Dilatometric Study of U1−xAmxO2±δ Sintering: Determination of Activation Energy

In the framework of minor actinide bearing blanket fuel fabrication, the sintering of uranium/americium (U_0.85Am_0.15O_2 ± δ) homogeneous mixed‐oxide pellets was studied. After the characterization of the U_0.85Am_0.15O_2 ± δ starting powder (granulometry, X‐ray diffraction, and scanning electron microscopy), its densification under a reducing atmosphere was monitored through dilatometric measurements and compared to a similar experiment with UO_2+δ. During the heating phase, up to 2173 K, the U_0.85Am_0.15O_2 ± δ pellet shrinks from 70 to 95%TD (theoretical density). Neck formation and growth initiate the densification of this pellet at 1200 K. Two subsequent mechanisms were also evidenced, and a maximum shrinkage rate was observed between 2000 and 2100 K, suggesting an optimal sintering temperature within this range. The associated activation energy (Q) of sintering process was also determined using three different models which yield values noticeably higher than those reported or measured for UO₂.     

UO2 + 5 ​vol% ZrB2 nano composite nuclear fuels with full boron retention and enhanced oxidation resistance

The boron isotope (¹⁰B) can be used as a neutron absorber in UO₂ to control the reactivity of nuclear fuel pellets, however, the boron source can react with oxygen source in UO₂ to form B₂O₃ that vaporize readily at temperatures above 1200 °C. Unfortunately, the sintering of UO₂ fuel requires hours holding at high temperature (>1700 °C), resulting in an inevitable B loss during sintering and unpredictable B concentration in final product. It is challenging to incorporate boron through a conventional sintering method. In this work, we demonstrated that spark plasma sintering (SPS), a field assisted sintering technology, can effectively densify UO₂ ​+ ​5 ​vol% ZrB₂ composite fuel pellets by rapid consolidation at 1600 °C for a short duration of 5 min under an applied pressure of 40 MPa. Thermogravimetric analysis (TGA) measurements confirm that ZrB₂ is fully retained inside the composite fuel pellets. Inside the composite fuel pellets, nano sized ZrB₂ particles are uniformly distributed along the grain boundaries of the UO₂ matrix. The ZrB₂ particle transforms to a glassy B₂O₃ phase covering the sample surface and grain boundaries of UO₂ matrix after a simple post-sintering annealing at 1000 °C in flowing Argon gas for 4 h. The formed glassy B₂O₃ slows down the diffusion of oxygen ions and postpones the onset temperature for oxidation of UO₂ from 400 °C to 550 °C. This study demonstrates the capability of SPS, an advanced fuel manufacturing technique, to achieve a full retention of ZrB₂ in UO₂ oxide fuel and increase oxidation resistance through a simple post-sintering annealing. The reported work holds great engineering potential for development of advanced oxide fuel for nuclear application.     

Densification behaviour of UO2/Mo core-shell composite pellets with a reduced coefficient of thermal expansion

UO₂/Mo composite pellets with enhanced thermal conductivity have been considered for the novel construction of high-safety fuel systems. UO₂/Mo core-shell composite pellets with a reasonable porosity of 4–5% were fabricated by spark plasma sintering (SPS). In the SPS UO₂/Mo composite, the majority of pores remained homogeneous in the UO₂ matrix, while it had a dense morphology in the continuous Mo channel for the heat conduction. The sintering behaviour of the UO₂/Mo composite indicated that the incorporating Mo impeded densification of UO₂, and the primary densification temperature ranged from 903 K to 1270 K. To introduce 2 vol% Mo to UO₂, the thermal conductivity (TC) was enhanced to 4.02 W/m·K at 1073 K. The above result represented a 23.31% improvement over the value of 3.26 W/m·K (Fink) for a pure UO₂ pellet, which was approximated by the revised Hasselman-Johnson model. In particular, the coefficient of thermal expansion (CTE) was reduced to 10.0 × 10⁻⁶/K from 298 K to 1673 K, representing an 8.83% reduction from the value of 10.98 × 10⁻⁶/K (Martin) for a pure UO₂ pellet. The reduction effect on the CTE was superior to that of other UO₂ matrix composites fuel systems and, hence, offers an external safety aspect for reactors at the elevated temperatures close to accident conditions. These results provide a feasible method for the fabrication of a UO₂/Mo core-shell pellet as an accident tolerant fuel.     

