellets depends on the oxygen potential in the sintering atmosphere ( Figure 4). In case of the pure UO 2 samples after the pre-treatment at 600 • C for both calcination steps, an increasing sintered density was observed with increasing oxygen potential from 94.5 % at −510 kJ/mol O 2 to 96.3% at −420 kJ/mol O 2 . sity ( Figure 3b). Increasing the second step of thermal treatment from 600 °C to 800 °C led to an increase in the green density ( Figure 3a), but to a decrease in the sintered density ( Figure 3b).
Chromium doping led to green densities similar to those of pure UO2 treated at the same temperatures (600 °C/600 °C, Figure 3a), but to much higher sintered densities (e.g., 97.24% for CPM and 98.55% for WCM at 637 MPa, Figure 3b). Additionally, WCM Crdoping results in a less pronounced pressure dependence compared to pure UO2 and material doped by CPM.  In addition to the sintering temperature and the compaction pressure, the sintered density of pure UO2 pellets depends on the oxygen potential in the sintering atmosphere ( Figure 4). In case of the pure UO2 samples after the pre-treatment at 600 °C for both calcination steps, an increasing sintered density was observed with increasing oxygen potential from 94.5 % at −510 kJ/mol O2 to 96.3% at −420 kJ/mol O2.

Adaption of Green Body Preparation and Pellet Density for Cr2O3 Doped UO2
The relationship between compaction pressure and sintered density was similar for the samples doped with 1000 ppm Cr2O3 obtained by the co-precipitation method and for pure UO2 (Figure 3). However, the relative densities of the 1000 ppm Cr2O3-doped UO2 increased on average by 3% compared to pure UO2. An increase in the Cr2O3-doping level from 1000 to 2500 ppm raised the sintered density from 97.3 to 98.1 % at P = 637 MPa ( Figure 4). For the samples doped with 1000 ppm Cr2O3 by CPM, the sintered densities of the pellets were slightly dependent on the compaction pressure, but densities were significantly higher than for pure UO2 and reached 98.6% at P = 637 MPa. All samples doped by WCM, independently on the doping level (1000, 1500, and 2500 ppm), showed similar sintered densities of nearly 98.4%. The sintering at different oxygen potentials (−510 or −420 kJ/mol O2) was found to have very little effect on the density of Cr2O3-doped pellets, independent of the doping level and the synthesis route ( Figure 4). Figures 3b and 4b demonstrate that the highest sintered densities are achievable using pre-treatment at 600 °C for both calcination steps, via the WCM approach and with an oxygen potential of −420 kJ/mol O2. The effect of the oxygen potential on microstructural

Adaption of Green Body Preparation and Pellet Density for Cr 2 O 3 Doped UO 2
The relationship between compaction pressure and sintered density was similar for the samples doped with 1000 ppm Cr 2 O 3 obtained by the co-precipitation method and for pure UO 2 ( Figure 3). However, the relative densities of the 1000 ppm Cr 2 O 3 -doped UO 2 increased on average by 3% compared to pure UO 2 . An increase in the Cr 2 O 3 -doping level from 1000 to 2500 ppm raised the sintered density from 97.3 to 98.1 % at P = 637 MPa ( Figure 4). For the samples doped with 1000 ppm Cr 2 O 3 by CPM, the sintered densities of the pellets were slightly dependent on the compaction pressure, but densities were significantly higher than for pure UO 2 and reached 98.6% at P = 637 MPa. All samples doped by WCM, independently on the doping level (1000, 1500, and 2500 ppm), showed similar sintered densities of nearly 98.4%. The sintering at different oxygen potentials (−510 or −420 kJ/mol O 2 ) was found to have very little effect on the density of Cr 2 O 3 -doped pellets, independent of the doping level and the synthesis route ( Figure 4).
Figures 3b and 4b demonstrate that the highest sintered densities are achievable using pre-treatment at 600 • C for both calcination steps, via the WCM approach and with an oxygen potential of −420 kJ/mol O 2 . The effect of the oxygen potential on microstructural features other than the density will be described in the following sections. The microstructure and grain size of samples sintered at 1700 • C with varying oxygen potential were analyzed in detail. The microstructures of samples prepared at an oxygen potential of −420 kJ/mol O 2 and of the AREVA reference material are presented in the backscattered electron (BSE) SEM images in Figure 5. In this SEM mode, the individual grains can be distinguished due to their orientation contrast. Pores appear as black round features, with some of them filled with Cr, as detected by EDS (see Figure 5).

