Effect of Nd and Pr addition on the thermal and mechanical properties of (U, Ce)O2

The thermal conductivity, Young’s modulus, and hardness of (U_0.65−xCe_0.3Pr_0.05Nd_x)O₂ (x = 0.01, 0.08, 0.12) were evaluated and the effect of Pr and Nd addition on the properties of (U, Ce)O₂ were studied. The polycrystalline high-density pellets were prepared with solid state reactions of UO₂, CeO₂, Pr₂O₃, and Nd₂O₃. We confirmed that all Ce, Pr, and Nd dissolved in UO₂ and formed solid solutions of (U, Ce, Pr, Nd)O₂. We revealed that the thermal conductivity of (U_0.65−xCe_0.3Pr_0.05Nd_x)O₂ (x = 0.12) was up to 25% lower than that of x = 0.01 at room temperature. The Young’s modulus of (U_0.65−xCe_0.3Pr_0.05Nd_x)O₂ decreased with x, whereas the hardness values were constant in the investigated x range.     

Thermal and mechanical properties of (U,Er)O2

Erbium is considered as a slow burnable poison suitable for use in light water reactors (LWRs). Addition of a small amount of Er₂O₃ to all UO₂ pellets will make it possible to develop super high burnup fuels in Japanese nuclear facilities which are now under the restriction of the upper limit of ²³⁵U enrichment. When utilizing the (U,Er)O₂ fuels, it is very important to understand the thermal and mechanical properties. Here we show the characterization results of (U_1−xEr_x)O₂ (0 ? x ? 0.1). We measured their thermal and mechanical properties and investigated the effect of Er addition on these properties of (U,Er)O₂. All Er completely dissolved in UO₂, and the lattice parameter decreased linearly with the Er content. Both the thermal conductivity and Young’s modulus of (U,Er)O₂ decreased with the Er content. These results would be useful for us in evaluating the performance of the (U,Er)O₂ fuels in LWRs.     

Thermal conductivity evaluation of high burnup mixed-oxide (MOX) fuel pellet

The thermal conductivity formula of fuel pellet which contains the effects of burnup and plutonium (Pu) addition was proposed based on the Klemens’ theory and reported thermal conductivities of unirradiated (U, Pu) O₂ and irradiated UO₂ pellets. The thermal conductivity of high burnup MOX pellet was formulated by applying a summation rule between phonon scattering parameters which show the effects of plutonium addition and burnup. Temperature of high burnup MOX fuel was evaluated based on the thermal conductivity integral which was calculated from the above-mentioned thermal conductivity formula. Calculated fuel temperatures were plotted against the linear heat rates of the fuel rods, and were compared with the fuel temperatures measured in a test reactor. Since both values agreed well, it was confirmed that the proposed thermal conductivity formula of MOX pellets is adequate.     

Thermophysical properties of Th1−xUxO2 pellets prepared by spark plasma sintering technique

Th_1?x U _x O₂ solid solutions were synthesized by solid-state reaction and pelletized using the spark plasma sintering (SPS) technique. Pellets with >90% theoretical density were easily obtained within 40 min of sintering without any additive. The thermal conductivity, Young's modulus, Debye temperature, Vickers hardness, and heat capacity were systematically investigated, and the values for ThO₂ agree with the literature data. The thermal conductivity of Th_1?x U _x O₂ decreased with increasing U content up to x =～0.5. This tendency corresponds to phonon-point defect scattering theory. The Young's modulus and Debye temperature linearly decreased with increasing U content. The Debye temperature and standard molar entropy derived from the low-temperature heat capacity agree with the reported values.     

Mechanical properties at sub-microscale and macroscale of polycrystalline uranium mononitride

The indentation hardness, Vickers hardness, fracture toughness, and Young’s modulus of polycrystalline uranium mononitride (UN) at sub-microscale and macroscale were evaluated by an indentation test, Vickers hardness test, and the ultrasonic pulse echo method. The Young modulus and Vickers hardness were in good agreement with the literature values. The fracture toughness of UN was about three times that of UO₂. In addition, we revealed the indentation size effect on the indentation hardness of UN.     

