[en] The advent of high-voltage (1000 kV) electron microscopy has put the study of irradiated uranium in the same position as that of most other metals some ten years ago. Foils of thickness 100 to 200 nm can be examined so that representative distributions of precipitates in unirradiated uranium and the fission gas bubbles in irradiated uranium (both approximately 30 to 50 nm diameter) can be obtained. There are three populations of UAl₂ precipitate in magnox fuel bars, and these arise as a consequence of the casting technique. The effect of irradiation to doses <= 1000 MWd/t on these precipitate distributions is discussed. The fuel bars have a variable swelling resistance depending on the alloying additions and heat treatment. The differences is fission gas bubble and subgrain size and distribution have been studied in fuel bars of high and low swelling resistance at doses >= 6000 MWd/t. The microstructure of those fuel bars showing breakaway swelling has been characterized. (author)