Cr 2 O 3 -UO 2 Solid Solution Formation: Consequences on Lattice Parameters and
The pure UO 2 reference pellet as prepared by our synthesis route (Figure 5a) contained grains in a size range between 1 µm (min) and~25 µm (max) with an average grain size of 12 µm. Typical for this sample is a uniform grain size distribution and well-formed triple points with an angle of 120 • . Small pores with about a 1 µm diameter are homogeneously distributed. Additionally, some pores with a size of up to 10 µm are visible. No systematic relationship between the location of the pores and grain boundaries or triple points was observed. Pure UO 2 pellets sintered at more reducing conditions (−510 kJ/mol O 2 ) showed the same microstructure ( Figure S1 in supplementary materials).
In contrast to the pure UO 2 pellet, the grains of the AREVA reference UO 2 -Cr 2 O 3 ceramic (1500 ppm Cr 2 O 3 as initial composition; Figure 5b) were significantly larger, with sizes in the range between 8 and 70 µm and an average grain size of 37 µm (Figures 5 and 6, Table 1, Table S1 in supplementary materials). The porosity was lower than that of the pure UO 2 reference, whereas the general microstructure concerning triple points, etc. was similar. Similarly, the WCM and CPM samples, starting from 1000 ppm Cr 2 O 3 , also had increased average grain sizes of 20 µm and of 24 µm, respectively (Figure 5c,d and 6). For direct comparison with the AREVA reference, a sample with 1500 ppm Cr 2 O 3 was prepared via WCM, which showed a similar microstructure and average grain size ( Figure S2 in supplementary materials). Increasing the initial Cr 2 O 3 doping level further to 2500 ppm led to an additional coarsening effect and average grain sizes of 56 (CPM) and 69 µm (WCM; Figures 5 and 6). In general, the co-precipitation samples exhibited a slightly higher porosity and a smaller grain size than the wet coating samples. tained grains in a size range between 1 µm (min) and ~25 µm (max) with an average grain size of 12 µm. Typical for this sample is a uniform grain size distribution and well-formed triple points with an angle of 120°. Small pores with about a 1 µm diameter are homogeneously distributed. Additionally, some pores with a size of up to 10 µm are visible. No systematic relationship between the location of the pores and grain boundaries or triple points was observed. Pure UO2 pellets sintered at more reducing conditions (-510 kJ/mol O2) showed the same microstructure (Figure 1 in supplementary materials). In contrast to the pure UO2 pellet, the grains of the AREVA reference UO2-Cr2O3 ceramic (1500 ppm Cr2O3 as initial composition; Figure 5b) were significantly larger, with lower grain size than the corresponding samples sintered at -420 kJ/mol O2. The grains of the WCM samples were systematically larger than the grains of the CPM samples. The AREVA pellet had an identical average grain size as the WCM sample initially doped with 1500 ppm Cr2O3 and sintered at −420 kJ/mol O2. Compared to the pure UO2 sample, the addition of 1000 ppm Cr2O3 only had minor effects on the average grain size for the samples sintered at −510 kJ/mol O2, and a more pronounced effect at −420 kJ/mol O2 ( Figure  6). To understand where the Cr is located within the samples, EDS mappings were carried out for the detection of Cr-rich precipitates (Figure 7). A comparison of the EDS mappings and the respective BSE-SEM images indicated no preferred precipitation along the microstructural features (e.g., pores or grain boundaries, cf. example overlay of EDS and BSE image in Figure 3 in supplementary materials). Typically, the ceramics prepared in this study had small Cr-rich precipitates that appeared not to agglomerate to larger particles. The image analyses carried out on the EDS mappings ( Table 2) indicated similar precipitate sizes for all samples (i.e., the average sphere equivalent radius calculated from the areas of the precipitates varied between 0.3 and 0.47 µm). The number of Cr-rich precipitates was observed to increase with the doping level, but the total amount of Cr2O3 in the precipitates remained negligible compared to the total mass doped into the UO2 ceramics. The Cr-precipitates in the AREVA pellet were found to be coarser with an average sphere equivalent radius of 0.82 µm.  The mean grain sizes obtained in this study and the available literature data are compiled in Figure 6. The data from the present wet chemical syntheses show a systematic shift in the average grain size toward larger grains, resulting from the variation of oxygen potential to lower values. In general, the samples prepared at −510 kJ/mol O 2 exhibited a lower grain size than the corresponding samples sintered at −420 kJ/mol O 2 . The grains of the WCM samples were systematically larger than the grains of the CPM samples. The AREVA pellet had an identical average grain size as the WCM sample initially doped with 1500 ppm Cr 2 O 3 and sintered at −420 kJ/mol O 2 . Compared to the pure UO 2 sample, the addition of 1000 ppm Cr 2 O 3 only had minor effects on the average grain size for the samples sintered at −510 kJ/mol O 2 , and a more pronounced effect at −420 kJ/mol O 2 ( Figure 6).
