[en] The modelling of fuel behaviour during irradiation and the tailoring of an irradiation stable fuel require an accurate description of the stress free volume changes due to fuel densification and fuel swelling. The influence of fuel flows due to creep and hot pressing should be considered separately. This paper presents detailed densification models which describe the dependence of densification on time or burnup, temperature, density, pore size distribution, grain size, and fission rate. UO₂ matrix swelling due to fission product accumulation and densification due to pore shrinkage are considered to operate simultaneously and are linearly superposed. Four temperature regions can be identified which describe the dependence of irradiation induced fuel densification on temperature. The shrinkage rate of small pores strongly controls the initial densification rate. Experimental verification of the densification models is obtained from numerous constant temperature thermal sinter tests conducted between 1200⁰C and 1900⁰C as well as in isothermal irradiation tests between 1000⁰C and 1200⁰C and around 700⁰C. Although the kinetics of thermal and in-reactor densification are different, standarized thermal resinter tests can be established and used to predict the in-reactor densification. (Auth.)