Influence of fuel microstructure on in-pile densification

Early in 1972, confirmation was obtained that gaps observed in the columns of fuel pellets in the cores of several pressurized water reactors were the result of densification of the fuel during operation. The implications of these gaps with regard to fuel rod integrity and reactor safety stimulated a substantial effort to understand in-pile densification at low temperatures and to provide corrective action. Data obtained in the course of irradiation and by postirradiation examinations have disclosed that in-pile densification is controlled by the microstructure of the fuel, particularly its pore size distribution and porosity. These factors, in turn, were found to be controlled by fabrication parameters of which the sintering conditions were most important. The background, data, and theory of densification are reviewed. As a consequence, appropriate controls have been placed on fuel density, microstructure, and sintering conditions to reduce in-pile densification to insignificant levels.