Upon irradiating tantalum at 500°C in the Dounreay Fast Reactor (DFR), it was found that the dimensions of the metal increased initially, reaching a maximum of approximately 0.2 percent linear at a neutron dose of 10²² neutrons (n)/cm² (E > 0.11 MeV) (1.5 displacements per atom (dpa) Half-Nelson model (N/2), tantalum). At higher neutron exposure, the dimensions of the specimens gradually decreased. The initial swelling observed has been shown to be due to the formation of a high concentration of small voids. Possible mechanisms to explain the contraction found at the higher levels of irradiation have been considered, and it is concluded that a combination of void-induced swelling and shrinkage due to the formation of the tungsten by transmutation accounts for the observed dimensional behavior of tantalum during irradiation at 500°C.