Radiation-enhanced diffusion and fission gas release from recrystallised grains in high burn-up \hbox{UO}_{2} nuclear fuel

The effective diffusion coefficient that gives a steady-state xenon concentration of 0.2-0.3wt% in the recrystallised grains of high burn-up UO 2 fuel is calculated to lie in the range 10 m 24 to 10 m 22 m 2 s m 1 . These values are one to three orders of magnitude lower than the value currently accepted for the radiation-enhanced diffusion coefficient. The time required to reach the steady-state concentration depends on the local fission rate, the grain size distribution and the precise magnitude of the radiation-enhanced diffusion coefficient, and can take from 2 to 10 years. Additional calculations reveal that substantially less than 10% of the fission gas inventory is released from the original UO 2 grains in the outer region of the fuel prior to recrystallisation. In contrast, with a diffusion coefficient of 10 m 22 m 2 s m 1 more than 80% of the fission gas is released from the recrystallised grains of the high burn-up structure in one year.     

Diffusion and segregation along grain boundary at the electrolyte-anode interface in IT-SOFC

The atomic level chemical and microstructural features of grain boundaries in gadolinium-doped ceria (GDC) electrolyte thin film supported by Ni-GDC cermet anode were characterized by high resolution transmission electron microscope (HR-TEM) and scanning TEM (STEM). It was found that metallic Ni can diffuse from the anode into the thin film electrolyte along grain boundaries. In addition, Ce and Gd can also diffuse and segregate at grain boundaries between Ni grains in the anode substrate. HR-TEM observations revealed that Ni diffusion and segregation at grain boundaries between GDC grains enhanced the inhomogeneity and led to microstructural changes at grain boundary regions, i.e. the formation of superstructure. The observations also indicated that enhanced inhomogeneity at grain boundaries might play a significant role in the conductivity of GDC electrolyte film in solid oxide fuel cells.     

Advantages of Production of New Fissionable Nuclides for the Nuclear Power Industry in Hybrid Fusion-Fission Reactors

A concept of a large-scale nuclear power engineering system equipped with fusion and fission reactors is presented. The reactors have a joint fuel cycle, which imposes the lowest risk of the radiation impact on the environment. The formation of such a system is considered within the framework of the evolution of the current nuclear power industry with the dominance of thermal reactors, gradual transition to the thorium fuel cycle, and integration into the system of the hybrid fusion-fission reactors for breeding nuclear fuel for fission reactors. Such evolution of the nuclear power engineering system will allow preservation of the existing structure with the dominance of thermal reactors, enable the reprocessing of the spent nuclear fuel (SNF) with low burnup, and prevent the dangerous accumulation of minor actinides. The proposed structure of the nuclear power engineering system minimizes the risk of radioactive contamination of the environment and the SNF reprocessing facilities, decreasing it by more than one order of magnitude in comparison with the proposed scheme of closing the uranium–plutonium fuel cycle based on the reprocessing of SNF with high burnup from fast reactors.     

EFFECT OF NUCLEAR VISCOSITY ON FISSION PROCESS

According to the fission diffusion model,the deformation motion of fission nucleues is regarded as a diffusion process of quasi-Brownian particles under fission potential,Through simulating such Brownian motion in two dimensional phase space by Monte-Carlo method,the effect of nuclear viscosity on Brownian particle diffusion is studied,Dynamical quantities,suchas fission rate,kinetic energy distribution on scission,and so on are numerally calculated for various viscosity coefficients,The results are reasonable in physics,This method can be easily extended to deal with multi-dimensional diffusion problems.     

Irradiation behaviour of nuclear fuels

This paper gives a brief review of the important phenomena observed in metallic uranium and ceramic nuclear fuels during irradiation in reactors. The mechanism of irradiation growth, irradiation creep and swelling which are responsible for the dimensional instability of uranium has been described. Important phenomena observed in ceramic nuclear fuels,e.g. fuel densification, fuel restructuring, plutonium segregation, oxygen and fission product migration, irradiation creep, fission gas release and swelling have been discussed. A brief note is included on computer modelling for prediction of fuel element irradiation behaviour.     

Fokker-Planck方程的数值解

采用扩散模型研究核裂变,需要求解Fokker-Planck方程。本文提出一个数值计算方法-平均隐式差分方法。对具有粘滞性的核体系的有关裂变动力学量,如几率分布、裂变率、断点处的平均动能以及鞍点到断点的平均扩散时间等一系列物理量做了计算,并与适合大粘滞性的Kramers的解析解做了比较。通过与解析解的比较及对归一常数的检验,证明计算结果精确可靠。     

Grain boundary self-diffusion of tracer <Superscript>18</Superscript>O atoms in nanocrystalline oxide LaMnO<Subscript>3 + δ</Subscript>

Diffusion coefficients of tracer ¹⁸O atoms at boundaries of nanograins of LaMnO_{3 + δ} oxide have been measured in the temperature range of 400–500°C. The samples of the nanocrystalline oxide are prepared with the use of the shockwave loading method. The concentration profile of the tracer atoms after diffusion annealing is measured with the use of the nuclear microanalysis method. The activation energies of the grain boundary diffusion amounts to about 2 eV and the boundary width is ～0.05 nm. The measured coefficients of the grain boundary diffusion at 500°C exceed the corresponding coefficients of the volume diffusion by seven orders of magnitude.     