To understand where the Cr is located within the samples, EDS mappings were carried out for the detection of Cr-rich precipitates (Figure 7). A comparison of the EDS mappings and the respective BSE-SEM images indicated no preferred precipitation along the microstructural features (e.g., pores or grain boundaries, cf. example overlay of EDS and BSE image in Figure S3 in supplementary materials). Typically, the ceramics prepared in this study had small Cr-rich precipitates that appeared not to agglomerate to larger particles. The image analyses carried out on the EDS mappings ( Table 2) indicated similar precipitate sizes for all samples (i.e., the average sphere equivalent radius calculated from the areas of the precipitates varied between 0.3 and 0.47 µm). The number of Cr-rich precipitates was observed to increase with the doping level, but the total amount of Cr 2 O 3 in the precipitates remained negligible compared to the total mass doped into the UO 2 ceramics. The Cr-precipitates in the AREVA pellet were found to be coarser with an average sphere equivalent radius of 0.82 µm.   ; and (f) WCM with 2500 ppm Cr2O3. All samples were sintered at an oxygen potential of -420 kJ/mol O2. For better contrast, the EDX mappings were transformed into grey scale. Bright spots represent chromium-rich grains, black spots refer to pores, and grey refers to the Cr-doped UO2 matrix. Scale was the same for all images. Figure 7c shows the Cr-grain distribution of a commercially available Cr doped UO2 pellet (AREVA). With an average grain size of 37 µm (Figure 5b, Table 1 in supplementary materials), the microstructure was similar to the pellets produced in the frame of this work doped with 1500 ppm Cr using the wet-coating method and sintered at an oxygen potential of -420 kJ/mol O2. Figure 8 shows the UO2 lattice parameter (a) of different pellets synthesized in this work as a function of the initial Cr2O3 doping level obtained via Rietveld refinement analysis. A Rietveld profile is presented in Figure 4 in supplementary materials for reference. It can be observed that the incorporation of Cr into the UO2 lattice led to a subsequent contraction of the lattice parameter, but is dependent on the Cr2O3-doping level in addition to the oxygen potential used during the sintering process. In contrast, the lattice parameters of pure UO2 pellets showed an insignificant change with the variation in the oxygen potential during sintering. In the range of Cr2O3-doping levels presented here, the increase in the initial Cr2O3-doping level leads to a nearly linear decrease in the lattice All samples were sintered at an oxygen potential of −420 kJ/mol O 2 . For better contrast, the EDX mappings were transformed into grey scale. Bright spots represent chromium-rich grains, black spots refer to pores, and grey refers to the Cr-doped UO 2 matrix. Scale was the same for all images.  Figure 7c shows the Cr-grain distribution of a commercially available Cr doped UO 2 pellet (AREVA). With an average grain size of 37 µm (Figure 5b, Table S1 in supplementary materials), the microstructure was similar to the pellets produced in the frame of this work doped with 1500 ppm Cr using the wet-coating method and sintered at an oxygen potential of −420 kJ/mol O 2 . Figure 8 shows the UO 2 lattice parameter (a) of different pellets synthesized in this work as a function of the initial Cr 2 O 3 doping level obtained via Rietveld refinement analysis. A Rietveld profile is presented in Figure S4 in supplementary materials for reference. It can be observed that the incorporation of Cr into the UO 2 lattice led to a subsequent contraction of the lattice parameter, but is dependent on the Cr 2 O 3 -doping level in addition to the oxygen potential used during the sintering process. In contrast, the lattice parameters of pure UO 2 pellets showed an insignificant change with the variation in the oxygen potential during sintering. In the range of Cr 2 O 3 -doping levels presented here, the increase in the initial Cr 2 O 3 -doping level leads to a nearly linear decrease in the lattice parameters (i.e., a higher amount of Cr 2 O 3 -doping leads to a higher structural uptake of Cr into the lattice of UO 2 ). The change in lattice parameters of UO 2 was less pronounced at lower oxygen potential (−510 kJ/mol O 2 ) compared to the samples sintered at higher oxygen potential (−420 kJ/mol O 2 ). That general lattice contraction, as observed with increasing initial Cr content, is consistent with previous experimental studies of and Cr doping of UO 2 [9,21,22,41]. In these references, the lattice contraction was attributed to U(V) formation due to charge balancing of the dopant cations. Accordingly, it can be argued that the greater contraction observed at higher initial Cr content is due to the formation of an increased amount of oxidized U. parameters (i.e., a higher amount of Cr2O3-doping leads to a higher structural uptake of Cr into the lattice of UO2). The change in lattice parameters of UO2 was less pronounced at lower oxygen potential (-510 kJ/mol O2) compared to the samples sintered at higher oxygen potential (−420kJ/mol O2). That general lattice contraction, as observed with increasing initial Cr content, is consistent with previous experimental studies of and Cr doping of UO2 [9,21,22,41]. In these references, the lattice contraction was attributed to U(V) formation due to charge balancing of the dopant cations. Accordingly, it can be argued that the greater contraction observed at higher initial Cr content is due to the formation of an increased amount of oxidized U. The effect of the doping method on the structural uptake appears to play no significant role depending on the oxygen partial pressure. In the case of −510 kJ/mol O2, no discernible difference could be observed between the different doping methods used with increasing Cr initial addition. At higher oxygen partial pressure, slight differences could be observed between the lattice parameter when using the different doping methods. At 1000 ppm Cr2O3 (i.e., below the solubility limit), WCM led to a greater contraction and presumed higher incorporation of Cr, whereas at 2500 ppm Cr2O3 (i.e., above the solubility limit of Cr), CPM led to greater contraction and presumed higher incorporation of Cr. A mechanistic understanding was beyond the scope of the present investigation, nevertheless, it can be readily observed that the doping methods and controlled oxygen partial pressure greatly assist in incorporation into the UO2 matrix as opposed to the sample produced by AREVA via powdering mixing routes where its doping of 1500 ppm Cr2O3 seemed to have little effect on the UO2 lattice and it is argued that the matrix contains little Cr. Figure 8. UO2 lattice parameters versus initial Cr2O3 doping level. Two different doping methods and oxygen potentials during the sintering process were investigated. -510: Sinter atmosphere was an Ar/4% H2 mixture with an oxygen potential (ΔG° = -RT ln PO2) of -510 kJ/mol O2. -420: Sinter atmosphere was an Ar/4% H2 + Ar/1% H2 mixture with an oxygen potential of -420 kJ/mol O2. The sintering temperature of all pellets was 1700 °C.

Discussion
As already discussed in the introductory part of this paper, the scope of this work is the provision of a "construction kit" for all kinds of UO2 model systems useable for single effect studies with the possibility of adjusting parameters such as density/porosity, The effect of the doping method on the structural uptake appears to play no significant role depending on the oxygen partial pressure. In the case of −510 kJ/mol O 2 , no discernible difference could be observed between the different doping methods used with increasing Cr initial addition. At higher oxygen partial pressure, slight differences could be observed between the lattice parameter when using the different doping methods. At 1000 ppm Cr 2 O 3 (i.e., below the solubility limit), WCM led to a greater contraction and presumed higher incorporation of Cr, whereas at 2500 ppm Cr 2 O 3 (i.e., above the solubility limit of Cr), CPM led to greater contraction and presumed higher incorporation of Cr. A mechanistic understanding was beyond the scope of the present investigation, nevertheless, it can be readily observed that the doping methods and controlled oxygen partial pressure greatly assist in incorporation into the UO 2 matrix as opposed to the sample produced by AREVA via powdering mixing routes where its doping of 1500 ppm Cr 2 O 3 seemed to have little effect on the UO 2 lattice and it is argued that the matrix contains little Cr.

