Microbehavior of U3Si during deformation

Metallography and X-ray diffraction were used to investigate the texture and microstructure of the intermetallic compound U₃Si after bulk and surface loading. It was found that there is neither an order-disorder transition nor a phase transformation as a result of cold-working of UaSi. Changes in the X-ray and in the metallographic patterns in deformed martensitic U₃Si were interpreted as being due to detwinning of the {112} 〈111〉 systems. Detwinning was moderate in the elastic range and considerable in the plastic range.     

Counter-diffractometer parameter determination of polycrystalline U3Si

The present study deals with an X-ray diffraction analysis of U₃Si. The U₃Si lattice parameters have been determined to a more accurate degree by means of a counter diffractometer step-scan apparatus. The intensities of the reflections were also recorded and compared with the calculated values. The discrepancies between calculated and observed intensités are discussed; they are probably caused by periodic faulting.     

Post-irradiation examination of AlFeNi cladded U3Si2 fuel plates irradiated under severe conditions

Three full size AlFeNi cladded U₃Si₂ fuel plates were irradiated in the BR2 reactor of the Belgian Nuclear Research Centre (SCK·CEN) under relatively severe, but well defined conditions. The irradiation was part of the qualification campaign for the fuel to be used in the future Jules Horowitz reactor in Cadarache, France. After the irradiation, the fuel plates were submitted to an extensive post-irradiation campaign in the hot cell laboratory of SCK·CEN. The PIE shows that the fuel plates withstood the irradiation successfully, as no detrimental defects have been found. At the cladding surface, a multilayered corrosion oxide film has formed. The U-Al-Si layer resulting from the interaction between the U₃Si₂ fuel and the Al matrix, has been quantified as U(Al,Si)_4.6. It is found that the composition of the fuel particles is not homogenous; zones of USi and U₃Si₂ are observed and measured. The fission gas-related bubbles generated in both phases show a different morphology. In the USi fuel, the bubbles are small and numerous while in U₃Si₂ the bubbles are larger but there are fewer.     

Comportement du compose U3Si sous irradiation aux neutrons: Microscopie et diffraction electroniques

In-pile irradiation of the U₃Si compound was carried out at different temperatures (200, 300 and 600 °C) and to fluences of 10¹⁵ and 10¹⁷ fissions/cm³. The experimental results could best be understood by assuming the occurrence of a simple cubic lattice originating from the tetragonal ág cubic phase transformation. Actually, the temperature dependence of swelling in the explored irradiation range could be accounted for by the related volume changes. Thereby a new source of dimensional instability was evidenced, lowering still further the chances of U₃Si ever being used as a nuclear fuel.     

Heats of formation of (U,Mo)Al3 and U(Al,Si)3

The heats of formation of (U,Mo)Al₃ intermetallic compounds were obtained by measuring the reaction heats of U-Mo/Al dispersion samples by differential scanning calorimetry. Based on literature data for the reaction heats of U₃Si/Al and U₃Si₂/Al dispersion samples, the heats of formation of U(Al,Si)₃ as a function of the Si content were calculated. The heat of formation of (U,Mo)Al₃ becomes less negative as the Mo content increases. Conversely, the heat of formation of U(Al,Si)₃ becomes more negative with increasing Si content.     

Temperature and dose dependence of fission-gas-bubble swelling in U3Si2

Large fission gas bubbles were observed during metallographic examination of an irradiated U₃Si₂ dispersion fuel plate (U0R040) in the Advanced Test Reactor (ATR). The fuel temperature of this plate was higher than for most of the previous silicide-fuel tests where much smaller bubble growth was observed. The apparent conditions for the large bubble growth are high fission density (6.1 × 10²¹ f/cm³) and high fuel temperature (life-average 160 °C). After analysis of PIE results of U0R040 and previous ANL test plates, a modification to the existing athermal bubble growth model appears to be necessary for high temperature application (above 130 °C). A detailed analysis was performed using a model for the irradiation-induced viscosity of binary alloys to explain the effect of the increased fuel temperature. Threshold curves are proposed in terms of fuel temperature and fission density above which formation and interconnection of bubbles larger than 5 μ are possible.     

