[en] Due to the extension of the permanence time of the nuclear fuels within the reactor, physical and chemical modifications take place in the fuel material, especially in the external ring of the fuel pellet. The codes for simulating the rod behaviour during reactor operation need adequate models to describe these phenomena and be capable of making accurate predictions in the whole burnup range. A complex group of subroutines has been included in DIONISIO to represent the radial distribution in the pellet of the power density, burnup, porosity and concentration of diverse nuclides, particularly those capable of undergoing fissions, in terms of overall parameters like initial enrichment and average burnup. In this work we summarize the models recently developed, related to the high burnup scenario. On the one hand, empirical expressions representing the absorption and capture cross sections of several uranium and plutonium isotopes, as functions of the initial enrichment in "2"3"5U, average burnup and radial coordinate are presented. In addition, new models that give the distribution of porosity, fission gas retention and release in the pellet edge are described. Moreover, an empirical formula that relates the thermal conductivity of the fuel material with the burnup and the content of gadolinium, usually added as burnable poison, is presented. Subroutines corresponding to each of these models have been incorporated to the DIONISIO code. With these improvements the code was used to simulate data provided by the FUMEX I/II/III NEA data bank. The results presented here make evident the good agreement between experiments and simulations. (author)