Dispersions of (UTh)O2 in beryliia, containing 1.7 per cent to 25 per cent (UTh)C>2 in three fuel particle sizes, coarse (150 —200μ)> medium (33 — 35μ and fine (<10 and <5º) were irradiated to burnups of 3—10 per cent of heavy metal atoms in the range 300-900ºC, in both fast and thermal fluxes. Changes in volume, lattice parameter, line breadth, and modulus of rupture were measured. Volume changes in the fine dispersions were ascribed wholly to fission fragment damage and were about 50 per cent greater than those caused by fast neutrons alone; they increased with increasing fission fragment flux, and decreased as irradiation temperature increased. Volume changes in medium and coarse dispersions were about 25 per cent greater than those caused by fast neutrons alone; the enhancement of the damage is attributed to the additional β flux. As fuel particle size increased, deterioration in strength under irradiation was more marked. This was attributed to more intense fission fragment damage in the recoil zone around larger particles causing volume increases which exceeded those of the remainder of the matrix. For maximum initial strength and retention of strength under irradiation the fuel particle size should not exceed 5μ, and the inter-particle spacing should not exceed 30.