Improvement of UO2 Pellet Properties by Controlling the Powder Morphology of Recycled U3O8 Powder

Powder morphology evolution of recycled U₃O₈ according to the thermal treatments has been studied. The defective UO₂ pellets are oxidized to U₃O₈ powders at a conventional temperature of 350 or 450°C in air. Those powders are pressed into green pellets and then sintered at 1,500 and 1,730°C in H₂ gas flow. Final reoxidized U₃O₈ powers are obtained by reoxidizing those sintered pellets at 450°C in air. This paper shows that the reoxidized U₃O₈ powder morphology and the BET surface areas are greatly dependent on the density of sintered UO₂ pellets before reoxidation. Reoxidized U₃O₈ powders are added to virgin UO₂ powders to fabricate UO₂ pellets and the effect of such addition on the UO₂ pellet properties is investigated. The reoxidized U₃O₈ powders having a certain range of BET surface area significantly promote the grain growth of UO₂ pellets.     

New Phase Transitions in Non-Stoichiometric U3O8-x at High Temperatures, (I)

The phase equilibrium of non-stoichiometric U₃O_8-x has been studied by thermogravimetry in the range 765°≦T≦995°C and 10^?4≦Po₂≦1 atm. The results suggest the presence of six phases within U₃O_8-x the phase, separated by second (or higher) order transitions. The relative partial molar free energies, enthalpies and entropies within an each of the six phases, as well as the standard free energies, enthalpies and entropies for the phase changes are calculated and compared with previous works.     

Synthesis and thermal conductivity of Y6UO12

Y₆UO₁₂ was synthesized by solid-state reactions of Y₂O₃ and U₃O₈. The high-density pellet of Y₆UO₁₂ was prepared by the spark plasma sintering followed by heat treatment in air for oxygen supplementation. The thermal conductivity (κ) was evaluated using the laser flash method from room temperature to 1173 K. The κ of Y₆UO₁₂ decreased with increasing temperature in the whole temperature range, indicating that the phonon contribution was predominant. The room temperature κ value of Y₆UO₁₂ was 4.90 Wm^?1K^?1. The magnitude relationship of κ among Y₆UO₁₂, Y₆WO₁₂, and Yb₆WO₁₂, i.e. κ of Yb₆WO₁₂ < κ of Y₆UO₁₂ < κ of Y₆WO₁₂, was discussed based on the general lattice thermal conductivity theory.     

Dissolution Behavior of Uranium Oxides with Supercritical CO2 Using HNO3-TBP Complex as a Reactant

Dissolution behavior of U₃O₈ and UO₂ using supercritical CO₂ medium containing HNO₃-TBP complex as a reactant was studied. The dissolution rate of the oxides increased with increasing the HNO₃/TBP ratio of the HNO₃-TBP complex and the concentration of the HNO₃-TBP complex in the supercritical CO₂ phase. A remarkable increase of the dissolution rate was observed in the dissolution of U₃O₈ when the HNO₃/TBP ratio of the reactant was higher than ca. 1, which indicates that the 2:1 complex, (HNO₃)₂TBP, plays a role in facilitating the dissolution of the oxides. Half-dissolution time (t_½ ) as an indication of the dissolution kinetic was determined from the relationship between the amount of uranium dissolved and the dissolution time (dissolution curve). A logarithmic value of a reciprocal of the t_½ was proportional to the logarithmic concentration of HNO₃, C_HNO3, in the supercritical CO₂. The slopes of the (l/t_½ ) vs. ln C_HNO3 plots for U₃O₈ and UO₂ were different from each other, indicating that the reaction mechanisms or the rate-determining steps for the dissolution of U₃O₈ and UO₂ are different. A principle of the dissolution of uranium oxides with the supercritical CO₂ medium is applicable to a method for the removal of uranium from solid matrices.     

New Phase Transitions in Non-Stoichiometric U3O8-x at High Temperatures, (II)

Electrical conductivity and X-ray diffraction studies on non-stoichiometric U₃O_8-x phase were carried out simultaneously in the range 765°≦T≦995°C and 10^?4≦Po₂≦1 atm. The plot of logσ vs. logPo₂ showed many refractions which corresponded with the phase transitions determined by thermogravimetry reported in the preceding paper. Based on the data of both electrical conductivity and thermogravimetry, the non-stoichiometric defect structures of various U₃O_8-x phases are interpreted as consisting of singly charged oxygen interstitials (O_l′) and doubly charged oxygen vacancies (Vo^.)? Some of the X-ray diffraction lines were found to undergo splitting with decreasing oxygen partial pressure. These splits are qualitatively discussed in reference to the out-of-step structure model. The mechanism of electrical conduction in the high temperature hexagonal U₃O_8-x phases is surmised to be the hopping of small polarons.     