Interface reaction of U3Si2-UO2 composite pellets during spark plasma sintering

U₃Si₂-UO₂ composite fuel combines the advantages of high uranium density and thermal conductivity of U₃Si₂ and excellent stability of UO₂ under light water reactor (LWR) condition. Since commercialized pressureless sintering of UO₂ is usually performed above the melting point of U₃Si₂, spark plasma sintering (SPS) is considered as a convenient method to produce U₃Si₂-UO₂ composite fuel. However, the potential interaction between U₃Si₂ and UO₂, an important aspect for composite materials, has not been sufficiently investigated. In this report, U₃Si₂-UO₂ pellets with 1:1 wt ratio were fabricated by SPS. Next, the interaction between U₃Si₂ and UO₂ during the sintering process was examined with X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that U₃Si₂ and UO₂ reacted to yield USi as the main product. The reaction occurred at a relatively low temperature of 1100 °C, at which the composite fuel was less than 90% theoretical density (TD).     

Single-step,high pressure,and two-step spark plasma sintering of UO2 nanopowders

Three different spark plasma sintering (SPS) treatments were applied to highly sinteractive, near-stoichiometric UO_2.04 nanocrystalline (5 nm) powders produced by U(IV) oxalate hydrothermal decomposition at 170 °C. The sintering conditions for reaching 95 % theoretical density (TD) in regular SPS, high pressure SPS (HP-SPS), and, for the first time, two-step SPS (2S-SPS), were determined. Densification to 95 % TD was achieved at 1000 °C in regular SPS (70 MPa applied pressure), 660 °C in HP-SPS (500 MPa), and 650?550 °C in 2S-SPS (70 MPa). With the goal of minimising the grain growth during densification, the sintering treatments were optimised to favour densification over coarsening, and the final microstructures thus obtained are compared. Equally dense UO₂ samples of different grain sizes, ranging from 3.08 μm to 163 nm, were produced. Room-temperature oxidation of the powders could not be avoided due to their nanometric dimensions, and a final annealing treatment was designed to reduce hyperstoichiometric samples to UO_2.00.     

Densification kinetics and sintering behavior of UO2 and 0.5 wt.%MnO-doped UO2

In this work, the sintering kinetics of pure UO₂ and 0.5 wt.%MnO-doped UO₂ was studied by a high-temperature dilatometer heated up to 1500°C. In addition, the sintering behavior of pure UO₂ and 0.5 wt.%MnO-doped UO₂ was studied by pressureless sintering technique. The results showed that MnO doping enhanced the grain boundary diffusion of UO₂, which can effectively decrease the densification temperature and promote grain growth. The sintering temperature of UO₂ was significantly reduced by about 200°C with the addition of 0.5 wt.%MnO. The microscopic morphology studies showed that there were still fine particles agglomerated, forming sintered spheres in the matrix even if no severe agglomeration and bimodal size distribution were observed in raw UO₂ powder. The microstructure evolution of the sintered sphere and UO₂ matrix during the densification process was studied by isothermal sintering. Finally, the present analyses indicated that the densification of UO₂ matrix can be accelerated by adding MnO or increasing the sintering temperature, thus improving the densification inhomogeneity of UO₂ matrix.     

Surface electrochemistry of UO2 in dilute alkaline hydrogen peroxide solutions

The reaction of H₂O₂ on SIMFUEL electrodes has been studied electrochemically and under open circuit conditions in 0.1 mol l⁻¹ NaCl (pH 9.8). The composition of the oxidized UO₂ surface was determined by X-ray photoelectron spectroscopy (XPS). Peroxide reduction was found to be catalyzed by the formation of a mixed U^IV/U^V (UO_2+x) surface layer, but to be blocked by the formation of U^VI (UO₂²⁺) species on the electrode surface. The formation of this U^VI layer blocks both H₂O₂ reduction and oxidation, thereby inhibiting the potentially rapid H₂O₂ decomposition process to H₂O and O₂. Decomposition is found to proceed at a rate controlled by desorption or reduction of the adsorbed O₂ species. Reduction of O₂ is coupled to the slow oxidative dissolution of UO₂ and formation of a corrosion product deposit of UO₃·yH₂O.     

