Nuclear Technology / Volume 150 / Number 2 / May 2005 / Pages 189-195
Technical Paper / Materials for Nuclear Systems
Nitrogen content, above the specified limit of 75 g(gU)-1, was encountered in sintered uranium dioxide in the course of its manufacture. The cause was traced to the sintering process, wherein carbon, a degradation product of the die wall or admixed lubricant, was retained in the compact as a result of inadvertent reversal of gas flow in the sintering furnace. In the presence of carbon, the uranium dioxide reacted with nitrogen from the furnace atmosphere to form nitride. The compacts with high nitrogen were also those with low sintered density, arising from low green density. The low green density was due to filling problems of an inhomogeneous powder. The experiments carried out establish the causes of high nitrogen to be the carbon residue from lubricant when the UO2 is sintered in a cracked ammonia atmosphere.