Thermal Conductivity Of UO2 Research Articles

Modelling of UO2 thermal conductivity: Improvement of the irradiation defects contribution and uncertainty quantification

Thermal conductivity of the fuel is a key parameter to assess the temperatures in nuclear fuel. Several empirical or mechanistic models exist in the literature. The objective of this work is to improve one of the empirical formulation by introducing a kinetic evolution of the irradiation defects density in the irradiated fuel. This evolution is calibrated amongst some dedicated experimental data. An inverse probabilistic method is also applied to quantify the modelling uncertainty of the obtained formulation, considering the scattering and the uncertainty of the experimental database available for both un-irradiated and irradiated materials.

Neutronic and Thermalhydraulic Evaluation of Annular Fuel in PWR with Three Different Nanofluids

In pressurized nuclear reactors, one of the fuel design principles is the fuel centerline temperature limit. Since the thermal conductivity of UO2 is not high, the temperature increases rapidly from the fuel surface to the fuel center. To overcome this limitation, the use of annular type fuels instead of solid fuels is one of the improvement efforts. In this study, the effects of using annular fuel in a typical PWR are investigated. At the same time, the effects of adding nanoparticles (〖Al〗_2 O_3,Cu and TiO_2) to the coolant water in the case of solid and annular fuels are also investigated. For both cases, neutronic and thermalhydraulic calculations are made and compared. In neutronic point of view, it has been shown that the use of annular fuel does not cause a significant change in the k_eff value but the presence nanoparticles reduces the k_eff. This reduction is maximum for Cu+water and minimum for 〖Al〗_2 O_3+water nanofluids. In thermalhydraulic point of view, it has been shown that the annular fuel positively affects the fuel temperature limits and the addition of nanoparticles could provide an additional contribution to this, provided that it is in very small amounts (φ

Suppressed thermal conductivity in hyperstoichiometric uranium dioxide controlled by phonon lifetimes

Thermal transport in nuclear fuels used for nuclear energy applications is directly tied to performance and reliability. Uranium dioxide (UO2), one of the most important nuclear fuels, can accumulate excess oxygen atoms as interstitial defects, which significantly impacts thermal transport properties. In this study, thermal conductivities and inelastic neutron scattering (INS) measurements on UO2+x were performed at low temperatures (2–300 K). The thermal conductivity of UO2+x (x = 0, 0.03, 0.04, and 0.11) is significantly suppressed compared to UO2 except near the Néel temperature TN = 30.8 K, where it is independent of x. INS measurements demonstrate that the heat capacities and phonon group velocities of UO2 and UO2.08 are similar, and the suppressed thermal conductivity results from smaller phonon lifetimes. These perceptions advance our understanding of thermal transport properties in advanced nuclear fuels and guide safe and economic utilization of nuclear energy.

Atomistic Level Molecular Dynamics Analysis and its Integrity in the Interim of Irradiation

In this work, molecular dynamics (MD) simulation was utilized in relation to access the thermal conductivity of UO2, PuO2 and (U, Pu)O2 in temperature range of 500–3000 K. Diffusion study on mixed oxide (MOX) was also performed to assess the effect of radiation damage by heavy ions at burnup temperatures. Analysis of the lattice thermal conductivity of irradiated MOX to its microstructure was carried out to enhance the irradiation defects with how high burnup hinders fuel properties and its pellet-cladding interaction. Fission gas diffusion as determined was mainly modelled by main diffusion coefficient. Degradation of diffusivity is predicted in MOX as composition deviate from the pure end members. The concentration of residual anion defects is considerably higher than that of cations in all oxides. Depending on the diffusion behavior of the fuel lattice, there was decrease in the ratio of anion to cation defects with increasing temperature. Besides, the modern mixed oxide fuel releases fission gas compared to that of UO2 fuel at moderate burnups.

Characterization of high thermal conductivity fuel surrogates before and after ion irradiation

