Mutsumi Hirai

Rim structure formation and high burnup fuel behavior of large-grained UO2 fuels

Irradiation-induced fuel microstructural evolution of the sub-divided grain structure, or rim structure, of large-grained UO2 pellets has been examined through detailed PIEs. Besides standard grain size pellets with a grain size range of 9–12 μm, two types of undoped and alumino-silicate doped large-grained pellets with a range of 37–63 μm were irradiated in the Halden heavy water reactor up to a cross-sectional pellet average burnup of 86 GWd/t. The effect of grain size on the rim structure formation was quantitatively evaluated in terms of the average Xe depression in the pellet outside region measured by EPMA, based on its lower sensitivity for Xe enclosed in the coarsened rim bubbles. The Xe depression in the high burnup pellets above 60 GWd/t was proportional to d−0.5–d−1.0 (d: grain size), and the two types of large-grained pellets showed remarkable resistance to the rim structure formation. A high density of dislocations preferentially decorated the as-fabricated grain boundaries and the sub-divided grain structure was localized there. These observations were consistent with our proposed formation mechanism of rim structure, in which tangled dislocation networks are organized into the nuclei for recrystallized or sub-divided grains. In addition to higher resistance to the microstructure change, the large-grained pellets showed a smaller swelling rate at higher burnups and a lower fission gas release during base irradiation.

Effect of grain size on recrystallization in high burnup fuel pellets

The effect of grain size on recrystallized structure formation in the outer region of high burnup UO2 fuel pellets was studied by optical microscopy, SEM, EPMA, XRD and TEM. Specimens were prepared from three kinds of fuels with different grain size (the standard pellet (grain size: 9 μm), the undoped large-grained pellet (51 μm) and the alumino-silicate-doped large-grained pellet (45 μm)), irradiated up to an average pellet burnup of 60 GWd/t in the Halden Reactor. The TEM observations showed that recrystallized structures were formed in a region from the middle to the edge (relative radius: r/r0= 0.7–1.0) of all fuel pellets, though they were less likely to form in the undoped large-grained pellet and the alumino-silicate-doped large-grained pellet than in the standard one. This result agreed qualitatively with the results obtained from optical microscope observations of the whole pellet region, SEM fractographs, and measurements of Xe concentration in the fuel matrix by EPMA. The effects of grain size and irradiation temperature on recrystallized structure formation were discussed in connection with fission damage accumulation.

Thermal conductivity measurements on 10 wt% Gd203 doped U02 + x

Abstract In order to evaluate the thermal conductivity of oxidized fuel pellets of leaker fuel rods, 10 wt% Gd 2 O 3 doped UO 2 + x samples with x between 0 and 0.15 were prepared by the oxidation method and their thermal conductivities were estimated from their thermal diffusivity measurements. The thermal conductivity decreased with increasing hyperstoichiometry. The summation rule was valid for the phonon scattering parameter of U 5+ ions and that of Gd 3+ ions. The thermal conductivities of (U, Gd)O 2 + x pellets were evaluated as a function of their hyperstoichiometry.

Heat capacity measurements of U1-yGdyO2 (y=0-0.27) from 325 to 1,673 K

Molar heat capacities were measured for U1-yGdyO2 (y=0–0.27) by differential scanning calorimetry (DSC) in the temperature region from 325 to 1,673 K. The molar heat capacities of U1-yGdyO2 (0<y&;lt;0.27) were close to, or slightly lower than, the values calculated by using the summation rule, and anomalous increases of heat capacities were not clearly observed at temperatures above 1,000 K. The heat capacity analysis for (U, Gd)O2 showed that the excess heat capacities of (U, Gd)O2 tended to decrease with increasing cation concentration excluding U4+ ions in crystals. This suggested that the Schottky term contribution to the heat capacity, which was mainly caused by the excitation of f electrons in the 5f 2 configuration in U4+ ions, decreased in (U, Gd)O2, compared to that in undoped UO2 due to the formation of U5+ and Gd3+ ions in (U, Gd)O2 by addition of Gd2O3. Thermal functions of (U, Gd)O2 were also evaluated.

The effects of oxidation on the thermal conductivity of (U, M)O2 pellets (M = Gd and/or simulated soluble FPs)

Abstract 10 wt% Gd 2 O 3 doped UO 2 + x samples with x being between 0 and 0.15 and simulated soluble FP doped UO 2 + x (simulated burnups: 30 and 60 GWd/tU), with x between 0 and 0.02, were prepared with an oxidation method. Sample thermal diffusivities were measured by using a laser flash method from 300 to 1400 K and sample thermal conductivities were evaluated by multiplying the thermal diffusivities by the sample densities and the specific heat capacities derived from the literature. The thermal conductivities of (U,M))O 2 + x (M = Gd and/or simulated soluble FPs) decreased with increasing hyperstoichiometry and they were expressed as a function of hyperstoichiometry and the concentration of impurities such as Gd 3+ and FP ions.

