Evolution of defects in micropellet fuel during the fabrication of HTGR compacts

Abstract

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 .