On the micromechanical modelling of the effective diffusion coefficient of a polycrystalline material

This study focuses on calculation of the effective diffusion coefficient of a polycrystalline material accounting for the grain size and shapes. Polycrystal is modelled as a composite consisting of a matrix with high diffusivity (grain boundaries and triple junctions) and inhomogeneities with low diffusivity (bulk grains including crystal defects like dislocations). The segregation at the grain boundaries is accounted for. Typical micromechanical models are re-written for diffusivity assuming that the grains have the shape of ellipsoids of revolution (spheroids). The results are compared with (1) numerical results for hydrogen diffusion in an imaginary polycrystalline material and (2) experimental results for diffusion of hydrogen in nickel polycrystal available in the literature. The approach can be used for extraction of information on diffusivity along the grain boundaries.     

Short-Circuit Diffusion in Ceramics

This article discusses grain boundary diffusion in ceramics. It gives a brief review of the experimental data available for ionic oxides and the problems of interpretation associated with it. The fundamental differences between grain boundary diffusion in metals and ceramics are noted. Calculations of the segregation of defects and impurities to grain boundaries are discussed together with methods of calculating diffusion coefficients in these boundaries. New results for alumina and chromia are presented. The problem of defining a grain boundary width is discussed with respect to new calculations on nickel oxide.     

Improvement of the zirconium diffusion barrier between lanthanide (La–Ce) and a clad material by hydrothermal crystallization

Hydrothermal crystallization was conducted to improve the diffusion barrier performance of Zr thin films to prevent a fuel clad chemical interaction (FCCI) at the interface between a clad material and lanthanide elements (Mischmetal: 75Ce–25La; intermetallic compound) as fission product. The crystalline phase and size of Zr thin films deposited on a HT9 disk by RF magnetron sputtering were varied by hydrothermal crystallization in an autoclave at 393 K, 423 K and 453 K. Diffusion couple tests of the clad with and without a Zr diffusion barrier were performed at 933 K for 25 h with mischmetal, which have diffusion properties similar to uranium metal composite fuel. While substantial FCCI occurred at the interface between the mischmetal and clad in the specimen without hydrothermal crystallization, the Zr barrier with hydrothermal crystallization showed excellent resistance to FCCI. The performance of the Zr FCCI barrier was improved due to a decrease in interdiffusion by the grain boundary, which can increase the FCCI in the Zr barrier.     

Diffusion through a polycrystalline thin film

Analytical solutions to Fick’s second law of diffusion have been simultaneously derived without the restrictions of parabolic profiles along the x-axis in grain boundaries and expressed in a series for both grain interior and grain boundary diffusing through a polycrystalline thin film. The analysis takes segregation of diffusion species at grain boundaries into account. The analytic solutions lead to the concentration profiles in the grain interior and in the grain boundary, to the average-integrated amount of diffusion species at the exit surface, and to the time lag, which can be technologically used for depth profile studies and kinetic accumulation measurements.     

Laser ultrasonics detection of an embedded crack in a composite spherical particle

Laser ultrasonics was applied to the manufacturing control of the integrity (no failure) of coated spherical particles designed for High Temperature Reactors (HTR). This control is of major importance, since the coating of the nuclear fuel kernel is designed to prevent from the diffusion of fission products outside the particle during reactor operation. The SiC layer composing the coating is particularly important, since this layer must be an impenetrable barrier for fission products. The integrity of the SiC shell (no crack within the shell) can be assessed by the ultrasonic vibration spectrum of the HTR particle, which is significantly changed, compared to the reference spectrum of a defect-free particle. Spheroidal vibration modes of defect-free dummy particles with a zirconium dioxide (ZrO₂) core were observed in the 2-5 MHz range. A theoretical analysis is presented to account for the observed vibration spectra of defect-free or cracked HTR particles.     

