[en] The initial rate of contraction has been measured during auto irradiation of three delta-phase (face centred cubic) PuSc alloys containing 5, 12 and 18 at.% Sc. The strongly negative value of the volume of formation of a Frenkel pair, deduced by extrapolation, for pure delta-plutonium can be explained by supposing that the interstitial plutonium atom can recover part of the lattice distortion energy by formation of bonds with its neighbours by a local increase of the d-f hybridization and more importantly be causing a strengthening of bonds among its neighbours. (Auth.)