Chemical Vapor Transport of Iron by Iodine Liberated due to Radiolysis of Cesium Iodide

The cladding component chemical transport (CCCT) is one of the important modes of the fuel-cladding chemical interaction (FCCI) of LMFBR. In order to explain this phenomenon, a model based on the vapor phase chemical transport of cladding components by iodine was proposed by Johnson et al. and Calais et al. In this study, experimental work has been done to check whether such a mechanism can work due to the free iodine generated by the radiolysis of Csl vapor. As a result, it was confirmed that a significant amount of Fe can be transported via vapor phase from the Fe sheet heated at 430°C to the Mo plate heated at 720 or 800°C. Preliminary comparisons between this study and the in-pile irradiation tests have been made. This result qualitatively supports the appropriateness of the model for the CCCT mechanism based on the vapor phase transport of cladding components by radiation-induced iodine.

In-Pile and Out-of-Pile Grain Growth Behavior of Sintered UO2 and (U, Gd)O2 Pellets

Grain growth behavior of UO2 and (U, Gd)O2 fuel pellets was investigated with the data from the out-of-pile isothermal heating experiments and the irradiation test at the Halden Boiling Water Reactor. The laboratory data gave best-fitted equations by employing the following fourth power rate equations : UO2 : D2-D4 0=3.79×1018 exp(-142,000/RT)t, (U, Gd) : D2-D4 0=4.98×1017 exp(-140,000/RT)t, where, D 0 and D are initial and final three-dimensional diameters (μm), respectively, R the gas constant (=1.987 cal/mole/K), T the absolute temperature (K) and t the time (h) (gadolinia content: 3~10%, temperature range: 1,700~2,000°C). The calculated grain diameter with the above equations revealed an overestimation on specimens which involved noticeable fission gas bubbles on their grain boundaries. It was demonstrated that the in-pile grain growth model, as was given in the following equation, which took account of the retarding effects of growth by precipitated intergranular bubbles could describe the grain grow...

Effects of pellet microstructure on irradiation behavior of UO2 fuel

In-reactor tests and post-irradiation examinations (PIEs) were performed for standard and large-grained pellets with and without additives being soluble in a matrix and/or precipitated in a grain boundary, to confirm the effects of large grain structure on decreasing fission gas release (FGR) and swelling and to evaluate the influence of the additives in the matrix/grain boundary on them. The standard and large-grained pellets were loaded into small-diameter rods equipped with a pressure gauge. These rods were irradiated to about 60 GWd/t U at a linear heat rate of about 30–40 kW/m in the Halden reactor and then subjected to PIEs. Large-grained pellets showed a smaller FGR compared with standard pellets. Post-irradiation annealing tests suggested that swelling during transient power was decreased for large-grained pellets, except for those with additive enhancing cation diffusion.

Thermal conductivities of granular UO2 compacts with/without uranium particles

The thermal conductivities of granular UO2 compacts with and without uranium particles were measured to evaluate the thermal performance of vibro-packed granular MOX fuels containing metallic fine particle oxygen getters. The thermal conductivity of the compact with 10 wt% of uranium particles was higher than that of the compact without uranium particles. After heating beyond 1,408 K, the melting point of the uranium particles, the thermal conductivity increased further. The evaluation model for analyzing such phenomena was developed. The model predicted that once the UO2 compact with uranium particles was exposed to a temperature beyond 1,408 K, the uranium particles should melt and provide interconnecting areas between the UO2 granules and uranium particles, and between other uranium particles. The resulting increase of the thermal conductivity was reasonably expressed by the effect of necks in the compact on the heat conduction.

Vibro-Packing Experiment of Non-Spherical Uranium Dioxide Particles with Spherical Metallic Uranium Particles

Japan Atomic Energy Agency has developed vibro-packed fuel as one of the candidates for commercial fast breeder reactor fuels. In this study, vibro-packing experiments were carried out to investigate particle movement during vibro-packing as well as particle distribution after the completion of vibro-packing. Non-spherical uranium dioxide particles and spherical metallic uranium particles were used to simulate mixed oxide particles and oxygen getter particles, respectively. These experiments revealed the following facts. It is important to prevent segregation by means of feeding each size of the fuel particles uniformly into a cladding tube in order to disperse oxygen getter particles uniformly. “Simultaneous feeding” with volumetric powder feeders is useful to realize the above.