  The low energy structures of irradiation-induced defects have been studied in
detail, as these determine the available modes by which a defect can diffuse or
relax. As a result, there are many studies concerning the relative energies of
possible defect structures, and empirical potentials are commonly fitted to or
evaluated with respect to these energies. But recently [Dudarev et al Nuclear
Fusion 2018], we have shown how to determine the stresses, strains and swelling
of reactor components under irradiation from the elastic properties of
ensembles of irradiation-induced defects. These elastic properties have
received comparatively little attention. Here we evaluate relaxation volumes of
irradiation-induced defects in tungsten computed with empirical potentials, and
compare to density functional theory results where available. Different
empirical potentials give different results, but some potential-independent
trends in relaxation volumes can be identified. We show that the relaxation
volume of small defects can be predicted to within 10% from their point-defect
count. For larger defects we provide empirical fits for the relaxation volume
of as a function of size. We demonstrate that the relaxation volume associated
with a single primary-damage cascade can be estimated from the primary knock-on
atom (PKA) energy. We conclude that while annihilation of vacancy- and
interstitial- character defects will invariably reduce the total relaxation
volume of the cascade debris, empirical potentials disagree whether coalescence
of defects will reduce or increase the total relaxation volume.