Fabrication and mechanical characterization of zirconium and gadolinium hydrides

We prepared three kinds of metal hydrides: Zr hydrides, Gd hydrides, and the hydrides of Zr-Gd alloys (Zr:Gd = 10:1, 8:1, 6:1), and characterized their mechanical properties. It was confirmed that the hydrides of Zr-Gd alloys were composed of Zr hydrides and Gd hydrides mixtures. We evaluated the Vickers hardness and the Young’s modulus of the hydrides. We succeeded in proposing empirical equations describing the density, Vickers hardness, and Young’s modulus of the hydrides of Zr-Gd alloys, as functions of the hydrogen content and the Gd content.     

Thermal conductivities of irradiated UO2 and (U, Gd)O2 pellets

Thermal diffusivities of UO₂ and (U, Gd)O₂ pellets irradiated in a commercial reactor (maximum burnups: 60 GWd/t for UO₂ and 50 GWd/t for (U, Gd)O₂) were measured up to about 2000 K by using a laser flash method. The thermal diffusivities of irradiated UO₂ and (U, Gd)O₂ pellets showed hysteresis phenomena: the thermal diffusivities of irradiated pellets began to recover above 750 K and almost completely recovered after annealing above 1400 K. The thermal diffusivities after recovery were close to those of simulated soluble fission products (FPs)-doped UO₂ and (U, Gd)O₂ pellets, which corresponded with the recovery behaviors of irradiation defects for UO₂ and (U, Gd)O₂ pellets. The thermal conductivities for irradiated UO₂ and (U, Gd)O₂ pellets were evaluated from measured thermal diffusivities, specific heat capacities of unirradiated UO₂ pellets and measured sample densities. The difference in relative thermal conductivities between irradiated UO₂ and (U, Gd)O₂ pellets tended to become insignificant with increasing burnups of samples.     

Physical properties of europium sesquioxide

Europium sesquioxide (Eu₂O₃) is a neutron-absorbing material of potential use in reactor control rods and is being evaluated for use in fast reactors. This paper presents the results of physical and mechanical property measurements performed on unirradiated europia. The material exists in two useful crystallographic forms. Both the monoclinic form and a cubic variety, stabilized by the addition of 17 wt.% molybdenum trioxide (MoO₃), have been examined. The properties reported are density, specific heat, thermal diffusivity and conductivity, thermal expansivity, Young's modulus, and strength. The data are compared with similar information in the literature.     

Investigation of ThO2 and (U, Th)O2

The effect of the properties of ThO₂ and (U, Th)O₂ powders, prepared with different technological regimes, on the properties of the finished items is investigated. The work includes detailed investigations of ThO₂ and (U, Th)O₂ powders (x-ray phase analysis, electron-microscope investigation) and sintered fuel pellets (determination of density, study of microstructure, thermophysical investigations). The temperature dependences of the crystal lattice parameters and the sizes of the crystallites in ThO₂ and (U, Th)O₂ powders with different UO₂:ThO₂ ratio are obtained. The temperature dependences of the thermal conductivity of sintered ThO₂ and (U, Th)O₂ pellets with different UO₂:ThO₂ ratio are studied.     

Thermal Diffusivities and Thermal Conductivities of UO2-Gd2O3

Thermal diffusivities of samples of UO₂ and UO₂ doped with 3, 5, 7 and 10 w/0 Gd₂O₃ were measured over the temperature range of 298~2,023 K by a laser flash method. Then thermal conductivities were calculated from them. The thermal conductivity decreased with increasing Gd₂O₃ content at low temperatures, while it was independent of Gd₂O₃ content at high temperatures. An expression of the thermal conductivity was proposed for (U, Gd)O₂ solid solution as a function of Gd₂O₃ content and temperature by applying Klemens' model.     

Fracture stress and elastic modulus of uranium dioxide including excess oxygen

The effects of porosity, grain size and excess oxygen on Young's modulus and fracture strength for sintered pellets of compositions from UO_2.00 to UO_2.25 were investigated at room temperature. Young's modulus was determined using the ultrasonic method and fracture strength was measured in compression. The results were shown in numerical formulae and were interpreted by the authors' theory or by published theories.     

Dynamic Young’s moduli of tungsten and tantalum at high temperature and stress

Recently reported results of the long lifetime of the tungsten samples under high temperature and high stress conditions expected in the Neutrino Factory target have strengthened the case for a solid target option for the Neutrino Factory. In order to study in more detail the behaviour of the material properties of tungsten, a dynamic method has been used for measurement of Young’s modulus at high stress, high-strain-rates (>1000 s⁻¹) and very high temperatures (up to 2650 °C). The method is based on measurements of the surface vibration of thin wires, stressed by a pulsed current, using a Laser Doppler Vibrometer. The measured characteristic frequencies under the thermal excitation have been used to obtain Young’s modulus as a function of applied stress and temperature. The same procedure has been used to measure Young’s modulus of tantalum up to 2500 °C.     