Treatment of UO2 pellets used for preparing fuel elements of HTR-10 followed by supercritical fluid extraction

International interest in high temperature gas-cooled reactor (HTGR) has been increasing in recent years. It is important to study on reprocessing of spent nuclear fuel from HTGR for recovery of nuclear resource and reduction of nuclear waste. Treatment of UO₂ pellets used for preparing fuel elements of the 10 MW high temperature gas-cooled reactor (HTR-10) followed by supercritical fluid extraction was investigated. When UO₂ pellets were dissolved and extracted with tri-n-butyl phosphate (TBP)–HNO₃ complex in supercritical CO₂ (SC-CO₂), the extraction efficiency was less than 7% under experimental conditions. After UO₂ pellets were ground into UO₂ fine powders, the extraction efficiency of the UO₂ fine powders with TBP–HNO₃ complex in SC-CO₂ could reach 92%. After UO₂ pellets broke spontaneously into U₃O₈ powders under O₂ flow and 600 °C, the extraction efficiency of the U₃O₈ powder with TBP–HNO₃ complex in SC-CO₂ could reach more than 98%.     

Fundamental Study of the Sulfide Reprocessing Process for Oxide Fuel (I) Study on the Pu,MA and FP Tracer-Doped U3O8

For the recovery of fuel materials from spent nuclear fuel, a novel reprocessing process based on the selective sulfurization of fission products (FP) has been proposed, where FP and minor actinides (MA) are first sulfurized by CS₂ gas, and then, dissolved by a dilute nitric acid solution. Consequently, the fuel elements are recovered as UO₂ and PuO₂. As a basic research of this new concept, the sulfurization and dissolution behaviors of U, Pu, Np, Am, Eu, Cs, and Sr were investigated by γ-ray and α spectrometries in this paper using ²³⁶Pu-, ²³⁷Np-, ²⁴¹Am-, ¹⁵²Eu-, ¹³⁷Cs-, and ⁸⁵Sr-doped U₃O₈ samples. The dependence of the dissolution ratio of each element on the sulfurization temperature was studied and reasonably explained by combining the information of the sulfide phase analysis and the chemical thermodynamics of the dissolution reaction. The sulfurization temperature ranging from 350 to 450°C seems to be promising for the separation of FP and MA from U and Pu, since a clear difference in the dissolution ratio between FP and U was derived by the sulfurization treatment in this temperature range.     

Relative Reactivities of Radioiodine Released from U3O8 in Oxygen and Helium Atmospheres toward Propane

Chemical form of radioiodine released from U₃O₈ will be UI_x in helium and I₂ in oxygen atmospheres. Relative reactivities of these iodines toward organic compounds have been studied at a temperature range of 250~500°C, by choosing propane as a reactant. The UI_x has a higher reactivity than I₂, particularly at a temperature range of 300~400°C. Above that temperature, both iodine species show a similar reactivity. In the reactions, CH₃I, C₂H₅I, i, and n-C₃H₇I are formed: relative yields depend on both temperature and atmosphere. Two reaction processes participate in the formation of organic iodides. One is an iodine-initiation reaction and is relatively important at a lower temperature. The other is a radical-initiation reaction and become predominant as the temperature rises.     

EPR Study of Uranium (V) Species in Photo-and Electrolytic Reduction Processes of UO2 NO3)2-2TBP

Abstract

Electron Paramagnetic Resonance (EPR) and optical spectra of uranium(V) species were observed in both processes of photo- and electrolytic reduction of UO₂(NO₃)₂-2tributylphosphate (TBP) in 80%TBP-n-dodecane solution. The formation of U(V) species was detected by an optical spectrum (λ_max: 770, 970 and 1,420 nm). EPR signal with the value of ff-factor –2.3 and a linewidth of approximately 1,100 Gs was observed during the electrolytic reduction. On the other hand, during the photoreduction the signal with the value of fil-factor –1.94 was observed and there was found a superhyperfine structure with the intensity ratio of 1:2:1, that is caused by the superhyperfine coupling with nuclear spin, I=1/2, of the strongly coordinated ³¹P to the central uranium through oxygen atom. The superhyperfine coupling constant was estimated to be 27 Gs. Moreover, the signal with the value of g-factor –2.00 due to an organic radical was observed. The residue after the thermo-gravimetric analysis of UO₂(NO₃)₂-2TBP was identified as α-UP₂O₇ by the powder X-ray diffraction analysis, indicating the strong coordination of TBP to the central uranium atom.     