Boro/carbothermal reduction synthesis of uranium tetraboride and its oxidation behavior in dry air

Uranium tetraboride (UB₄) was successfully synthesized by boro/carbothermal reduction of UO₂ with B₄C and carbon as combined reduction agents under flowing argon. The effects of processing temperature and mole ratios of starting materials on phase evolution were studied. XRD results demonstrated that UB₄ was obtained with 3.75 mol% B₄C in excess based on the UO₂/B₄C/C molar ratios of 1:1:1 at 1500°C. SEM observation revealed that submicrometer-sized quasi-spherical UB₄ particles cemented together to be an aggregate. The laser particle size analysis showed that the particle size was in the range of 1-10 μm. The oxidation behavior of UB₄ was also investigated by TG and XRD. The oxidation of UB₄ started at about 500°C and it showed better oxidation resistance than other basic uranium nuclear fuels (UO₂, UC, UN and U₃Si₂). The oxidation chemical process of UB₄ was presented as a three-step process: (a) the formation of U₃O₈ and B₂O₃ oxidation products; (b) the formation of UB₂O₆ intermediate product by the interaction of U₃O₈ and B₂O₃; (c) the decomposition of UB₂O₆ to get U₃O₈.     

Dense nanocrystalline UO2+x fuel pellets synthesized by high pressure spark plasma sintering

Nanocrystalline UO_2+x powders are prepared by high‐energy ball milling and subsequently consolidated into dense fuel pellets (>95% of theoretical density) under high pressure (750 MPa) by spark plasma sintering at low sintering temperatures (600°C‐700°C). The grain size achieved in the dense nano‐ceramic pellets varies within 60‐160 nm as controlled by sintering temperature and duration. The sintered fuel pellets are single phase UO_2+x with hyper‐stoichiometric compositions as derived by X‐ray diffraction, and micro‐Raman measurements indicate that random oxygen interstitials and Willis clusters dominate the single phase nano‐sized oxide pellets of UO_2.03 and UO_2.11, respectively. The thermal conductivities of the densified nano‐sized oxide fuel pellets are measured by laser flash, and the fuel stoichiometry displays a dominant effect in controlling thermal transport properties. A reduction in thermal conductivity is also observed for the dense nano‐sized pellets as compared with micron‐sized counterparts reported in the literature. The correlation among the SPS sintering parameters—microstructure control—properties is established, and the nano‐sized UO_2+x pellets with controlled microstructure can serve as the model systems for fundamental understandings of fuel behaviors and obtaining critical experimental data for multi‐physics MARMOT model validation.     

Influence of the PuO2 content on the sintering behaviour of UO2-PuO2 freeze-granulated powders under reducing conditions

The sintering behaviour of freeze-granulated UO₂-PuO₂ powders containing 33 and 15 mol% Pu/(U + Pu) was investigated under reducing conditions up to 1700 °C. For both compositions, the “grain size versus relative density” trajectory was constructed. All the experimental points form a single trajectory meaning that a relative density/grain size pair obtained after sintering seems independent of the thermal path (heating rate, soak time, soak temperature) and of the Pu content. Exploiting the “grain size versus relative density trajectory” enabled also to propose that densification was controlled by grain boundary diffusion and grain growth by the grain boundaries whatever the Pu content. An activation energy around 510 kJ/mol was obtained for densification, which was close to the value reported for the grain boundary diffusion of plutonium cations in U_1-xPu_xO₂ polycrystals. Whatever the Pu/(U + Pu) content, the sintered microstructure of 98 % dense samples possesses a homogeneous distribution of plutonium and uranium cations.     