Experimental studies of mechanical properties of solid zirconium hydrides

Zirconium alloy Zr-2.5Nb has been hydrided to ZrH_x (x = 1.15-2.0), and studied using microhardness and unconfined and confined compression techniques. At room temperature, results on Young’s modulus and yield strength of solid hydrides show that these mechanical properties remain about the same as the original zirconium alloy for hydrogen compositions up to about ZrH_1.5. The levels of these properties start to drop when δ hydride becomes the major phase and reaches a minimum for the ε hydride phase. Between room temperature and 300 °C, Young’s modulus of solid hydrides decreases with temperature at about the same rate as it does for the original zirconium alloy.     

Thermal Conductivity of UO2-BeO Pellet

Beryllium oxide(BeO)-doped (0.3, 0.6, 0.9, 1.2 and 13.6 wt%) UO₂ pellets were fabricated to evaluate the effects of BeO precipitate shape on thermal conductivity. Precipitate distributions were of two types: BeO precipitated almost continuously along a grain boundary (designated BeO continuous type) and spherical BeO randomly dispersed within the matrix (designated BeO dispersed type). Thermal diffusivity was measured by a laser flash method and thermal conductivity was evaluated. The thermal conductivity increased with the BeO content. The thermal conductivity of the BeO continuous type was higher than that of the BeO dispersed type at lower temperatures, while their difference became smaller at higher temperatures. The thermal conductivities of UO₂-1.2 wt% BeO at 1,100K were higher than that of UO₂ by about 25 % for the BeO continuous type and by about 10 % for the BeO dispersed type. The thermal conductivities of both types were expressed by a semi-empirical equation as a function of volume fraction and shape of the BeO precipitates.     

Structural-phase transformations in near-surface layers of alumina-zirconium ceramics induced by low-energy high-current electron beams

The effect of low-energy high-current electron beams (LEHCEBs) at E = 15 keV on mechanical characteristics of near-surface layers of alumina-zirconium ceramics (AZC) of the 20Al₂O₃-80ZrO₂(Y) composition (in mass %) is investigated by the method of dynamic indentation. It is shown that electron-beam treatment reduces Young’s modulus and nanohardness of this ceramic material. The action of LEHCEBs on the structural-phase state of modified subsurface layers of AZC specimens is analyzed. Their elemental and phase compositions are examined using several techniques: electron-probe X-ray spectral microanalysis (EPMA), secondary-ion mass spectrometry (SIMS), and X-ray diffractometry. It is found out that irradiation of the ceramic material by LEHCEBs stimulates the transitions of an m-phase of zirconium dioxide into a tetragonal modification, and results in a considerable decrease in the alumina phase. Based on the analysis of the data obtained, a conclusion is drawn that it is the processes resulting from irradiation and giving rise to formation of a subsurface layer with a phase composition different from that of the bulk material, which are responsible for the effect of modification of the material mechanical property.     

Elastic properties of cubic, tetragonal and monoclinic ZrO2 from first-principles calculations

In terms of first-principles calculations, elastic stiffness constants C_ij’s as well as the polycrystalline aggregates including the bulk, shear, Young’s moduli, Possion’s ratio, and anisotropy factors have been predicted for three technologically important polymorphs of ZrO₂, i.e., monoclinic m-ZrO₂, tetragonal t-ZrO₂, and cubic c-ZrO₂. Here, both the strain vs. stress (S-S) and the strain vs. strain energy (S-E) methods are adopted. In the first-principles calculations, both the local density approximation (LDA) and the generalized gradient approximation (GGA) are utilized. It is found that the more accurate structural and elastic properties are determined by LDA in comparison with experimental results and the S-S method is more effective than the S-E method although the two methods predict the similar results. The predicted negative values for C₁₆, C₃₆, and C₄₅ of m-ZrO₂ suggest that the certain normal or shear stress corresponds to an opposite shear strain for reducing the total energy. Small differences of shear and Young’s modulus between m-ZrO₂ and t-ZrO₂ suggest that their mechanical properties are comparable.     