Discussion
As already discussed in the introductory part of this paper, the scope of this work is the provision of a "construction kit" for all kinds of UO 2 model systems useable for single effect studies with the possibility of adjusting parameters such as density/porosity, grainsize, Cr-distribution in the matrix. The flow chart in Figure 9 displays the optimized processing steps for the fabrication of pure UO 2 and Cr 2 O 3 doped UO 2 pellets with well controlled microstructure, sintered density, and distribution of Cr within the pellets. A careful preparation of the three solid phases (powders) ADU, U 3 O 8 , and UO 2 before the sintering of the ceramic is an essential prerequisite for successful pellet fabrication. In the following section, the parameters that have a strong influence on the sinterability are discussed for the production of pure UO 2 and of the UO 2 + Cr 2 O 3 ceramic via wet chemical methods. In addition, the effects of doping UO 2 with Cr 2 O 3 upon the microstructure and crystal structure are discussed in comparison with studies following the powder synthesis route. grainsize, Cr-distribution in the matrix. The flow chart in Figure 9 displays the optimized processing steps for the fabrication of pure UO2 and Cr2O3 doped UO2 pellets with well controlled microstructure, sintered density, and distribution of Cr within the pellets. A careful preparation of the three solid phases (powders) ADU, U3O8, and UO2 before the sintering of the ceramic is an essential prerequisite for successful pellet fabrication. In the following section, the parameters that have a strong influence on the sinterability are discussed for the production of pure UO2 and of the UO2 + Cr2O3 ceramic via wet chemical methods. In addition, the effects of doping UO2 with Cr2O3 upon the microstructure and crystal structure are discussed in comparison with studies following the powder synthesis route. Figure 9. Flow chart of the synthesis route for the production of pure and chromium doped UO2 pellets developed in this work.

Processing Parameters
The present results for pure UO2 indicate that there was a significant effect with respect to the two thermal treatments and the compaction pressure upon sinterability and density of the final ceramic. The optimum compaction pressure for all samples was determined as 637 MPa because it is well below the identified limit of the middle pressure region-independently from the other processing steps [32]. For pure UO2, conversion

Processing Parameters
The present results for pure UO 2 indicate that there was a significant effect with respect to the two thermal treatments and the compaction pressure upon sinterability and density of the final ceramic. The optimum compaction pressure for all samples was determined as 637 MPa because it is well below the identified limit of the middle pressure region-independently from the other processing steps [32]. For pure UO 2 , conversion temperatures from ADU to U 3 O 8 of 600 • C or 800 • C result in a good sinterability. This is well in line with the XRD measurements indicating a full transition to U 3 O 8 already at 600 • C and the findings of Hung et al. (2001) [27], who also suggested these two temperatures based on their ADU to UO 2 pellet processing study.
To minimize the known chromium loss due to volatilization during thermal treatment [9,10], 600 • C was the temperature chosen for the conversion of ADU to U 3 O 8 when the Cr-doped ceramics were prepared. For the reduction step, a temperature of 600 • C was determined to be sufficient. It is known that higher temperatures during thermal treatment lower the active surface of powders and thereby their reactivity due to the fusion of grains and beginning the sintering processes [42,43]. As no milling or grinding steps should be included in the synthesis route, this effect could change and deteriorate the density/porosity of the sintered pellets [44]. To optimize the sintered density, the sintering atmosphere was optimized in the last step to −420 kJ/mol O 2 . At these conditions, pure UO 2 pellets with a density of higher than 96 % were prepared, providing the starting point for the optimized Cr-doped pellet route.
Cr-doping was found to increase the sintered density in all cases when compared with the respective UO 2 reference. For the Cr-doped materials sintered at −510 kJ/mol O 2 , as usually done in other studies [21,22], the relationship between the processing parameters and the final density was more similar to pure UO 2 in the case of the CPM approach than with the WCM approach. At optimized conditions, the highest densities were reached with the WCM method, independent from the compaction pressure, whereas the CPM method yielded lower sintered densities. The Cr-doped pellets of Silva et al. (2021) [21] reached about 96% of the theoretical density at comparable conditions, close to the pellets prepared via CPM at −510 kJ/mol O 2 . In contrast, the samples prepared by Milena-Perez et al. (2021) [22], which were sintered at an oxygen potential of about −510 kJ/mol O 2 , showed increasing porosity with increasing doping, as observed via SEM.
