Patent Number: 042645409
Section: summary

BACKGROUND TO THE INVENTION This invention relates to the production of nuclear fuel pellets. Nuclear fuel pellets may be produced from powdered oxides of uranium, plutonium or thorium, or mixtures of these oxides, by granulation of the powders followed by pressing of the granules into compacts and sintering the compacts. Additives are often mixed with the oxide powders and for a variety of reasons for example, to control the porosity of the sintered pellets, to act as a binder for the compacts or to aid sintering. Additives may also be used to control grain size in the sintered pellets, particularly to increase it, and this is now considered desirable because it increases gas atom diffusion length and, hence, reduces gaseous fission product release from the fuel pellets during irradiation. STATEMENT OF THE INVENTION According to the invention in the production of nuclear fuel pellets consisting essentially of oxides of uranium, or mixtures of these oxides with oxides of thorium or plutonium, formed into compacts and sintered, niobium pentoxide is included in the oxide compact in sufficient quantity to encourage grain growth in the oxide compact during sintering, and the sintering and other process parameters are adjusted so that no impurities are trapped in the sintering pellets which would prevent a high matrix density and grain size being obtained. Retention of some impurities, notably carbon, which form gaseous compounds on reacting within the pellets, leads to the development of porosity within the pellets which will prevent high matrix density being attained, reduces the grain size which is achievable and may lead to pellet fragmentation. Since niobium pentoxide as well as being a grain growth promoter also increases the sintering rate the time before closure of pores during sintering may be insufficient for impurity removal and although complete elimination of the impurity is not essential removal down to very low levels is desirable, say less than ten parts per million by weight. The sintering rate and the impurity removal process may be controlled by control of the sintering atmosphere, in particular the oxygen potential and the heating rate during sintering. The level at which the oxygen potential of the sintering atmosphere is controlled may affect both matrix density and grain size, and also the reproducibility of results. The extent of the effect depends not only on the absolute oxygen potential but on the density of compacts before sintering, the size of the compacts and the heating rate during sintering, indicating an impurity trapping mechanism is operating. It is an important effect of the addition of niobium pentoxide in accordance with the invention that it does not significantly increase the diffusion rate of the fission products, notably the gases, in the ceramic matrix of the fuel pellets. Such an increase would, of course, offset the advantage which can be achieved by larger grain sizes. With hydrogen as the sintering atmosphere the oxygen potential may be supplied by water vapour or by carbon dioxide (which leads to the production of water vapour).