Fission product release in high-burn-up UO2 oxidized to U3O8

Results of oxidation experiments on high-burn-up UO₂ are presented where fission-product vaporisation and release rates have been measured by on-line mass spectrometry as a function of time/temperature during thermal annealing treatments in a Knudsen cell under controlled oxygen atmosphere. Fractional release curves of fission gas and other less volatile fission products in the temperature range 800-2000 K were obtained from BWR fuel samples of 65 GWd t⁻¹ burn-up and oxidized to U₃O₈ at low temperature. The diffusion enthalpy of gaseous fission products and helium in different structures of U₃O₈ was determined.     

Structural and Electrochemical Studies on Uranium(VI) Nitrato Complex with n-Octyl(phenyl)-N,N-Diisobutylcarbamoylmethylphosphine Oxide in Non-aqueous Solvents

The structure of uranyl nitrato complex with CMPO [n-Octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide] in solid state and in non-aqueous solvents without containing free CR/IPO has been studied by using IR spectrophotometer, ¹³C- and ³¹P-NMR. The carbonyl(vcO) and phosphoryl(vpO) stretching bands of coordinated CMPO were observed at lower wavenumber than the corresponding bands of free CMPO in both the states. The ¹³C and ³¹P peaks assigned to the carbonyl carbon and phosphoryl phosphine of coordinated CMPO was detected in the lower field than that of free CMPO. From these results, it was concluded that the uranyl nitrato complex with CMPO in both the states has the structure with two nitrate and one CMPO coordinated as bidentate in the equatorial plane of uranyl ion, i.e., UO₂(NO₃)₂·CMPO. Furthermore, the electrochemical studies of UO₂(NO₃)₂·CMPO complex in CH₃CN have been carried out using cyclic and normal pulse voltammetric methods. It was found that the UO₂(NO₃)₂·CMPO complex is reduced to U(V) complex at around ?1.22V vs. Fc/Fc⁺ (ferrocene/ferrocenium) and that the resulting reductant is oxidized to U(VI) at around +0.04V vs. Fc/Fc⁺.     

Recovery of neutron-irradiation-induced defects of Al2O3, Y2O3, and yttrium-aluminum garnet

SiC fiber-reinforced SiC matrix composites (SiC_f/SiC) are considered as one of the candidates for blanket materials in future fusion reactors and as an advanced fuel cladding material for next-generation fission reactors. Generally, the densification of SiC needs sintering additives and oxides such as Al₂O₃, Y₂O₃, and yttrium-aluminum garnet (YAG, Y₃Al₅O₁₂), which are frequently added to SiC. However, the effects of neutron irradiation on sintering additives are still unclear. In this study, we performed the neutron irradiation of Al₂O₃, Y₂O₃, and YAG at fluences up to 2.0–2.5 × 10²⁴ n/m² (E > 0.1 MeV) at 60–90 °C. The isochronal recovery of the macroscopic volume of Al₂O₃ against annealing temperature showed smooth and continuous shrinkage at a temperature of up to 1200 °C, and the volume slightly increased above that temperature. In contrast, the volume of Y₂O₃ showed quick shrinkage at the low temperature range, and slower and smooth recovery was observed up to ～1100 °C. In the case of YAG, the recovery of volume occurred in a step-wise manner at 600–750 °C, and continuous shrinkage occurred at temperatures lower and higher than that temperature range. The activation energies for the macroscopic volume recoveries of three oxides were obtained from the Arrhenius plots of the rate coefficients. Two-stage recovery was observed for Al₂O₃, whereas more complicated recovery processes were suggested for Y₂O₃ and YAG.     

Experimental Determination of Spectral Indices in Low Enriched UO2-H2O Lattices

To get information about the neutron spectrum in low enriched UO₂-H₂O lattices, the spectral indices SI(U^8c/Dy) and SI(U^8c/U^5f) were measured on the basis of the parallel irradiation technique, which basically irradiates activation foils both in a neutron field to be investigated and in a reference field of thermal neutrons. In the present study, a fuel pellet of UO₂ was used for the measurement of activities caused by the neutron capture of ²³⁸U and the fission of ²³⁵U. Besides the technical details of the measurements, the origins of experimental errors are listed with the method how to eliminate them. The measurements were carried out in lattices of different fuel enrichment to demonstrate the capability of the present method, and the experimental results were compared with the calculated ones. It was found that the results of the present measurements are useful to assess the validity of the cell calculations.     