Test irradiations of full-sized U3Si2-Al fuel plates up to very high fission densities

In the course of the licensing procedure of the ‘Forschungsneutronenquelle Heinz Maier-Leibnitz’, i.e. the new 20 MW high-flux research reactor FRM II in Garching near Munich, extensive test irradiations have been performed to qualify the U₃Si₂-Al dispersion fuel with a relatively high density of highly enriched uranium (93 wt% of ²³⁵U) up to very high fission densities. Two of the three FRM II type fuel plates used in the irradiation tests contained U₃Si₂-Al dispersion fuel with HEU densities of 3.0 gU/cm³ or 1.5 gU/cm³ (‘homogeneous plates’) and one plate had two adjacent zones of either density (‘mixed plate’). They were irradiated in the French MTR reactors SILOE and OSIRIS in the years before 2002. The local plate thickness was measured on certain tracks along the plates during interruptions of the irradiation. The maximum fission density obtained in the U₃Si₂ fuel particles was 1.4 × 10²² f/cm³ and 1.1 × 10²² f/cm³ in the 1.5 gU/cm³ and 3.0 gU/cm³ fuel zones, respectively. In the course of the irradiations, the plate thickness increased monotonically and approximately linearly, leading to a maximum plate thickness swelling of 14% and 21% and a corresponding volume increase of the fuel particles of 106% and 81%, respectively. Our results are discussed and compared with the data from the literature.     

Irradiation of U3Si-based compounds in the high-voltage electron microscope

U₃Si and U-3.5 wt% Si-1.5 wt% Al have been irradiated in a high-voltage electron microscope (HVEM) at 500–1180 keV and 300–660 K. The creation of ‘black spot’ damage and removal of deformation twins are observed. Defects about 10 nm diameter averaging 4 × 10²¹ m^?3 are seen only in samples pre-injected with 10^?5 atomic fraction argon and are tentatively identified as voids. The atomic displacement rate during HVEM irradiation of U₃Si-based compounds is about two orders of magnitude higher than that for fuel at power reactor ratings. It is inferred that displacement of silicon, and possible uranium, atoms in U₃Si-based compounds occurred in the HVEM at accelerating voltages in the range 700–1180keV.     

Reactivity feedback coefficients of a material test research reactor fueled with high-density U3Si2 dispersion fuels

The reactivity feedback coefficients of a material test research reactor fueled with high-density U₃Si₂ dispersion fuels were calculated. For this purpose, the low-density LEU fuel of an MTR was replaced with high-density U₃Si₂ LEU fuels currently being developed under the RERTR program. Calculations were carried out to find the fuel temperature reactivity coefficient, moderator temperature reactivity coefficient and moderator density reactivity coefficient. Nuclear reactor analysis codes including WIMS-D4 and CITATION were employed to carry out these calculations. It is observed that the average values of fuel temperature reactivity feedback coefficient, moderator temperature reactivity coefficient and moderator density reactivity coefficient from 20 °C to 100 °C, at the beginning of life, followed the relationships (in units of Δk/k × 10⁻⁵ K⁻¹) −2.116 − 0.118 ρ_U, 0.713 − 37.309/ρ_U and −12.765 − 34.309/ρ_U, respectively for 4.0 ≤ ρ_U (g/cm³) ≤ 6.0.     

Formation of U2C3 in the reaction of UC2 with UO2

The formation of U₂C₃ by the reaction of UC₂ with UO₂ has been studied by chemical and X-ray analyses at temperatures between 1400 and 1700 °C in vacuo. The reaction is represented by 7 UC₂ + UO₂ → 4 U₂C₃ + 2 CO.     

An investigation on the green properties of U-10wt.%Mo/Al and U3Si2/Al powder compacts

The effects of fuel powder volume fraction and fuel particle shape on green properties of compacts, which were produced by processing the blended U-10wt.%Mo and U₃Si₂ with Al powders were investigated respectively, with respective to the compacting pressure range of 50–400 MPa. The relative density of the compacts increases with decreasing volume fraction of fuel powder. The compressibility of comminuted powder compacts was larger than that of the atomized powder compacts due to the fragmentation of comminuted particles, and the compressibility of the compacts of U-10wt.%Mo was larger than that of the compacts of U₃Si₂ due to the deformation of U-10wt.%Mo particles. The green strength of the comminuted powder compacts is higher than that of the atomized powder compact. This seems to have resulted from the smaller pore size and the larger contact area between the comminuted fuel powders and Al powders. It is suggested that the compacting condition adjustment be required to fabricate the atomized powder compacts having comparable green strength.     

