Evaluation of the TRIGA reactor fuel burn-up by means of Monte Carlo codes and measurements

Knowledge of fuel composition as a function of irradiation time is of paramount importance both for reactor operation and fuel management. During reactor operation, the evaluation of each fuel element composition, allows to accomplish the constraints of the plant operation licence while optimizing the core management. At the discharge of fuel, the knowledge of the fuel composition is a necessary information in order to plan and to actuate a correct fuel handling and management for the final disposal. Due to the variety of core geometries and fuel compositions of research reactors, it is useful to approach the problem of the evaluation of the time-dependent composition of the nuclear fuel by developing a flexible method of analysis based on Monte Carlo codes. In this work we present a methodology of analysis based on Monte Carlo codes MCNP (vers. 4C) and MCB that was developed to evaluate the fuel burn-up and poisoning of the TRIGA Mark II nuclear research reactor of the Applied Nuclear Energy Laboratory (LENA) of the University of Pavia. This methodology, extendable to other TRIGA reactors, has been validated by comparing the results of the Monte Carlo simulation with the reactivity measurements performed at the reactor during several years of operation. The results of the fuel composition analysis were also used to design a new reactor core configuration in order to increase the core reactivity control margin using the available irradiated fuel elements