Effects of swift heavy ion irradiation on the structure of Er2O3-doped CeO2

In order to simulate the effects of burnable poison doping on the fission fragment damage of UO₂ nuclear fuels, Er₂O₃-doped CeO₂ pellets were irradiated with 200 MeV Xe¹⁴⁺ ions. The irradiation effect was measured by means of X-ray diffraction (XRD). The expansion of lattice and the disordering of atomic arrangement due to the irradiation become more remarkable with increasing the concentration of the Er₂O₃ dopant.     

Study on effects of swift heavy ion irradiation on the crystal structure in CeO2 doped with Gd2O3

To simulate the effects of Gd₂O₃-doping and high-energy fission products in UO₂, Gd₂O₃-doped CeO₂ pellets were irradiated with 200-MeV Xe¹⁴⁺ ions. Doping and irradiation effects were analyzed using X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS). The lattice constant of CeO₂ decreases and the local structure is disordered with increased doping levels. However, the irradiation induces an expansion of the lattice and a disordering of atomic arrangement near the Gd atoms. The effects of the irradiation become more pronounced with increasing Gd₂O₃-dopant levels. Our results are compared with those of a study involving Er₂O₃-doped CeO₂.     

Electrical conductivity measurement and thermogravimetric study of pure and niobium-doped uranium dioxide

The electrical conductivity and nonstoichiometric composition of UO_2+x and (U_1?yNb_y)O_2+x (y = 0.01, 0.05 and 0.10) were measured in the range $\text{1282 ≦ T ≦ 1373}$ K and Pa by tie four inserted wires method and thermogravimetry, respectively. The electrical conductivity of (U_1?yNb_y)O_2+x plotted against the oxygen partial pressure indicated a minimum corresponding to the transition between $\text{n}$- and $\text{p-}\text{type}$ cone uction. The band-gap energy of (U_1?yNb_y)O_2+x was calculated to be $\text{(248 ± 12)}$ kJmol.^?1, independent of niobium content, which is nearly the same as that of UO_2+x. From the oxygen partial pressure dependences of both the electrical conductivity and the deviation $\text{x}$ of UO_2+x and (U_1?yNb_y)O_2+x, the defect structures in these oxides were discussed with the complex defect model consisting of oxygen vacancies and two kinds of interstitial oxygens.     

Trend in criticality calculation results of borated-water-moderation experimental cores relating to 10B(n, α) cross section

ABSTRACT

In connection with the accuracy of the ¹⁰B(n, α) cross section in the thermal- and epithermal-neutron energy regions, criticality calculation results were examined for six benchmark sets of light-water-moderation critical experiments of UO₂ and MOX fuel lattice cores with un-borated and borated water. Two of the benchmark sets were those implemented in the Tank-Type Critical Assembly (TCA). The others were taken from the International Handbook of Evaluated Criticality Safety Benchmark Experiments (ICSBEP), and the International Handbook of Evaluated Reactor Physics Benchmark Experiments (IRPhEP). The enrichments of the UO₂ fuel range from 1.9 wt% to 2.6 wt%, and the Pu contents of the MOX fuel do from 2.0 to 6.6 wt%. The boron concentrations in water are up to 1511 ppm. The effective neutron multiplication factors (k_eff ) were taken from the published documents. They were calculated with continuous-energy Monte Carlo calculation codes in combination with JENDL-4.0, and other evaluated nuclear data libraries. It was confirmed that the k_eff values of the critical cores increased with the boron concentrations, which indicates that the ¹⁰B(n, α) cross section in the thermal- and epithermal-neutron energy regions should be larger than those in JENDL-4.0 and other libraries.     