Thermoelectric power studies of ferromagnet U2ScB6C3

The thermoelectric power (TEP) of a ferromagnet U₂ScB₆C₃ (T_C = 61 K) has been measured in the temperature range 5-300 K. The TEP is positive over the whole measured temperature range and reaches a relatively large value at room temperature of 29 μV/K. Below 30 K and above 200 K the TEP follows a straight line S(T) ∼AT, with slope of 0.23 and 0.085 μV/K², respectively. The change in the slope can be explained by the electron-phonon interaction renormalization effects or spin-reorientation associated with a change in the electronic structure. Analysing the temperature dependence of the ratio [S(T)/T]/[S_300 K/300] and taking into account the specific heat data, we suggest that spin fluctuations are another important factor in determining the thermoelectric power behaviour of U₂ScB₆C₃.     

The U0.8Pu0.2C-W system

A phase diagram of the U_0.8Pu_0.2C-W system has been established based on X-ray diffraction measurements and metallographic observations. A peritectic four-phase reaction has been found to occur at 2100 ± 40°C:

The peritectic point is close to U_0.8Pu_0.2C-27 wt.%W. Composition of the peritectic liquid is near 48(U + Pu)/18W/34C (at %) by analogy with the UC-W system. The liquid phase transforms, during rapid cooling, to a metastable mixture [(U, Pu)C + (U, Pu)metal + W] with accompanied retention of (U, Pu)WC_1.75. The (U, Pu)WC_1.75 phase crystallizes in a UWC_1.75-type monoclinic structure. Essentially single-phase (U_0.8Pu_0.2)WC_1.75 has been produced by sufficient annealing of the arc-melted specimen, with lattice parameters: a₀ = 5.6257 ± 0.0008 Å, b₀= 3.2498 ± 0.0005Å, c₀ = 11.623 ± 0.002 Å, β = 109.61 ± 0.01°. This compound perhaps melts incongruently as:

The terminal solid solubility limit of tungsten in U_0.8Pu_0.2C increases from about 0.8 wt%W at 1500°C to a maximum of about 3.2 wt%W at 2100°C. The partial molar heat of solution of tungsten in U_0.8Pu_0.2C is approximately 14.4 kcal/mole. Un diagramme de phase du système U_0,8Pu_0,2C-W a étée'tabli en se basant sur les mesures par diffraction aux rayons X et par examen métallographique. Une réaction péritectique a quatre phases a été observée à 2100 ± 40°C:

     

Cuboctahedral oxygen clusters in U3O7

When UO₂ is oxidised to U₃O₇, the positions in the crystal lattice of all the uranium atoms and of about 70% of the oxygen atoms are hardly affected. The remaining oxygen atoms occupy new sites which are located 310 pm along 〈1 1 0〉 vectors from the holes in the fluorite framework of UO₂. These results, which are based on the analysis of neutron diffraction powder data, are consistent with the concept that excess oxygen in U₃O₇ is accommodated in cuboctahedral anionic clusters.     

Formation of columnar and equiaxed grains by the reduction of U3O8 pellets to UO2+x

The reduction of U₃O₈ pellets to UO_2+x has been investigated at 1300 °C in H₂, Ar and CO₂ gas atmospheres by TGA, SEM, and X-ray diffraction. The selected U₃O₈ pellet was prepared by sintering a U₃O₈ powder compact. The TGA results show that the reduction rate is fastest in H₂ gas, and X-ray diffraction indicates that U₃O₈ reduces to UO_2+x without any intermediate phase. The reduced pellet, UO_2+x, has a special grain structure that consists of equiaxed grains at the surface, columnar grains in the middle, and equiaxed grains in the center. The equiaxed grains and columnar grains are much smaller in H₂ gas than in Ar or CO₂ gas. The reducing gases significantly influence the morphology of the grain structure. This difference can be explained in terms of a relation between oxygen potential and critical nucleus size during the reduction.     

Thermodynamic properties of the O-U system. II - Critical assessment of the stability and composition range of the oxides UO2+x, U4O9−y and U3O8−z

The published data concerned with the determination of the composition ranges of uranium oxides, UO_2+x, U₄O_9−y and U₃O_8−z, which have been determined using thermogravimetric, X-ray diffraction and electrochemical techniques are critically assessed. U₄O₉ and U₃O₈ have quite small domains of composition and the assessment of such data has carefully considered the uncertainties in the experimental determinations. In addition, the thermodynamic properties of U₄O₉ and U₃O₈, enthalpies of formation and transformation, entropies, and thermal capacities are analyzed and selected to build a primary data base for compounds.     