In situ control of oxygen partial pressure in a buffered UO2 fuel: A manufacturing process

An innovative laboratory process making use of the specific properties of solid redox buffers was developed for producing O₂ self-regulated UO₂ fuel samples. The method was optimized using Molybdenum and Niobium oxide buffers. The starting materials were first mixed with the UO₂ powder, then pressed into pellets and finally sintered in a quasi-closed vessel at 1670 °C under strictly controlled oxygen potential using appropriate solid redox buffers. Speciation of Mo and Nb was characterized using Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-ray spectroscopy (EDX) as well as X-ray Diffraction (XRD). Using this optimized procedure, both oxido-reducing forms of the O₂ buffer incorporated into the UO₂ specimens were preserved during sintering, allowing an in situ control of the O₂ partial pressure inside the material. The processing methodology, the laboratory experimental set-up, the sintering procedure and the characteristics of the final oxygen buffered UO₂ fuel are described. This work opens the path to representative laboratory studies related to the redox dependent behavior of irradiated fuels in reactor.     

Effect of niobium doping on the densification and grain growth in alumina

The effect of niobium doping on the densification and grain growth of nano-sized α-Al₂O₃ powders during sintering has been investigated. The dopant concentration added ranged from 0.1 to 0.5 mol%. It was observed that addition of niobium oxide could improve the densification of the pure alumina with a lower sintering temperature, a shorter sintering time. The effect is strengthened by increasing the amount of dopant. It also demonstrated that niobium dopant significantly promotes the grain growth of alumina during sintering and the grain size of alumina increases with increasing the amount of dopant in the added range.     

Influence of processing parameters on thermal conductivity of uranium dioxide pellets prepared by spark plasma sintering

High density uranium dioxide (UO₂) pellets with grain sizes between 0.9 μm and 9 μm were produced by spark plasma sintering (SPS). A systematic study was performed by varying the sintering temperature between 750 °C and 1450 °C and hold time between 0.5 min and 20 min to obtain UO₂ pellets with a range of theoretical densities (TD) and grain sizes. The microstructure development in terms of grain size, density and porosity distribution was investigated. The oxygen/uranium (O/U) ratio of the resulting pellets was found to decrease after SPS. The thermal conductivity of UO₂ pellets increased with the theoretical density but the grain size in the investigated range had no significant influence. The measured thermal conductivity values up to 900 °C were consistent with the reported literature for conventionally sintered UO₂ pellets. The benefits of using SPS over the conventional sintering of UO₂ are summarized.     

Reactive sintering of U1−yAmyO2±x in overstoichiometric conditions

In the framework of Partitioning & Transmutation (P&T), U_1?yAm_yO_2±x materials are promising fuels for Am recycling. In this context, these materials were fabricated in the ATALANTE facility by a process which consists of pelletizing and reactive sintering. Since it was effective in studies conducted on UO_2+x, sintering in overstoichiometric conditions was investigated. In this work, three Am contents (10, 20 and 30%) and four temperatures (from 1000 to 1300 °C) were studied. It was shown that low-density and multiphasic compounds were obtained. Moreover, XRD and XAFS analyses pointed out that the total reduction of Am(+IV) to Am(+III) and solid solution formation occur during sintering at temperatures inferior to 1300 °C. Although previous studies on UO_2+x showed that high oxygen potential enhanced the diffusion process, this work proved that this effect is clearly modified by the presence of Am. Finally, sintering in overstoichiometric conditions is not yet suitable for Am-bearing fuel fabrication.     

On the origin of the sigmoid shape in the UO2 oxidation weight gain curves

Cracking and spalling are known to occur during the oxidation of UO₂. However, these phenomena are not considered by the existing kinetic models of the oxidation of UO₂ into U₃O₈. In this study the oxidation of UO₂ samples of various sizes from the single crystal to nanopowders, has been followed by isothermal and isobaric thermogravimetry, environmental scanning electron microscopy and in situ X-ray diffraction at temperatures ranging from 250 to 370 °C in air. It has been shown that cracking occurs once a critical layer thickness of intermediate oxide has been reached, which corresponds to the beginning of the sigmoid kinetic curve. Cracking contribution to the sigmoid kinetic curve is then discussed as a function of temperature, and on the basis of nucleation and growth processes.     