Neutron cross section sensitivity analysis of UO2 and MOX core physics experiments on light water reactors

The perturbation theory based on the transport calculation has been applied to study sensitivity of neutron multiplication factors (k_eff's) to neutron cross sections used for the reactivity analysis of UO₂ and MOX core physics experiments on light water reactors. The studied cross sections were neutron capture, fission and elastic scattering cross sections, and a number of fission neutrons, ν. The obtained sensitivities were multiplied to relative differences in the cross sections between JENDL-4.0 and JENDL-3.3 in order to estimate the reactivity effects. The results show that the increase in k_eff, 0.3%Δk/kk′, from JENDL-3.3 to JENDL-4.0 for the UO₂ core is mainly attributed to the decreases in the capture cross sections of ²³⁸U. On the other hand, there are various contributions from the differences in the cross sections of U, Pu, and Am isotopes for the MOX cores. The major contributions to increase in k_eff are decreases in the capture cross sections of ²³⁸U,²³⁸Pu, ²³⁹Pu, and those to decrease in k_eff are decreases in ν of ²³⁹Pu and increases in the capture cross sections of²⁴¹Am. They compensate each other, and the difference in k_eff between JENDL-3.3 and JENDL-4.0 is less than 0.1%Δk/kk′ and relatively small.     

Vaporization study on UO2 and (U1−yNby)O2+x by mass-spectrometric method

The vapor pressures over UO_2.000 and (U_1?yNb_y)O_2+x (y = 0.01, 0.05, x = 0.000–0.022) were measured by the mass-spectrometric method in the temperature range 2025–2343 K. The main gas species over UO_2.000 were observed to be UO₃(g) and UO₂(g) and those over (U_1?yNb_y)O_2+x were NbO₂(g), NbO(g), UO₃(g) and UO₂(g). The partial vapor pressures of almost all gas species over (U_1?yNb_y)O_2+x increased with increasing O/M (M = U + Nb) ratio. With increasing Nb content in (U_1?yNb_y)O_2.000, the partial vapor pressures of UO₂(g) and UO₃(g) decreased and those of NbO(g) and NbO₂(g) increased. The congruently vaporizing composition in the (U_1?yNb_y)O_2+x phase was estimated to be ($\text{U}{}_{\mathrm{0.985\pm 0.005}}\text{Nb}{}_{\mathrm{0.015\pm 0.005}}\text{)O}{}_{2.000}$ from the compositional dependence of the total vapor pressures. The partial molar enthalpy and entropy of oxygen of (U_1?yNb_y)O_2+x calculated from the partial pressures of gaseous species NbO₂(g) and NbO(g) were in fairly good agreement with those previously obtained by the present authors with a thermobalance.     

A Nuclear Magnetic Resonance Study on Ligand Exchange Reactions in La(III), Nd(III), and U(VI) Nitrato Complexes with n-Octyl(pheny1)-N,N-Diisobutylcarbamoylmethylphosphine Oxide in Non-aqueous Solvents

The exchange reactions of n-octyl(pheny1)-N, N-diisobutylcarbamoylmethylphosphine oxide (CMPO) in La(III), Nd(III), and U(VI) nitrate complexes with CMPO (La(III)-, Nd(III)-, and U(VI)-CMPO complexes) have been studied in CD₃COCD₃ by means of ³¹P NMR method. The number of CMPO coordinated to the first coordination sphere of La(III) ion was directly determined to be 3 by the area integrations of ³¹P NMR signals of free and coordinated CMPO molecules. The same coordination number of 3 was also obtained for the U(VI)-CMPO complex. The coordination number was not determined for the Nd(III)-CMPO complex, because of its paramagnetic behavior. The exchange rate constants of CMPO in La(III)- and U(VI)- CMPO complexes were obtained by the two-site exchange model. Paramagnetic line broadening was observed in the Nd(III)-CMPO complex and the rate constant for the exchange of CMPO was determined by the line-broadening method. The exchange rates of CMPO in La(III)- and Nd(III)-CMPO complexes depend on the free CMPO concentration ([CMPO]), while that in U(VI)-CMPO complex is independent of [CMPO]. The dissociative (D) and dissociative interchange (I_d ) mechanisms were proposed for the exchange reactions in the La(III)- and Nd(III)-CMPO complexes, and dissociative (D) or I_d mechanism was proposed for the U(VI)-CMPO complex. The dissociative rate constants (s^?1) at 25°C and activation parameters ΔH^# (kJ·mol^?1) and ΔS^# (J·K^?1·mol^?1) are 4.76x10³, 28.7±0.1, ?78.4±0.2 for La(III)-CMPO complex, 4.72x10³, 42.6±0.4, ?31.7±1.3 for Nd(III)-CMPO complex, and 3.20x10³, 46.9±0.6, ?20.5±2.2 for U(VI)-CMPO complex, respectively.