MgO-pyrochlore composite as an inert matrix fuel: Neutronic and thermal characteristics

Inert matrix fuels are an important component of advanced nuclear fuel cycles, as they provide a means of utilizing plutonium and reducing the inventory of ‘minor’ actinides. We examine the neutronic and thermal characteristics of MgO-pyrochlore (A₂B₂O₇: La₂Zr₂O₇, Nd₂Zr₂O₇ and Y₂Sn₂O₇) composites as inert matrix fuels in boiling water reactors. By incorporating plutonium with resonance nuclides, such as Am, Np and Er, in the A-site of pyrochlore, the k_infvs. burn-up curves are shown to be similar to those of UO₂, although the Doppler coefficients are less negative than UO₂. The Pu depletion rates are 88-90% (²³⁹Pu) and 54-58% (total Pu) when the inert matrix fuels experience a burn-up equivalent of 45 GWd/tHM UO₂. Because of the high thermal conductivity of MgO, the center-line temperatures of the MgO-pyrochlore composites at 44.0 kW/m are lower than those of UO₂ pellets. After burn-up, the A-site cation composition is 15-35 at.% lower than that of the B-site cations in pyrochlore (e.g., A_1.84B_2.17O_7.00) due to the fission of Pu in the A-site and the presence of fission product elements in the A- and B-sites of the pyrochlore structure.     

The influences of Pu and Zr on the melting temperatures of the UO2–PuO2–ZrO2 pseudo-ternary system

Specimens of (U, Pu, Zr)O₂ were prepared as simulated corium debris that were assumed like debris generated in the severe accident of the Fukushima Daiichi Nuclear Power Plant and their melting temperatures were measured by the thermal arrest technique in order to evaluate the influence of plutonium and zirconium content on the melting temperature of the corium debris. From the evaluation, it was found that the influence of zirconium on the melting temperatures of both (U, Pu, Zr)O₂ and (U, Zr)O₂ was similar and that the melting temperature of (U, Pu, Zr)O₂ had a local maximum value in the Pu-content between 0 and 20 mol%. The UO₂–PuO₂–ZrO₂ pseudo-ternary phase diagram at 2900 and 3000 K was evaluated from the present experimental results and previously reported results.     

The effect of processing on the thermal diffusivity of MgO-Nd2Zr2O7 composites for inert matrix materials

Oxides possess many of the required properties suitable for an inert matrix fuel in light water reactors, however, their primary disadvantage is low thermal conductivity. Composites are being investigated to maximize the thermal conductivity of the inert matrix fuel by using thermally conductive MgO as the primary phase while improving its hot water corrosion resistance through the addition of a second phase acting as a hydration barrier. Inert matrix fuel candidate MgO-Nd₂Zr₂O₇ composites were synthesized with multiple processing methods, the composite powders were characterized, the resulting microstructures quantitatively analyzed, and the thermal diffusivity of the composites was measured. Among the four processing methods investigated, ball milling and high-energy shaker blending produced the most homogeneous microstructures with a negligible amount of MgO and Nd₂Zr₂O₇ heterogeneities. An effect of processing on the properties of the composites manifests as a larger variation in the thermal diffusivity in pellets processed by methods that produce a higher quantity and frequency of MgO and Nd₂Zr₂O₇ heterogeneities than in methods that produce negligible amounts of heterogeneities.     

Development of Impregnated Agglomerate Pelletization (IAP) process for fabrication of (Th,U)O2 mixed oxide pellets

Impregnated Agglomerate Pelletization (IAP) technique has been developed at Advanced Fuel Fabrication Facility (AFFF), BARC, Tarapur, for manufacturing (Th,²³³U)O₂ mixed oxide fuel pellets, which are remotely fabricated in hot cell or shielded glove box facilities to reduce man-rem problem associated with ²³²U daughter radionuclides. This technique is being investigated to fabricate the fuel for Indian Advanced Heavy Water Reactor (AHWR). In the IAP process, ThO₂ is converted to free flowing spheroids by powder extrusion route in an unshielded facility which are then coated with uranyl nitrate solution in a shielded facility. The dried coated agglomerate is finally compacted and then sintered in oxidizing/reducing atmosphere to obtain high density (Th,U)O₂ pellets. In this study, fabrication of (Th,U)O₂ mixed oxide pellets containing 3–5 wt.% UO₂ was carried out by IAP process. The pellets obtained were characterized using optical microscopy, XRD and alpha autoradiography. The results obtained were compared with the results for the pellets fabricated by other routes such as Coated Agglomerate Pelletization (CAP) and Powder Oxide Pelletization (POP) route.