In the present paper it is assumed that the recrystallization temperature of uranium dioxide decreases with burn-up. Two opposing effects of enhancement and inhibition of irradiation damage introduced by fission effect on grain growth are described. Mathematical model of fission gas release from the UO_{2} fuel affected by grain growth is presented. Theoretical results are compared with the experimental data.
In the present paper it is assumed that in the fluence range of 4×10^{19} - 0.5×10^{21} fissions/cm^{3} the main contribution to the fission gas release from an uranium dioxide single crystal is from the bubble traps by knock-out process. An analysis of bubble distribution in the single crystal for different temperatures shows that some experimental peculiarities of the fission gas release during irradiation arise from different conditions of the thin surface layer and the interior of the considered solid. The thin surface layer is assigned by the fission fragment range.
In the present paper it is assumed that above a limiting value of fission fluency (burn-up) a more intensive process of irradiation induced chemical interaction occurs. A significant part of fission gas product is thus expected to be chemically bounded in the matrix of UO_2 fuel. The fission gas atoms substituting, for example, uranium atoms in the crystallographic lattice can form weak facets. At a certain saturation condition, division of the grains can occur at the weak facets and the increase in fission-gas-products release may be expected. The fact that the process of grain division for high burn-ups (70-80 MWd/kgU) forms an extremely fine structure up to a temperature as high as 1100^ºC and simultaneously the observed decrease in fission gas concentration in the fuel supports this concept. The analysis of fission gas concentration change due to the formation of nanostructures in UO_2 fuel at high burn-ups in terms of total surface area change in a function of burn-up and knock-out process is presented.
The electrical and optical properties of semiconductors are largely determined by the defects and impurities they contain. Without a doubt, hydrogen is the impurity which exhibits the most varied and exotic properties. In most semiconductors, it is found in three charge states and four configurations. It forms (at least) two types of dimers as well as small and large precipitates such as platelets. H also interacts with impurities and defects. It removes or changes the electrical activity of many shallow and deep centers, and catalyzes the diffusion of interstitial oxygen (in Si). Sometimes, it exhibits quantum tunneling and is associated with unusual effects such as Fermi resonances. But one of the most exotic forms of hydrogen in GaAs and Si is the interstitial H_2 molecule, which appears to play a critical role in processes such as the "smart cut". It is the only interstitial molecule observed (so far) in semiconductors. In GaAs, it behaves like a nearly-free rotator, with properties very much as one would expect them to be. But in Si, the early experiments were puzzling. No ortho/para splitting was observed, the symmetry appeared to be C_1, the single HD line was at the wrong place and had the wrong amplitude, and other features seemed strange as well. Recent experimental studies have now resolved many issues. However, the behavior of the simplest molecule in the Universe proved to be a tough nut to crack, which goes to show that devils can be a lot more fun than angels after all.
A study of the defect structure of heteroepitaxially grown Hg_{1-x}Cd_{x}Te (MCT) films was performed with the use of ion milling. Undoped and in situ As- (acceptor) or In- (donor) doped films with x=0.22, grown by molecular beam epitaxy on GaAs substrates, as-grown and annealed, were subjected to ion milling with subsequent electrical characterization. The results obtained on the MBE films were compared to those acquired on wafers cut from bulk crystals, and on epitaxial films grown by liquid and vapor phase epitaxy. In all the MBE films ion milling revealed a presence of a neutral defect with concentration ≈ 10^{17} cm^{-3}, formed at the stage of the growth. Residual donor concentration in the films was found to be of the order of 10^{15} cm^{-3}, which is typical of high-quality MCT.
We present a detailed investigation of the growth kinetics of aluminium-related shallow thermal donors: the K-donors. Constraints for the diffusion mechanism of oxygen in silicon at temperatures ≈ 470°C are found. A large entropy of the K-donors is considered as a possible explanation of high diffusivities and interaction radii found for the generation of the K-donors.
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