• Thermal conductivity of UO 2 surrogates was enhanced during sintering using aluminum oxide and silicon carbide additives. • Compositions were stable up to 5 dpa and 700 °C while a decrease in their mechanical properties was observed at 15 dpa. • Microcrack formation due to anisotropic swelling of the additives was not observed. High thermal conductivity nuclear fuels offer important potential advantages over traditional oxide-based fuels such as higher burnup, reduction in fission gas release, and better overall safety of the system. One proposed approach to high thermal conductivity fuels utilizes high thermal conductivity nonfissile additives with UO 2 fuel to lower the fuel operating temperature and thereby take advantage of the highly favorable radiation resistance of UO 2 at lower operating temperatures. However, differential swelling in the matrix and high conductivity additive phases during high dose irradiation could lead to internal cracking and poor performance. In the current study, ceria (CeO 2 ) and zirconia (ZrO 2 ) surrogate matrices were used to model UO 2 behavior. Additives of 10 vol. % Al 2 O 3 or SiC in the form of short fibers or platelets were used for the high conductivity second phase. The nuclear fuel surrogates were sintered to achieve densities greater than 93% of the ideal values. Scanning electron microscopy (SEM) imaging and X-ray diffraction confirmed the uniform distribution of the second phase and that no intermetallic second phase was formed during sintering. The thermal conductivity of the sintered samples was measured from 50 °C to 900 °C and confirmed the desirable increase compared to pure CeO 2 /ZrO 2 pellets. Samples were irradiated with 20 MeV Ni 6+ ions at midrange doses ranging from 1 to 15 displacements per atom (dpa) and temperatures from 300 °C to 700 °C. Post irradiation characterization revealed a good stability of the samples at low to medium doses with matrix lattice parameter swelling of < 0.14 % but showed a significant microstructural deterioration and decrease of the mechanical properties at 15 dpa.

UO2/BeO interfacial thermal resistance and its effect on fuel thermal conductivity

UO2/BeO interfacial thermal resistance (ITR) is calculated by diffuse mismatch model (DMM), and the effects of ITR on UO2-BeO thermal conductivity are investigated. DMM can predict UO2/dispersed-BeO ITR within the accuracy of orders of magnitude. However, UO2/continuous-BeO ITR is three to four orders of magnitude larger than DMM predictions. This indicates that UO2/dispersed-BeO ITR is mainly induced by the vibrational mismatch, while UO2/continuous-BeO ITR may be attributed to the contact resistance. The thermal conductivity of UO2 containing dispersed BeO decreases with the decrease in BeO size, and the thermal conductivity of UO2 containing continuous BeO decreases with the decrease in the size of UO2 granule surrounded by BeO. The conditions for achieving the targeted enhancement of UO2 thermal conductivity by doping with BeO are derived. These conditions can be used to design and optimize the distribution, content, size of BeO, and the size of UO2 granule.

Analysis of Burnup effects and Its Integrity Assessment in the Interim of Irradiation with Molecular Dynamics

Abstract:Burnups can cause major structural changes in the edge of the fuel rod and a general degradation of the thermal conductivity. In irradiated mixed oxide fuels of UO2, PuO2 with NpO2 as fission products (FP) various chemical states depending on the conditions of the fuel is developed. This work, we firstly applied the MD relation to obtain the thermal conductivity of UO2, PuO2, and (U, Pu) O2 in temperature range of 300–2000 K. Lattice parameter, Burnup and the thermal conductivity were then calculated for specified UO2 and PuO2. This calculation relates the degradation of thermal conductivity with a number of pores and increasing temperature. Finally, the migration energy barrier and the recovery energies of the obstruction type defects were calculated with molecular dynamics simulation.

Effect of Xe bubbles on the thermal conductivity of UO2: Mechanisms and model establishment

The thermal conductivities of UO2 containing Xe bubbles were investigated using non-equilibrium molecular dynamics (MD) simulations as a function of porosity, bubble radius and the ratio of Xe atoms to U vacancies (Xe:VU). It was found that when the pores are filled with Xe atoms to form bubbles, the porosity and radius of the bubble both increase with the increase in Xe:VU. Based on the equilibrium porosity and bubble radius, a previous model for porous UO2 was used to predict the thermal conductivity. This model predicted only a slight change in the thermal conductivity with Xe:VU, which is induced by the changes in porosity and bubble radius. However, according to MD simulations, the thermal conductivity of UO2 containing Xe bubbles decreases obviously with Xe:VU. This indicates that in addition to porosity and bubble size effects, there exists another mechanism for the degeneration in UO2 thermal conductivity due to the presence of Xe bubbles. MD details revealed that many defects are produced on UO2/bubble interface, the concentration of which increases with Xe:VU. This leads to the increase in interfacial thermal resistance (ITR), thus resulting in the decrease in overall thermal conductivity. Based on the previous model for porous UO2, a model considering the above mechanism was established for UO2 containing Xe bubbles through phenomenological phonon hydrodynamics. In this new model, the increase in ITR is reflected by multiplying the thermal-drag force by a correction factor. The proposed model can be implemented in fuel performance codes to improve calculations of fuel thermal conductivity.