The increase of the oxygen potential to −420 kJ/mol O 2 during sintering improved the densities to around 98%, confirming that the combination of 600/600 • C for the thermal treatments with this oxygen potential leads to better results. The densities were similar for WPM and CPM and independent of the doping level.

Microstructure, Grain Size, Chromium Distribution, and Lattice Parameters in Comparison with the Literature
After the optimization of parameters, the wet chemical synthesis route leads to ceramics with a very well-developed microstructure. All samples, pure and Cr-doped UO 2 , exhibited a uniform grain size distribution and 120 • triple points, indicating well equilibrated grain boundaries. Pores were homogeneously distributed with an average size of 1 µm. The mean grain size of the pure UO 2 was very similar to the samples prepared by Silva et al. (2021) [21] via the classical powder method. Depending on the doping method, during sintering at −510 kJ/mol O 2 , the grain size increased with the doping level of Cr 2 O 3 . In agreement with the observed increasing density, the grain size of the WCM sample was also higher. This was also true for sintering at −420 kJ/mol O 2 , but not as significantly. The values of Milena-Perez et al. (2021) [22] followed a very similar trend as the new data presented here, whereas Bourgeois et al. (2001) [23] produced larger grains in general, which may partly be explained by the advanced spray coating doping method used. Still, the trend of their data was also similar with increasing doping level. The effect of sintering time on the grain growth seems to be insignificant compared to the effect of doping level and oxygen potential, as the sintering time used in [22] and [23] was only 4 h, whereas the sintering time in this work was 10 h. The heating and cooling rates as well as the sinter temperature ( [22] 1675 • C [23], and this work 1700 • C) in all studies was in the same order of magnitude. Silva et al. (2021) [21] only saw a very slight change in the mean grain size with increasing Cr-doping. In particular, at higher doping levels above 2000 ppm, their trend in the mean grain size deviated from the trend of Bourgeois [23] and this study. However, the absolute numbers of the mean grain sizes deviated between all studies, indicating that the details of the preparation for the classical powder route also have a significant effect on the resulting microstructure.
Chromium precipitates are a common feature of Cr-doped UO 2 ceramics. The occurrence of Cr-precipitates and their size was evaluated by Cardinaels et al. (2012) [9] to correlate with the total amount of Cr doping, with a typical size of the precipitates of about 1 to 3 µm, often bound to pores. Similarly, the precipitates in the AREVA reference pellets had Cr-precipitates of about 1 µm in size. For the pellets synthesized in this work, the number and the size of pores was smaller and the number of Cr-precipitates lower. Additionally, the grain size distribution of Cr-grains was more even. The Cr precipitates were typically smaller than 1 µm when produced via wet-chemical routes.
The solubility of chromium in the UO 2 lattice is known to depend on the oxygen potential [8]. Consequently, the effect of Cr upon the lattice parameter was systematically examined at different Cr initial doping levels for the materials sintered at −420 kJ/mol O 2 compared to those sintered at −510 kJ/mol O 2 [9,10,21]. It was argued recently by Riglet-Martial et al. (2014) [8] that the incorporation of Cr into the UO 2 lattice depends on the oxygen partial pressure whereby higher oxygen potentials lead to higher Cr inclusion; however, volatility of Cr also plays a significant role. That we observed greater lattice contraction and by extension Cr inclusion at higher oxygen potentials is consistent with the argument given by [8]. A full mechanistic understanding was beyond the scope of the present investigation, nevertheless, it can be readily observed that the doping methods and controlled oxygen partial pressure greatly assisted in Cr incorporation into the UO 2 matrix. In contrast to this observation, the sample produced by AREVA via powder mixing routes with 1500 ppm initial Cr 2 O 3 doping only seemed to have little effect on the UO 2 lattice and it is argued that the matrix contained little Cr.