Helium behaviour in stoichiometric and hyper-stoichiometric UO2

Understanding the long-term behaviour of the UO₂ spent fuel in terms of helium build-up and oxidation is a very important issue for safety aspects of storage and disposal. Although helium behaviour in stoichiometric UO₂ has been studied by many authors, it has not been well established and there is a lack of experimental studies in non-stoichiometric UO₂. In this study, an infusion technique was chosen to introduce helium in stoichiometric and hyperstoichiometric single and polycrystalline UO₂ and polycrystalline U₃O₈ samples. Characterization of the samples before and after infusion and after thermal desorption measurements were performed by thermogravimetry, X-ray diffraction, laser flash technique and scanning electron microscopy. It was observed that the increase of stoichiometry in UO₂ affects barely the dissolved helium quantity (under the same infusion conditions as in stoichiometric), while it has a more pronounced influence on the helium diffusion. These two effects are much more pronounced for U₃O₈.     

A semi-empirical model for the air oxidation kinetics of UO<Subscript>2</Subscript>

UO₂ is readily oxidized to U₃O₈ at a high temperature, and this reaction has received considerable attention in the field of nuclear fuel cycles. A voloxidation process which makes use of the characteristics of a UO₂ oxidation has been developed to treat the spent fuels produced by irradiation of UO₂. In this work, semi-empirical kinetic models to describe the sigmoidal behavior of a UO₂ oxidation were selected and compared in order to obtain a kinetic expression with different temperatures. Two basic approaches of a nucleation-and-growth model and an autocatalytic reaction model were adequate enough to describe the S-shaped oxidation behavior, and an equation to correlate the model parameters with the temperature was introduced. The calculation results of the two models satisfy the experimental data for UO₂ spheres and the activation energy of a reaction rate constant was evaluated. The models were also adopted as a surface reaction time term for a UO₂ pellet.     

Competition between densification and microstructure development during spark plasma sintering of B4C–Eu2O3

The densification of nonoxide ceramics has been a known challenge in the field of engineering ceramics. The amount and type of sinter‐aid together with sintering conditions significantly influence the densification behavior and microstructure in nonoxide ceramics. In this perspective, the present work reports the use of Eu₂O₃ sinter‐aid and spark plasma sintering towards the densification of B₄C. The densification is largely influenced by the solid‐state sintering reactions during heating to 1900°C. Based on the careful analysis of the heat‐treated powder mixture (B₄C–Eu₂O₃) and sintered compacts, the competitive reaction pathways are proposed to rationalize the formation of EuB₆ as dominant microstructural phase. An array of distinctive morphological features, including intragranular and intergranular EuB₆ phase as well as characteristic defect structures (asymmetric twins, stacking faults and threaded dislocations) are observed within dense B₄C matrix. An attempt has been made to explain the competition between microstructure development and densification.     

Densification of liquid phase sintered silicon carbide

The densification and phase formation of liquid phase sintered silicon carbide (LPSSiC) with 10 wt.% additives were investigated. The ratio of the Al₂O₃/Y₂O₃-additives was changed between 4:1 and 1:2. Densification was carried out by hot pressing and gas pressure sintering. The different densification routes result in different major grain boundary phases—aluminates in gas pressure sintered materials and silicates in hot pressed samples. Thermodynamic calculations were carried out to determine the amount of liquid phase during densification and for the interpretation of the results.     

The effect of yttrium nitrate addition on the densification behaviour of Y2O3 ceramics during the cold sintering process

The densification behaviour and phase development of Y₂O₃ ceramics were investigated as a function of yttrium nitrate (Y(NO₃)₃·6H₂O) solution addition during the cold sintering process at 200 °C. Second phases such as Y₄O(OH)₉NO₃ and Y(OH)₃ were observed after the cold sintering process. The amount of Y₄O(OH)₉NO₃ increased with increasing amount of yttrium nitrate, while the amount of Y(OH)₃ decreased. The second phases were transformed to fine sized Y₂O₃ (∼30 nm) particles smaller than those of the raw powder (<400 μm) by sintering at 600 °C. The fine sized Y₂O₃ particles were located in the voids between the larger Y₂O₃ particles, thus increasing the packing density and enhancing the densification of the Y₂O₃ ceramics after the final sintering process.