[en] The present final report presents a part of the research work, done in the Research Contract RC 15164, at the Institute for Nuclear Research and Nuclear Energy (INRNE), Sofia in the frame of the CRP 'Improvement of Computer codes used for Fuel Behaviour Simulation-FUMEX-III' supported by IAEA. According to the working program of the project the INRNE had to analyse the fuel performance of WWER rods included into the CRP FUMEX-III as well as the Gd doped fuel and the transient experiment Riso3, (rods II5 and GE7) and Kola3-MIR. The rod H09 from experiment OSIRIS irradiated for 4 cycles in the EDF Cruas PWR to a final burnup of 46 MWd/kgU was simulated too. The US-PWR 16x16 LTA Extended Burnup Demonstration Program and the Siemens Corporation 14x14 lead fuel assemblies, irradiated in the Ginna PWR for 5 cycles up to 58 MWD/kgU were analysed too. The goal of these two programs is to demonstrate and evaluate the potential of annular pellets and barrier cladding for resisting fuel failures due to pellet- cladding interaction up to burnups of 60 MWd/kgU. During the first year of the contract the team has worked on MIR ramp tests on Kola3 rods (Kola3-MIR experiment) by using the latest TRANSURANUS-WWER version v1m1j09 on the basis of the IFPE-OECD/ IAEA-NEA database. Fuel rods included in the tests have been operated under normal conditions at Kola NPP up to maximum burnup of about 50 - 60 MWd/kgU. Nine re-fabricated rods have been cut from selected FA's and carried out under single ramp conditions (RAMP test) and 2 step-by-step power increase tests with instrumented rods. The attention was concentrated on: - Fuel central temperature during the ramp irradiation (FRG2-test). Comparison between TRANSURANUS assessments and thermocouple records; - Pin pressure (FGR1-test. In-pile pressure measurements for two rod with different accumulated burnup during base irradiation; - Fission gas release and gas mixture (from PIE); - Mcrostructure changes of the fuel-central hole closing and porosity distribution. The calculations were done for base irradiation as well as for the test irradiation, using restart option of TRANSURANUS code for account rod cutting and refilling. According to the working program of the project for the second year of contract INRNE had to simulate and analyse the fuel performance of Gd doped fuel rods (Experiment GAIN), PCMI cases (Riso3- GE7; OSIRIS-2, H09 ) as well as the transient experiment Riso3, rod II5, included into the CRP FUMEX-III. The fuel rods included in the tests have been operated under normal conditions up to average burnup of about 40MWd/kgU (GAIN rods and Riso3-rods) and local burnup of about 50 MWd/kgU in OSIRIS-2 experiment. One rod Riso3-II5 rod was re-fabricated, instrumented with thermocouple and pressure transducer, refilled up to 0.64MPa and carried out under ramp conditions. Main results of the study was: - Two rods with 3% and 7% Gd₂O₃ doped fuel have been simulated and analysed. The fuel stack length changes and the axial distribution of the cladding diameter as well as the fission gas release in the case of Gd doped fuel were predicted with reasonable agreement. - The pin geometry changes (length and diameter) in the PCMI conditions and outer clad oxide layer were simulated and compared. - The radial distribution of the retained Xe and other products after irradiation (EPMA and micro gamma scanning data) were reproduced by TRANSURANUS code and comparison with measured data revealed good agreement. - The TRANSURANUS assessments of the fuel central temperature during the ramp irradiation (Riso3-II5 irradiation) is very close to the thermocouple data. The present final report presents the TRANSURANUS prediction of the fuel and cladding properties for the two USA experiments with different pellet design as well as the idealized AREVA case from the FUMEX-III priority cases programme. (author)