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We have monitored the thermal evolution of the proton irradiated structure of W--5 wt% Ta alloy by in-situ annealing in a transmission electron microscope at fusion reactor temperatures of 500--1300 textdegreeC. The interstitial-type a/2 dislocation loops emit self-interstitial atoms and glide to the free sample surface during the early stages of annealing. The resultant vacancy excess in the matrix originates vacancy-type a/2 dislocation loops that grow by loop and vacancy absorption in the temperature range of 600--900 textdegreeC. Voids form at 1000 textdegreeC, by either vacancy absorption or loop collapse, and grow progressively up to 1300 textdegreeC. Tantalum delays void formation by a vacancy-solute trapping mechanism.