PRACTICAL EXPERIENCE IN THE PRODUCTION OF HIGH-DENSITY COMPONENTS TO ACCURATE DIMENSIONS FROM FINE POWDERS

Abstract

The paper traces the dependence of consistent quality in high-density components on the preparative history of the powder used in fabrication, taking uranium dioxide as an example. Consistent dimensions imply consistent densities, microstructures, &c., and the influence of powder quality on these properties is discussed. The chemistry of uranyl nitrate precipitation with ammonia is described with reference to the powder characteristics of the product and the way in which these are modified by subsequent thermal and mechanical treatments, such as hydrogen reduction and micronizing.

In the high-throughput production of uranium dioxide solid cylinders for nuclear fuel elements, automatic presses. are used; the requirements for granulation and binder addition to ensure a smooth and consistent feed to the presses are considered, together with the precautions necessary in removal of the binder to avoid subsequent distortion of the pellet on sintering.

In the U.K.A.E.A., such pellets are made on the ton scale and the typical dimensional spread on as-sintered pellets is ± 1% on components of ～0·5 in. dia.; this is a considerable achievement, considering that the shrinkage of a component on sintering is some 40 vol.-%. Final sizing, for instance on centreless grinders, reduces the dimensional spread to ± 0·0005 in.

The experience gained in studying uranium dioxide fabrication is translatable to other special (i.e. non-silicate) ceramics and to metal powders, as illustrated by work at Springfields on beryllia and alumina.