The stress distribution arising in micropellets and cylindrical fuel compacts during fabrication, the stress concentration in micropellets located near the surface of a compact, and the evolution of defects in micropellets as a function of the type of stress state are investigated. It has been found that an ensemble of micropellets with a large number of particles contains a continuous spectrum of defects in the range 10 -4 -10 2 µm. Mechanical stresses engender evolution of the defects according to the scheme accumulation of microdefects → microcracks → cracks → through defects. Recommendations are formulated for lowering the number of defects in micropellets during deposition of coatings on the micropellets and compaction.The process of obtaining uranium-graphite fuel elements (spherical, prismatic, cylindrical) for HTGR includes the operations of depositing pyrolytic carbon (PyC) and silicon carbide (SiC) coatings on fuel kernels in the fluidized-bed furnace, mixing the obtained micropellets with graphite and a binding agent, pressing the conpacts, polymerizing the binder, carbonization and high-temperature heat-treatment [1]. The samples were pressed under pressure 10 MPa at temperature 100°C, and then went through at a higher temperature a stage of polymerization of the binder (130-160°C), carbonization (200-800°C), and high-temperature heat treatment to remove the impurities (1800°C). Most technological operations involve cycles of heating and cooling of the micropellets and compacts. As a result of the low relaxational properties of pyrolytic carbon, graphite, and silicon carbide, residual stresses arise in the micropellets and compacts after each heat-treatment cycle [2]. In the present work, the distribution of stresses arising during fabrication of micropellets and cylindrical fuel compacts, the concentration of stresses in the micropellets located near the surface of a compact, and the evolution of defects in micropellet claddings were studied as a function of the type and level of the stress state.Calculation of the Stresses in a Matrix and the Micropellets of a Compact. The growth of SiC and dense PyC coatings on the curvilinear surface of micropellets with a PyC buffer layer deposited on the pellets is accompanied by stress localization on intergrain boundaries. The average grain size is 1-3 µm for PyC and 3-5 µm for SiC [3]. Intergrain stress relaxation occurs by a dislocation mechanism.Accumulations of microdefects cannot form microcracks under the conditions of isotropic compression if the stresses do not exceed the strength limit of the material σ B . At the same time, microcracks will form if the diagonal terms of the stress tensor differ in absolute value and sign. Under these conditions, a microcrack grows in length by the Griffith mechanism [4]:σ~ l -1/2 , where σ is the stress required for a crack of length l to grow. A crack can also grow by this mechanism if σ < σ B .
Progress in modern X ray techniques to a large extent is due to improvements in the performance char acteristics of X ray tubes. In addition to better X ray focusing, it is also very important to increase the service life of the X ray tube under conditions of increased ther momechanical load.Anode rotation leads to intensive electron bombard ment of its focal tracks. The heat generated at the focal spot is expanded all over the anode and emitted from its surface. As the focal spot is briefly exposed to the electron beam, its temperature rapidly increases, and then gradu ally decreases. Such temperature dynamics continues during the entire exposure session, whereas during the pause between the exposure sessions the mean tempera ture of the anode becomes lower. Usually, after 5 10 cycles the dynamics is leveled.Thus, the character of the first temperature cycle is due to the anode rotation and depends on the rotation period, while the second temperature cycle is determined by the operating mode of the X ray tube (namely, the exposure time and the pause duration).Typical operating modes of the 20 40BD40 125 X ray tube (Svetlana Roentgen, Ltd., St. Petersburg) are given in Table 1. This X ray tube anode has the following characteristics: anode diameter, 70 mm; substrate thick ness, 12 mm; target thickness, 1 mm; working surface slope angle, 17°. The mean radius of the working track is 27.5 mm. The anode rotation rate is 50 rpm; effective focal spot size, 2 × 2 mm. The substrate and target are made of molybdenum and tungsten, respectively. The service life of the X ray tube is 20,000 exposure sessions.The X ray tube service life was calculated based on the temperature fields and the number of thermal cycles before appearance of cracks in anodes made of monocrystalline and polycrystalline Mo and W. Calculation of Temperature Fields and Stress States of Rotating X Ray Tube AnodesThe anode temperature is increased above mean level near the focal spot. The anode size is not taken into account in calculation of the temperature variation amplitude. This is valid if the anode rotation rate is n > 0.17·а·d·(r 0 h 2 ) [1], where а is the temperature conductiv The X ray tube anode life (time interval before cracks in substrate or coating appear) was estimated for differ ent operating conditions. The calculations were based on the Manson equation and experimental results on the plastic properties of polycrystalline and monocrystalline materials. The limiting number of exposure sessions before appearance of cracks in the substrate or the monocrystalline coating was 80,000 250,000, which is 30 100 times greater than in the polycrystalline coating or 4 5 times greater than in the polycrystalline W-Re coat ing, respectively. It was demonstrated in this work that the exposure dose power for a monocrystalline anode was virtually invariable for ~5000 exposure sessions, whereas in the case of a polycrystalline anode ~1000 exposure sessions caused a decrease in the dose power. This can be regarded as evidence of larger damageability of p...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.