Unified Effect of Dispersed Xe on the Thermal Conductivity of UO2 Predicted by Three Interatomic Potentials

Thermal conductivity is a critical fuel performance property of uranium dioxide (UO2)-based nuclear fuels. Numerous studies have shown that xenon (Xe) fission gas plays a major role in fuel thermal conductivity degradation. It has also been shown that dispersed Xe atoms can cause a stronger phonon-scattering effect than their clustered form. In this work, molecular dynamics simulations are conducted to study the dispersed Xe-induced thermal conductivity reduction using three different interatomic potentials. It is found that although these potentials result in significant discrepancies in the absolute thermal conductivity values, the normalized values are very similar at a wide range of temperatures and Xe concentrations. By integrating this unified effect into the experimentally measured thermal conductivities, a new analytical model is developed to predict the realistic thermal conductivities of UO2 at different dispersed Xe concentrations and temperatures. Using this new model, the critical Xe concentration that offsets the grain boundary Kapitza resistance effect in a high burnup structure is revisited.

Effect of Xe bubble size and pressure on the thermal conductivity of UO2—A molecular dynamics study

Abstract

A feasibility study of using CeO2 as a surrogate material during the investigation of UO2 thermal conductivity enhancement

ABSTRACTA possible substitution of UO2 for research purposes is the cerium dioxide (CeO2) owing to its chemical and physical properties. Neutronic properties are different and fission is absent in the case of CeO2; however, similarities were studied recently to have a possibility to compare the neutronic influence of secondary additives into the matrix. This paper deals with increasing the thermal conductivity of UO2 nuclear fuel on surrogate material (CeO2); the main focus of the research is given on the sintering behaviour of CeO2. The incorporation of highly thermally conductive material (SiC) is the investigated concept of thermal conductivity enhancement. Conventional sintering and spark plasma sintering (SPS) were applied to compare the behaviour of CeO2 and UO2 reported in the literature. High temperature thermal conductivity measurements did not confirm the positive influence of SiC additive inside the CeO2 matrix mainly due to grain boundary disruptions. Similar behaviour was also previously reported for UO2 pellets with SiC.

Influence of porosity formation on irradiated UO2 fuel thermal conductivity at high burnup

Based on the existing low temperature high burnup gaseous swelling model for UO2 fuel, the matrix swelling terms are calculated and the formation of total volume porosity up to burnup of 120 MWd/KgU is computed. The irradiated UO2 thermal conductivity model based on the Maxwell-Eucken correlation for porosity factor is selected as a case study and the calculation of porosity evolution with burnup is carried out. It is shown that taking into account the formation of porosity with burnup compared to the case with constant porosity equal to as-fabricated value leads to a decrease in the UO2 fuel thermal conductivity up to 15% at high burnup values of 120 MWd/kgU. Results of the calculations are also compared with the available experimental data and good agreement was found. The conducted parametric study clearly demonstrated the impact of the key parameters on the results of the present investigation.

A comment on the thermal conductivity of (U,Pu)O2 and (U,Th)O2 by molecular dynamics with adjustment for phonon-spin scattering

AbstractA new approach for adjusting molecular dynamics results on UO2 thermal conductivity to include phonon-spin scattering has been used to improve calculations on UxPu1−xO2 and UxTh1−xO2. We demonstrate that by including spin scattering a strong asymmetry as a function of uranium actinide fraction, x, is obtained. Greater degradation is shown for UxTh1−xO2 than UxPu1−xO2. Minimum thermal conductivities are predicted at U0.97Pu0.03O2 and U0.58Th0.42O2, although the degradation in UxPu1−xO2 is negligible relative to pure UO2.

Thermal properties of (Al,Gd)O3 doped uranium dioxide

In this paper were defined thermal characteristics (thermal diffusivity up to 2000 °C, heat capacity up to 1250 ° C and thermal conductivity of up to 1250 °C) of oxide nuclear fuel with additives of burnable neutron absorber in the form of (Al,Gd)O3. Also for comparison samples UO2 with gadolinium Gd2O3 and pure uranium dioxide were studied. It is shown that the thermal diffusivity and thermal conductivity of UO2 with dopant of gadolinium in the form of (Al, Gd) O3 is considerably higher than those of Gd2O3 -doped uranium dioxide, due to the fact that (Al, Gd)O3 does not form a solid solution with UO2.

Molecular dynamics calculations of heat conduction in actinide oxides under thermal gradient

Thermal conductivities of UO2, PuO2 and (U0.8,Pu0.2)O2 have been investigated by non-equilibrium molecular dynamics (NEMD) simulation between 300 K and 2000 K. The thermal conductivity was directly calculated by the temperature gradient on the system according to Fourier's law in NEMD simulation. The thermal conductivity obtained from the NEMD simulation decreases with a decrease of the supercell size, which means the phonon scattering occurs at the system boundaries in the microsystem. In addition, the present NEMD simulation, as well as previous EMD simulation studies, clearly shows that the Umklapp process causes the decrease of thermal conductivity at high temperatures. When comparison is made with literature data, the calculated results obtained from the relatively small supercell are in good agreement with the measured ones for the above actinide dioxides.

