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.     

Al2O3/Y2O3/ZrO2 Ceramic Composite Properties Investigation by Plasma Focus Device

The Al₂O₃–Y₂O₃–ZrO₂ eutectic composition samples were prepared using the Al₂O₃, Y₂O₃, ZrO₂ powder treated at 900 °C for 30 min, pressed at 5 ton for 15 s and sintered at 1500 °C for 2 h. The locally made dense plasma focus (DPF) system with energy 2.8 kJ was used to surface modification of these samples. The samples, mounted at distance about 2 cm from the anode, were exposed to three shots of the DPF in Ar gas at a pressure of 0.8 mbar. The phase and elemental analysis of the untreated and plasma treated samples were conducted by the Raman and EDX spectroscopy. The Raman spectroscopy showed the formation of new phases (α-Al₂O₃ and c-ZrO₂) in the treated samples. The micro-hardness of the plasma treated samples was increased by about 280 % in comparison with the untreated sample.     

Ion guiding through capillaries in uncoated Al2O3 membranes

We made an experimental study on ion guiding through capillaries in uncoated Al₂O₃ membranes using a variety of ions such as O¹⁺, O³⁺, and O⁶⁺. The incident energy was varied within the range of 30-150 keV. The results were compared with others using coated PET and Al₂O₃ capillary membranes as well as with the so-called scaling law discovered by Stolterfoht and his co-workers. Good agreement of our results with the scaling law was found. However, our membranes showed extraordinarily strong guiding ability. The reason lies in that our membranes were uncoated. A slower charge drift speed along the insulating capillary wall and a much larger equilibrium charge Q_∞ seems to exist in our experiment.     

Al2O3-containing silver phosphate glasses as hosting matrices for radioactive iodine

Al₂O₃-containing silver phosphate glasses were synthesized to investigate the feasibility of phosphate glasses for the immobilization of radioactive iodine (¹²⁹I) present in spent nuclear fuel. Characterizations were performed by X-ray diffraction, Fourier transformed infrared spectroscopy, and scanning electron microscopy coupled with energy dispersive spectroscopy to examine structures, bonding properties, surface morphology, and elemental distribution of the synthesized glasses. The principal results showed that iodine became more strongly immobilized in the phosphate glasses with the addition of Al₂O₃, which was confirmed by the decrease of iodine leaching rates with approximately one order of magnitude. The present study would be helpful to decide whether Al₂O₃-containing silver phosphate glasses could be used as a candidate matrix to incorporate ¹²⁹I.     

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.     

Boiling and drying accident of high-level liquid waste in a reprocessing plant: Examination of the NO2 and NO generation using the simulated waste

ABSTRACT

NO₂ and NO generated during a boiling and drying accident, which affects the release of volatilized radioactive Ru into the atmosphere, were examined using various samples including simulated high-level liquid waste and a thermogravimetric analyzer. NO₂ and NO in the gas flowing out of the analyzer were measured separately using a NOx analyzer equipped with NO₂ and NO sensors. The samples were heated to 600°C at constant heating rates of mainly 0.2 and 1°C min^?1 that was adopted taking into account the decay heat of high-level liquid waste. It was found that under 180°C some nitrates in the liquid waste mainly separated their nitrate groups as HNO₃ without generating NOx (a mixture of NO and NO₂) and above 300°C the residual HNO₃ in the waste participated in thermal decomposition generating NOx. The generation rates of NO₂ and NO were obtained as a function of time using Arrhenius type equations, and the O₂ rate was derived from these equations using the stoichiometry of the reactions that generate NO₂, NO, and O₂.     

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.     

Compatibility of Eu2O3 and (Eu,Gd)2O3 with Type 316 Stainless Steel

The compatibility of Eu₃O₃ and (Eu,Gd) ₂O₃ mixed oxides which contain 0, 1, 5, 10, 50 and 100 mol% of Gd₂O₃, with type 316 stainless steel was examined by out-of-pile heatings. The heating temperature ranged from 550 to 1,000°C and heating time from 500 h to up to 5,000 h. Metallographic examinations of the stainless steel indicated that chemical reaction between Eu₂O₃ and stainless steel occurred at 650°C and that grain boundary penetration extended up to 40 μ m after heating at 1,000°C for 500 h. The penetration depth Δ was expressed as a function of heating temperature T as

Δ = 3.08 × 10⁴ exp(?1.72 × 10²RT)

where R is the gas constant and the activation energy is given in cal/mol. Electron probe microanalysis and X-ray diffraction analysis indicated that main reaction product is europium silicate. When Gd, which is a decay product of Eu, is mixed with Eu₂O₃, the degree of reaction between (Eu,Gd) ₂O₃ and stainless steel decreases as increase in concentration of Gd₂O₃. It was found that actually no grain boundary attack was observed even after heating at 1,000°C for 500 h when concentration of Gd₂O₃;i in (Eu,Gd) ₂O₃ exceeded 10 Discussion was made of reaction mechanism of Eu₂O₃ and (Eu,Gd) ₂O₃ with stainless steel.     

