Fuel burnup modelization with the Monte Carlo code MCNP5 and core reconfiguration prediction for the Triga Mark II reactor at the University of Pavia

A time evolution model was developed to study fuel burnup for the TRIGA Mark II reactor at the University of Pavia. The results obtained by this model were used to predict the effects of a complete core reconfiguration and the accuracy of this prediction was tested experimentally. The Monte Carlo code MCNP5 was used to reproduce system behaviour and to analyze neutron fluxes in the reactor core. Before the evolution analysis, the model was validated by reproducing experimental data from 1965, year in which the reactor reached its first criticality. The results showed good accuracy for both low and high power configurations. The software that took care of time evolution, completely designed in-house, used the neutron fluxes obtained by the Monte Carlo simulations to evaluate material consumption. This software was developed specifically to keep into account some features that differentiate experimental reactors from commercial reactors, such as the daily on/off cycle and long fuel lifetime; these effects could not be neglected to properly account for neutron poison accumulation. The model was used to evaluate the effects of 58 years of reactor operation and to predict several possible new configurations for the reactor core: the objective was to remove some of the fuel elements from the core and to obtain a substantial increase in the Core Excess value. The evaluation of fuel burnup and the reconfiguration results are presented in this paper.