Microstructural Changes in UN Fuel Elements under the Influence of a Temperature Gradient

Abstract

Microstructural changes UN fuel elements including the vibratory compact of arc-fused UN, the sintered pellets containing a high UO₂ impurity and the vibratory compact of the powder prepared by crushing the sintered pellets were investigated under the influence of a steep temperature gradient by center-line heating technique at high temperatures in a vacuum of 1.0×10⁻⁴ mmHg.
Main results obtained were as follows:
(1) In the central regions of UN fuels, where the temperature was above 1800°C, uranium metal was precipitated by the decomposition of UN.
(2) Columnar grain growth, which was usually observed in UO₂ fuel heated under a temperature gradient in vacuum, was not found to occur.
(3) There was no appreciable difference in the microstructural changes between the vibratory compact of arc-fused UN containing a low UO₂ impurity and the vibratory compact of the powder prepared by crushing sintered pellets containing a high UO₂ impurity.
In addition, the microstructure at the interface of UN-UO₂ contact pellets after center-line heating was investigated.