Observations of Kernel Migration in the Coated Fuel Particles under Temperature-Gradient Heating

Abstract

Several kinds of coated fuel particles, with their coating either intact or artificially cracked, were heated out-of-pile in such manner as to create a sharp temperature gradient across the particles (60°∼120°C per particle), at temperatures from 1, 500° to 1, 950°C. The purpose was to obtain information on the displacement of the kernel material relative to the coating. To examine this amoeba effect, the particles were observed, after heating, by both ceramography and X-ray radiography. The results revealed that:(1) In the case of UO2 kernel with artificially impaired coating, their kernels were found to move more readily toward the crack, regardless of the temperature gradient, as compared with UC2.(2) The amoeba effect is observed even in out-of-pile heating on intact coated particles with UO2 kernel which moves down the temperature gradient. This UO2 movement was given a new explanation based on the evaporation and subsequent condensation of the UO2 within the particle, when the coating is intact.(3) In case of UC2 kernel, which moves up the temperature gradient, the sealing-in of the kernel by the intact coatings appears to assume a controlling factor, and the occurrence of evaporation is negligible.