As described earlier, it was argued by Sun et al. that the incorporation of Cr into the UO 2 matrix resulted in the occurrence of Cr 2+ with U 5+ and an oxygen vacancy surrounding it, independent of Cr 2 O 3 precipitates. It was subsequently argued through this structural and redox mechanism of incorporation that it would result in a relative change of lattice parameter, ∆a/a, at 1000 ppm of −0.25 − 10 −3 compared to 0 ppm. At the same concentration from the present study, the experimentally determined ∆a/a derived from Rietveld refinements against XRD data of 0 and 1000 ppm Cr was −3.08 × 10 −3 for WCM −420 kJ/mol O 2 , -0.52 × 10 −3 for WCM −510 kJ/mol O 2 , −1.86 × 10 −3 for CPM −420 kJ/mol O 2 and −0.72 × 10 −3 for CPM −510 kJ/mol O 2 . Compared to the value reported by [24], the values were somewhat consistent for more reducing conditions, but an order of magnitude difference was observed for oxidizing. However as shown by this investigation, the solubility of Cr is oxygen potential dependent, such that varying the oxygen potential will result in variable amounts of Cr entering the matrix and subsequently effecting the lattice parameter to different degrees. Accordingly, it is difficult to corroborate the proposed structural redox scheme of Cr 2+ with U 5+ and an oxygen vacancy surrounding it by [24] via measurement of the lattice parameter due to its dependence on the oxygen potential. Subsequently, it is argued that for true determination of the structural and redox state within the matrix of Cr doped UO 2 , direct measurement of the crystal lattice ideally via grain or a single crystal samples is necessary.
The analysis here indicates that the structural uptake of Cr has no effect on the microstructure and sintered density. This indicates that Cr is surface active during the sintering, changing the behavior of the moving grain boundaries. However, the extent of Cr uptake was found to be strongly dependent upon treatment approach and the oxygen partial pressure.
A comparison of grain boundaries between pure and Cr-doped UO 2 pellets revealed a distinct curvature of the grain boundaries for the pure UO 2 pellets, whereas the chromium doped UO 2 pellets, especially the pellets doped with WCM, had straight grain boundaries. This is an indication for a wetting effect caused by the chromium dopant during sintering [42,43].

Conclusions
In this work, a wet chemical synthesis route for the production of pure and Cr-doped UO 2 pellets was demonstrated. Two different approaches for the Cr-doping were followed, namely doping by co-precipitation with the ADU precursor and doping by wet coating of UO 2 powders. Using an initial synthesis route involving precipitation of ADU as opposed to traditional powder mixing was demonstrated to be advantageous for producing high quality ceramics regarding the material properties and dopant homogeneity to be used later for single effect studies such as dissolution experiments on simulated (spent) nuclear fuel. Such a method can be readily applied to produce high quality ceramics involving other homogeneously distributed dopants relevant to model systems studies such as U-233 or Pu-238, among others.
Comparison with the literature shows similar trends for the influence of Cr-doping on the UO 2 grain growth in sintered pellets. This effect is pronounced in different degrees depending on the synthesis route. Choosing the wet chemical route of this work allows for a very good reproducibility of the results compared to routes with mechanical steps such as milling.
A recently discussed topic is the structural uptake of Cr into the UO 2 structure lattice. The theoretical approach of [8,24] describes that chromium incorporation is strongly dependent on oxygen partial pressure during the sintering process. This observation is supported by the findings of this work, although the solubility limit of Cr in the UO 2 lattice was not determined. The determination of the solubility of chromium in the UO 2 lattice is an important task for future work.
The precise knowledge of the influence of the synthesis parameters, for instance, the thermal treatment of precursors, method and amount of chromium doping, and oxygen potential during the sintering of the pellets facilitates the production of materials with a hand tailored microstructure with respect to the density, grain size, and chromium grain distribution.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/ma14206160/s1, Table S1: Values of all experiments; Figure S1: BSE images of a pure UO 2 pellets sintered at 1700 • C and an oxygen potential of −510 kJ/mol O 2 , for 10 h; Figure S2: BSE images of a Cr-doped UO 2 pellets with 1500 ppm Cr 2 O 3 WCM, sintered at 1700 • C and an oxygen potential of −420 kJ/mol O 2 , for 10 h; Figure S3: Example overlay of EDX and BSE image from sample 2500 ppm Cr 2 O 3 WCM, 600/600, −420 kJ_1; Figure S4: Example Rietveld profile of UO 2 doped with Cr at 1000 ppm initial using a WCM approach. Black crosses = observed; red solid line = calculated; red line below = difference curve; vertical black tick marks = positions of the space group allowed Bragg reflections.