Measurements of Fission and Radioactive Capture Reaction Rates Inside the Fuel of the Ipen/MB-01

This work presents the measures of the nuclear reaction rates along the radial direction of the fuel pellet by irradiation and posterior gamma spectrometry of a thin slice of fuel pellet of UO₂ at 4.3% enrichment. From its irradiation, the rate of radioactive capture and fission had been measured as a function of the radius of the pellet disk using a Ortec GMX HPGe detector. Lead collimators had been used for this purpose. Simulating the fuel pellet in the pin fuel of the IPEN/MB-01 reactor, a thin UO₂ disk is used, being inserted in the interior of a dismountable fuel rod. This fuel rod is then placed in the central position of the IPEN/MB-01 reactor core and irradiated during 1 h under a neutron flux of 5 ×10⁸ n/cm² s. In gamma spectrometry, 10 collimators with different diameters have been used; consequently, the nuclear reactions of radioactive capture that occurs in atoms of ²³⁸U and the fission that occurs on both ²³⁵U and ²³⁸U are measured in function of 10 different regions (diameter of collimator) of the UO₂ fuel pellet disk. Nuclear fission produces different fission products such as ¹⁴³Ce with a yield fission of 5.9% which decay is monitored in this work. Corrections in geometric efficiency due to introduction of collimators on HPGe detection system were estimated using photon transport of MCNP-4C code. Some calculated values of nuclear reaction rate of radioactive capture and fission along the radial direction of the fuel pellet obtained by Monte Carlo methodology, using the MCNP-4C code, are presented and compared to the experimental data showing very good agreement.     

Molecular dynamics investigation of the diffusion permeability of triple junctions of tilt and mixed-type boundaries in nickel

The diffusion permeability of triple junctions of high-angle tilt boundaries 〈111〉 and 〈100〉 and mixed-type boundaries in nickel has been investigated using the molecular dynamics method. It has been shown that the diffusion permeability of equilibrium triple junctions does not exceed the permeability of the grain boundaries forming them. The effective diffusion radius of the considered triple junctions and the width of the grain boundaries are determined.     

ENHANCED NEUTRON EM1SSION IN HEAVY ION REACTIONS BY DIFFUSION MODEL

An extensive Smoluchowski equation with neutron emission is suggested in this paper and is analytically solved by means of van Kampen technique.The average fission rate,fission probability,the ratio of neutron emission to fission are calculated for composite nucleus ²⁴⁰PU. The time evolution of these quantities is analysed and their essential dependence on fission potential height and nuclear temperature are discussed.A fair1y substantial particle evaporation prior to fission in heavy-ion reactions can be well understood in our diffusion model.     

Plasma-stimulated penetrability of hydrogen as a tool for studying phase transitions at grain boundaries

It is well known that the diffusion of hydrogen atoms through the intrinsic defects of a crystal lattice has characteristics different from those of bulk diffusion and, at certain parameters for some polycrystalline metals, ensures the determining contribution to the transfer of hydrogen atoms through the material. Grain boundaries (and dislocations) are the most important and shortest paths, the diffusion through which is much faster than bulk diffusion through a crystal lattice. It is particularly important to take into account this diffusion in materials with grains having sizes of about several nanometers. The possibility of using the method of the plasma-stimulated penetrability of hydrogen to analyze phase transitions at the grain boundaries is demonstrated on the example of polycrystalline niobium foils. In contrast to the existing methods, this method proposed for studying grain-boundary diffusion and phase transitions is simple and ensures control over the surface. The temperature characteristics of the diffusion of hydrogen atoms through niobium grain boundaries have been measured.     

同一燃耗区域核燃料包壳破损建模分析

核反应堆燃料包壳是构成反应堆安全的重要屏障, 一旦发生破损,放射性裂变产物就会释放到一回路的冷却剂中。本文通过一回路134Cs/137Cs比值确定破损燃料棒所在燃耗区域,对该燃耗区域裂变产物的产生、裂变产物进入芯块间隙、一回路核素平衡分别进行建模,然后,利用建立的数学模型对破口处裂变产物向一回路释放过程进行分析,得出破口的大小和破损根数及所在燃耗区域,并以压水堆核电站燃料包壳破损的数据进行验证,证实了模型的合理性。     

Diffusion coefficients of vacancies and interstitials along tilt grain boundaries in molybdenum

The diffusion coefficients of vacancies and interstitials along symmetrical tilt grain boundaries in molybdenum have been calculated using the molecular dynamics method. The migration energies of defects have been obtained. The activation energy and coefficients of grain boundary self-diffusion have been deter-mined. A comparison of the obtained results with the studies of other authors indicates that boundaries formed between particles in the powder in sintering experiments have a higher diffusion activity as compared to stable grain boundaries in polycrystals.     

Importance functions in calculation of fuel burnup in a VVER

Equations that describe fuel burnup in a VVER are given. Equations for the neutron flux density and the content of fission products are presented in the canonical Cauchy form. Such form of representation of equations lends itself well for their use in solving problems related to optimization of the process of fuel burnup in a nuclear reactor. Also given are equations for the importance functions of neutrons and fission products that correspond to the basic system of equation for phase variables.     

Energies of formation and structures of point defects at tilt grain boundaries in molybdenum

The structure and formation energy of vacancies and interstitials at symmetrical tilt grain boundaries in molybdenum have been investigated by means of classical molecular dynamics. The dependence of the formation energy of these boundaries on the grain misorientation angle has been calculated. The structures of defects, energies of their formation, and diffusion mechanisms have been determined for the most stable grain boundaries.