Investigation of grain size distribution in nuclear fuel pellets

Abstract

Demands for increased safety margins and better fuel economy in Light Water Reactors have led to the development of a new type of fuel called Advanced DOped Pellet Technology (ADOPT). ADOPT fuel has been designed to have a larger grain size compared to Standard UO2 fuel. It is believed the larger grain size is achieved by an increased diffusion caused by dopants. In this work the average grain size and distribution in ADOPT has been studied. An optical microscope has been used to take micrographs from which the grain size has been determined using the linear intercept method. Pellets from different parts of a sintering boat have been investigated to see if there is any variation in grain size. The grain growth has been studied by doing measurements before and after heat treatment. These results have been compared to simulations of growth during isothermal conditions. Results from an irradiation experiment are evaluated with respect to the grain size to study the grain growth behavior under in-pile conditions. A variation of grain size within a sintering boat was observed but judged to be within the measurement error. The grain growth model used showed good agreement with the measurements for both Standard fuel and ADOPT fuel during isothermal conditions at 1700°C. The evaluation of irradiation experiment showed that for a lower temperature some correction of the model may be needed.