Soliton microdynamics of multiphase thermal conductivity of fuel materials of the uranium dioxide type with the generation of new-type surface vibrations

The microdynamics of large-amplitude nonlinear atomic vibrations in crystals of the UO2, PuO2, and ThO2 types with the fluorite structure has been studied using the neutron spectrometry and computer simulation methods. Investigations performed on the DIN-2PI neutron spectrometer have revealed a fine structure of the multi-resonance spectral density of vibrations in UO2. The temperature dependence of the coefficient of thermal conductivity of UO2 with two maxima in the range from 500 to 3000 K and the multi-resonance density of vibrations has been interpreted according to the results of the computer simulation demonstrating the generation of single solitons and soliton beams at low and high temperatures. It has been shown that the maximum of the coefficient of thermal conductivity at a temperature of 500 K is determined by the energy transfer by solitons. A decrease in the coefficient of thermal conductivity in the range from 500 to 2000 K is determined by soliton-soliton scattering. An increase in the coefficient of thermal conductivity in the range from 2000 to 3000 K is determined by the generation of soliton beams with the formation of dynamic pores. It has been found that, in crystals of the UO2, PuO2, and ThO2 types, there are resonances of new-type surface vibrations between the dispersion branches of optical phonons. An additional resonance between the low-frequency optical branch and the acoustic branch has been revealed at finite temperatures. This resonance has been interpreted as a nonlinear local mode, in the framework of the quantum theory, possible, a biphonon. It has also been found that, with an increase in the excitation energy, there are soliton branches between this resonance and the acoustic branch in UO2, which in the phase plane cross the band of a nonlinear local mode with an increasing rate.

Thermal conductivity of UO2 and PuO2 from first-principles

Based on our previously calculated phonon spectra from first-principles, in the present work, we have applied the Slack relation to obtain the thermal conductivity of UO2, PuO2, (Th, Pu) O2, and (U, Pu) O2 in temperature range of 300–1500K. Also, the mode Grüneisen parameters of PuO2 and UO2 were presented to analyze the anharmonic properties. Rapid decrease of the thermal conductivity for UO2 at low-temperature zone was observed. As for PuO2, its larger phonon group velocity results in better thermal transfer ability than ThO2, UO2, and AmO2 at high-temperature zone.

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

High density uranium dioxide (UO 2 ) 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 UO 2 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 UO 2 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 UO 2 pellets. The benefits of using SPS over the conventional sintering of UO 2 are summarized.

Thermal conductivity of UO2+x and U4O9−y

Stoichiometries of UO2 specimens ranging from 2.000 to 2.210 were prepared in situ at 1673K within thermoanalytical equipment. Thermal conductivities were analyzed according to Klemens–Callaway defect model. Analysis of the data sets allows for extraction of the fundamental phonon–phonon and phonon-defect scattering contributions for the materials investigated. Low temperature thermal conductivity data was modeled using sigmoidal averaging to calculate the dual phase field UO2+x–U4O9−y. This allowed the thermal conductivity of U4O9−y to be calculated as a function of temperature up to 873K. This fundamental dataset provides a key contribution to ongoing efforts in the modeling and simulation communities in the areas of both light water reactor fuel performance and accident evolution.

Effect of ion irradiation on the thermal conductivity of UO2 and U3O8 epitaxial layers

The effect of 2MeV Ar+-ion irradiation on the thermal conductivity of epitaxial layers of UO2 and U3O8 was studied with time-domain thermoreflectance (TDTR) at temperatures between 323 and 658K. The thermal conductivity of unirradiated UO2 is 10.2Wm−1K−1 at 323K and scales with temperature like 1/T. Upon irradiation with doses >0.7×1015Ar+cm−2, the thermal conductivity decreases to a nearly constant value <4Wm−1K. By comparing the data to an analytic model, we report the point-scattering strength of irradiation induced defects in UO2. Unirradiated U3O8 has a thermal conductivity of 1.6Wm−1K−1 at 333K. Low and medium doses of Ar-ion irradiation decrease the thermal conductivity of U3O8 to <1.0Wm−1K−1. Unexpectedly, it is found that higher doses of Ar+ irradiation increased the conductivity of U3O8 to ∼2.0Wm−1K−1.