Metallurgical structure modification of UO2 pellet during Sintering

Abstract

Nuclear Fuel Complex (NFC), Department of Atomic Energy (DAE) produces sintered UO₂ pellets by compaction, high temperature sintering followed by centreless wet grinding method from the powder received through ADU-route. Enhancement of fuel burn up of the Indian PHWRs becomes very important in order to effectively utilize the fuel to the maximum extent inside the reactor. Burn up is mainly limited by increased fission gas release from the fuel during reactor operation. Without introducing much change in the design, rate of release of fission gas can be reduced through enlargement of UO₂ grain size. In Powder Metallurgical (PM) route of fuel fabrication, trials were taken by doping various oxide powder additives like TiO₂, Al₂O₃, SiO₂, Nb₂O₅ and Cr₂O₃. The dopant normally goes into the solid solution of parent matrix during sintering at 1700°C and thus enhance the rate of diffusion. Aliovalant dopant can alter the defect chemistry of the parent material either by creating vacancy or interstitial. It is apparently understood that the combination of above mechanisms are responsible for structural modification of UO₂. Hence selection of dopant remains largely empirical. It has been observed at NFC Hyderabad that the Cr₂O₃ is the most suitable for achieving average UO₂ grain size of about 70 micron and 98%TD of the sintered pellet. The paper discusses about the various experimental trials, sintered densities, metallographic examination, effect of various quantities, analysis and result obtained thereof.