In materials under irradiation Frenkel Pairs are produced at a rate proportional to the irradiating flux. Of these defects a fraction (1-α') suffer immediate recombination, and, under neutron or heavy ion irradiation a further fraction of the vacancies condense into vacancy dislocation loops. The effective defect production rates are therefore substantially less than those assumed in previous studies. Moreover the fraction α' is not the same for different types of irradiation. Under electron irradiation the effective biasBbecomesBeff= α'electronBwhile under ion or neutron irradiationBeffbecomesBeff= (α'hB—f) where α'electron # α'h.