[en] In a liberalised electricity market the improvement of nuclear fuel performance in terms of burn-up, reliability and safety directly affects the economic performance a fundamental factor in the assessment of nuclear energy sustainability (IAEA 2011). In this perspective, it is therefore relevant to have available fuel performance codes deeply verified and validated to perform with due accuracy safety assessment, fuel rod design and studies on fuel rod behaviour under different operating conditions, both normal and accidental. Fuel performance codes can be classified into national, with origins and histories of independent development, and international, distributed to multiple partners and then specialized according to the needs of individual users. On one hand codes are therefore different for history and stage of development, on the other hand new experimental data provide to researchers further information useful to improve the knowledge of fuel behaviour in an extended burn-up domain. Furthermore improvements of materials properties in parallel with investigations on new materials for nuclear applications are steadily proceeding. In the area of fuel modelling the IAEA launched a series of coordinated research projects (CRPs) since 1981 with the first project called DCOM followed by FUMEX and FUMEX II projects held in 1993-1996 and 2002-2006 respectively (IAEA 1998; Killeen et al. 2007).In 2006 and in 2007 the Technical Working Group on Fuel Performance and Technology (TWGFPT) of the IAEA recognized the need for a FUMEX III with the aim of supporting Member States in refining their codes in particular in the high burn-up domain of LWR applications (UO2, MOX). The Improvement of Computer Codes Used for Fuel Behaviour Simulation project (FUMEX III), coordinated by the IAEA, was an important opportunity for Member States to develop and extend the validation of their fuel performance codes on the basis of experimental data made available by the OECD/NEA and the IAEA itself (Killeen 2008). The specific objectives of FUMEX-III (2008-2012) are focused on various topics of fuel modelling amongst them: fission gas release, MOX, mechanical interaction (PCMI), pellet-clad interaction (PCI) and severe accidents (LOCA, RIA). The studied domain extends beyond 50 MWd/kgHM in the case of LWRs and 20 MWd/kgHM for HWRs. As mentioned, the project made available to participants qualified data from research and commercial reactors. The most important source of this information was the International Fuel Performance Experiments (IFPE), a database of the OECD/NEA (IFPE 2012; Killeen 2008). ENEA took part in the FUMEX III project in collaboration with POLIMI, devoting own efforts to the analysis of two aspects of LWR fuel performance: reactivity-initiated accidents (RIAs) and MOX fuel. In these domains ENEA took in charge the analysis of following priority cases: FK-1, FK-2 (RIA) and IFA-629.1 (MOX) (Killeen 2008; Vettraino et al. 2008). The main objective of ENEA was the reinforcement of in-house expertise in LWR fuel performance analysis. To this purpose selected cases proved to be an outstanding opportunity in fact while FK-1 and FK-2 deal with UO2 rodlets pre-irradiated under BWR conditions, IFA-629.1 deals with MOX under PWR conditions, furthermore different approaches to analyse severe transients and normal operating conditions are required (Vettraino et al. 2008). ENEA owns a licence, granted through agreements with the European Atomic Energy Community, for the use of the TRANSURANUS fuel performance code and is actively participating in the code users community (Lassmann 1992, Transuranus 2011). ENEA contribution was mainly devoted to testing code capabilities in the above mentioned areas of LWRs fuel performance. While for MOX the code already provides an extensive set of validation experiments for RIA development is on- going and up to now no purpose-developed modelling was implemented in the code versions distributed to users. The performance of recently developed models for transient fission gas release and helium release was, in particular, pointed out (Botazzoli et al. 2011; Transuranus 2011). This report summarizes the results obtained within the project as a contribution of ENEA to the final document of FUMEX III. (author)