[en] Irradiation induced volume swelling in reactor structural materials is of major concern to a reactor designer. Swelling causes dimensional instability and this time-, temperature- and flux density-dependent variation in materials properties can result in a serious problem for reactor design. Observations made during the course of resistivity recovery experiments on the swelling of molybdenum specimens which were irradiated to a very high fluence (approximately 1x10²² n/cm²,E>1MeV) at six relatively high temperatures lying between 460 and 1080⁰C, are reported. (Auth.)