[en] Microstructural examination of ferritic and austenitic commercial alloys neutron irradiated to /approximately/100 dpa confirms that ferritic alloys are very low swelling but precipitate development can be very complex. Austenitic alloys can be very high swelling but no clearly defined microstructural differences could be found between two alloys of similar composition but very different swelling response or between alloys of very different composition but with similar swelling. Differences are ascribed to differences in the onset of swelling. 5 refs., 4 figs., 2 tabs