[en] The creep of UO₂ containing small additions of Nb₂O₅ has been investigated in the stress range 0.5-90 MN/m² at temperatures between 1422 and 1573 K. The functional dependence of the creep rate of five dopant concentrations up to 0.8 mol% Nb₂O₅ has been examined and it was established that in all the materials the secondary creep rate could be represented by the equation epsilonkT = Asigmasup(n) exp(-Q/RT), where epsilon is the steady state creep rate per hour, Q the activation energy and A and n are constants for each material. It was observed that Nb₂O₅ additions can cause a dramatic increase in the steady state creep rate as long as the niobium ion is maintainde in the Nb⁵⁺ valence state. Material containing 0.4 mol% Nb₂O₅ creeps three orders of magnitude faster than the pure material. Analysis of the results in terms of grain size compensated viscosity suggest that, like pure UO₂, the creep rate of Nb₂O₅ doped fuel is diffusion-controlled and proportional to the reciprocal square of the grain size. A model is developed which suggests that the increase in creep rate results from suppression of the U⁵⁺ ion concentration by the addition of Nb⁵⁺ ions, which modifies the crystal defect structure and hence the uranium ion diffusion coefficient. (orig.)