LiMgPO4:Tb,B - A new sensitive OSL phosphor for dosimetry

Optically Stimulated Luminescence (OSL) technique has emerged as a serious competitor to Thermally Stimulated Luminescence (TSL) technique in various dosimetric applications, especially after the development of crystalline alumina (Al₂O₃:C) doped with carbon. Since then, several attempts are being made to develop other possible materials for OSL based dosimetric applications. Efforts conducted in our laboratory in this direction have led to the development of a new phosphor, Lithium Magnesium Phosphate doped with terbium and boron (LiMgPO₄:Tb,B). This phosphor is prepared by solid-state diffusion method involving conventional air furnaces with operating temperature 1000 °C and easily amenable to large scale production without compromising primary dosimetric advantages. In this work we present some of the dosimetric OSL characteristics of this phosphor. The phosphor exhibits a main TSL peak at 250 °C. The phosphor also emits OSL, when the irradiated phosphor is stimulated with 470 nm light with the OSL sensitivity 1.3 times that of commercially available Al₂O₃:C. Photoluminescence (PL) emission spectrum consists of sharp lines characteristics of Tb³⁺ emission. The OSL discs made out of this phosphor are reusable up to at least 50 cycles, the phosphor exhibits dose linearity up to 1 kGy. Minimum detectable dose is found to be 20 μGy and fading of the OSL signal is found to be about 16% in four days, after which the OSL signal stabilizes.     

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.     

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.     

Nanoparticles of Al2O3:Cr as a sensitive thermoluminescent material for high exposures of gamma rays irradiations

Aluminum Oxide (Al₂O₃) doped with proper activators is a highly sensitive phosphor commonly used for radiation dosimetry using thermoluminescence (TL) technique. Nanoparticles of this material activated with Chromium (Cr) have been synthesized using the propellant chemical combustion technique and studied for their TL response. They were characterized by X-ray diffraction and scanning electron microscope. The synthesized material has spherical nanoparticles with grain size around 25 nm. These nanoparticles were exposed to heavy doses from γ-rays of ¹³⁷Cs. The TL glow curves show a prominent peak at around 474 K. This peak is found to be sensitive for high exposures of γ-rays and has linear response in the range of 100 Gy-20 kGy without showing saturation. This remarkable result suggests that Al₂O₃:Cr nanoparticles might be used for the dosimetry of food and seed irradiations.     

Depth profiling of Al2O3 + TiO2 nanolaminates by means of a time-of-flight energy spectrometer

Atomic layer deposition (ALD) is currently a widespread method to grow conformal thin films with a sub-nm thickness control. By using ALD for nanolaminate oxides, it is possible to fine tune the electrical, optical and mechanical properties of thin films. In this study the elemental depth profiles and surface roughnesses were determined for Al₂O₃ + TiO₂ nanolaminates with nominal single-layer thicknesses of 1, 2, 5, 10 and 20 nm and total thickness between 40 nm and 60 nm. The depth profiles were measured by means of a time-of-flight elastic recoil detection analysis (ToF-ERDA) spectrometer recently installed at the University of Jyväskylä. In TOF-E measurements ⁶³Cu, ³⁵Cl, ¹²C and ⁴He ions with energies ranging from 0.5 to 10 MeV, were used and depth profiles of the whole nanolaminate film could be analyzed down to 5 nm individual layer thickness.     

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.     