The mechanism and kinetics of U2C3 formation by the synthetic reaction

In fast breeder reactors it is planned to use the fuel in the form of (U, Pu)C with a slight carbon hyperstoichiometry. It is therefore important to know under what conditions the synthetic reaction $\text{UC + UC}{}_2\text{U}{}_2\text{C}{}_3$ occurs, since the hyperstoichiometric carbon, which exists as a uranium dicarbide phase (UC₂) after manufacture, is to be converted to U₂C₃. The literature concerning the reaction conditions is partly contradictory. In this paper, the influence of thermal or mechanical pretreatments on the formation of U₂C₃ was investigated experimentally and is discussed in connection with other published data. It was found that the relative increase of the U₂C₃ nuclei density caused by grinding corresponds to the increase in surface of the ground material. A quantitative kinetic examination of the U₂C₃ formation was made and the activation energy for the synthetic reaction in powder was found to be $\text{394 ± 30}$ kJ/mol.     

Recent activities in the field of radiation measurement and nuclear data

The characteristics, that is, morphology, size distribution, alloy phase and microstructure of U₃Si and U₃Si₂ powders, solidified rapidly by a centrifugal atomization, were investigated. The atomized powders consist of spherical particles with a relatively narrow size distribution independent of the alloy composition. The particle size distribution can be controlled by adjusting the atomization parameters such as feeding rates of the melt and revolution speeds of the disk. The major phases of atomized U₃Si and U₃Si₂ powders are α-U and U₃Si₂ and U₃Si₂, respectively. The atomized U₃Si powder has a dendritic structure of very fine and non-faceted U₃Si₂ precipitates with less fibric and eutectic U₃Si₂ structure. The microstructure of U₃Si₂ powder shows a cellular structure with fine U₃Si₂ grains and finely dispersed silicon-rich precipitates. The time for complete peritectoid reaction of the atomized U₃Si particles and the resulting grain size are greatly reduced, due to the refinement of primary U₃Si₂ precipitates.     

The ternary system: silicon-uranium-vanadium

Phase equilibria in the system Si-U-V were established at 1100 °C by optical microscopy, EMPA and X-ray diffraction. Two ternary compounds were observed, U₂V₃Si₄ and (U_1−xV_x)₅Si₃, for which the crystal structures were elucidated by X-ray powder data refinement and found to be isotypic with the monoclinic U₂Mo₃Si₄-type (space group P2₁/c; a = 0.6821(3), b = 0.6820(4), c = 0.6735(3) nm, β = 109.77(1)°) and the tetragonal W₅Si₃-type (space group I4/mcm, a = 1.06825(2), c = 0.52764(2) nm), respectively. (U_1−xV_x)₅Si₃ appears at 1100 °C without any significant homogeneity region at x ∼ 0.2 resulting in a formula U₄VSi₃ which corresponds to a fully ordered atom arrangement. DTA experiments clearly show decomposition of this phase above 1206 °C revealing a two-phase region U₃Si₂ + V₃Si. At 1100 °C U₄VSi₃ is in equilibrium with V₃Si, V₅Si₃, U₃Si₂ and U(V). At 800 °C U₄VSi₃ forms one vertex of the tie-triangle to U₃Si and V₃Si. Due to the rather high thermodynamic stability of V₃Si and the corresponding tie-lines V₃Si + liquid at 1100 °C and V₃Si + U(V) below 925 °C, no compatibility exists between U₃Si or U₃Si₂ and vanadium metal.     

The use of U3Si2 dispersed fuel in Low-Power Research Reactors

The use of U₃Si₂ as a Low Enriched Uranium (LEU) dispersed fuel in Low-Power Research Reactors is investigated in this paper. The fuel proves to be usable if some of the original fuel rods (HEU UAl₄–Al fuel) are still simultaneously employed (mixed core) without changing the structure of the actual core. About 3.5712 mk Initial Excess Reactivity (IER) is procured. Although the worths of both the control rod and the reactivity devices decrease, the safety of these reactors is higher in the case of the new LEU fuel. If the dimensions of the meat and/or the clad are allowed to change these reactors can be run with a meat 2.15 mm outer radius, and a clad 0.58 mm thickness. The IER will then be 4.1537 mk, and both the control rod (CR) worth and the safety margins decrease.