An LMFBR fuel rod performance analysis system has been developed using the CYGRO-F computer code. The approach used was to model materials properties and phenomena using basic data and fundamental formulations as far as possible. Lack of data and understanding prevent a completely fundamental solution. The uncertainties were resolved by normalizing the code against selected experimental results and making appropriate adjustments to the models. The result of this work is an analytical system of models integrated into a central stress-strain analysis. Application of the system to independent experimental data gave satisfactory agreement between analysis and observation. Four highly significant models have emerged from this work. The analytical fuel swelling model simulates volume increase saturation and subsequent volume conservate growth which can result in center-hole growth independent of pore migration. The feasibility of a fundamental fuel swelling model has been demonstrated. This model describes the nucleation, growth, and migration of gas bubbles and produces mechanical effects similar to those in the analytical model. The cladding swelling model accounts for the effects of stress on swelling rate to reproduce the large volume changes observed in some fuel rod cladding. The results of this model are in qualitative agreement with fundamental analyses of the effect. Studies of startup and early life behavior resulted in a thermomechanical analysis system which describes in detail the complex interacting phenomena occurring under these transient conditions. The system may be applied to the determination of LMFBR core limiting operating conditions.