Electronic sputtering of Gd3Ga5O12 and Y3Fe5O12 garnets: Yield, stoichiometry and comparison to track formation

The results of present paper have shown that sputtering of yttrium iron garnet (Y₃Fe₅O₁₂) under swift heavy ions in the electronic energy loss regime is non-stoichiometric. Here we are presenting additional experimental results for gadolinium gallium garnet (Gd₃Ga₅O₁₂) as target. The irradiations were performed with different ions (⁵⁰Cr (589 MeV), ⁸⁶Kr (195 MeV) and ¹⁸¹Ta (400 MeV)) impinging perpendicularly to the surface. As earlier, the sputtering yield was determined by collecting the emitted gadolinium and gallium atoms on a thin aluminium foil, placed upstream above the target and analyzing the Al catcher by Rutherford backscattering. Also for Gd₃Ga₅O_12, the emission of Gd and Ga is non-stoichiometric. Sputtering appears above a critical electronic stopping power of S_th = 11.6 ± 1.5 keV/nm, which is larger than the threshold for track formation, in agreement with other amorphisable materials. In addition, the angular distribution of the sputtered species was measured for Y₃Fe₅O₁₂ and Gd₃Ga₅O₁₂ using 200 MeV Au ions impinging the surface at 20° relatively to the surface. For the two garnets the ratio of Y/Fe (and Gd/Ga) varies with the angle of emitted species and the stoichiometry seems to be preserved only for an emission perpendicular to the surface.     

Thermal shock properties of 2D-SiCf/SiC composites

This paper dealt with the thermal shock properties of SiC_f/SiC composites reinforced with two dimensional SiC fabrics. SiC_f/SiC composites were fabricated by a liquid phase sintering process, using a commercial nano-size SiC powder and oxide additive materials. An Al₂O₃–Y₂O₃–SiO₂ powder mixture was used as a sintering additive for the consolidation of SiC matrix region. In this composite system, Tyranno SA SiC fabrics were also utilized as a reinforcing material. The thermal shock test for SiC_f/SiC composites was carried out at the elevated temperature. Both mechanical strength and microstructure of SiC_f/SiC composites were investigated by means of optical microscopy, SEM and three point bending test. SiC_f/SiC composites represented a dense morphology with a porosity of about 8.2% and a flexural strength of about 160 MPs. The characterization of SiC_f/SiC composites was greatly affected by the history of cyclic thermal shock. Especially, SiC_f/SiC composites represented a reduction of flexural strength at the thermal shock temperature difference higher than 800 °C.     

Effect of carbide on Al2O3/B4C burnable poison sintered in Ar

Al₂O₃–14 wt%B₄C composites with 0–1 wt% C addition were sintered in Ar at 1550–1650 °C. The influence of the C additive on the B loss, densification behavior, and microstructure of the Al₂O₃–B₄C composites were investigated. The results show that there are B₂O₃, H₃BO₃ and Al₁₈B₄O₃₃ exist between Al₂O₃ and B₄C interface, which result in B loss because of B₂O₃'s high vapor tension at above 1500 °C. The presence of Al₁₈B₄O₃₃ grains formed by chemical reaction of Al₂O₃ with surface oxides on B₄C inhibit the densification of pellets by reducing the specific free surface energy of the Al₂O₃. However, the added C eliminates those oxides to reduce B loss because of its higher activity than B₄C, and it also coarsens Al₂O₃ grain although the density of pellets is decreased by gas products.     

Volume change of Al2O3 and MgAl2O4 induced by 14-MeV neutron irradiation

Volume change of Al₂O₃ and MgAl₂O₄ induced by irradiation with 14-MeV neutrons at 50°C has been measured. It is shown that the volume change of A1₂O₃ is anisotropic and is larger than that of MgAl₂O₄ about a factor of five. The result for MgAl₂O₄ is compared with that of fission neutron irradiation.     

Oxidation of Uranium Dioxide

The oxidation of UO₂ was studied by thermogravimetry and X-ray diffraction. It was clarified that the thermal history covering the first stage of the oxidation from UO₂ to U₃O UO₇ significantly influenced the rate of the oxidation of the second stage from U₃O₇ to U₃ O₈.

The entire oxidation reaction proceeded in what to all appearances, was a single stage when the specimen temperature was raised rapidly, whereas at slower rates of heating up, two distinct stages of oxidation were observed, separated by an intermediate induction period. These findings suggest the existence of a close connection between the rate of formation of the U₃O₇ phase and the rate of the subsequent oxidation of this phase: A slow formation of U₃O₇ would tend to prolong the induction period preceding the second stage of the oxidation. A similar effect was observed also with annealing of the intermediate U₃O₇ at 200°C: The increase of annealing time prolonged the induction stage.

The rate of the second stage oxidation was fairly well expressed by Johnson & Mehl's equation, log (1/(1-y/)=(1/2.303)kⁿtⁿ . The time exponent n in this equation varied in the range of 1.0~2.5, and the rate constant k of 1.15×10^?4~2.04 ×10^?1 min^?1, depending on the experimental conditions.