Study of the WWER core behavior during a Rod Ejection Accidentby coupled three dimensionalneutron kinetics / thermal-hydraulic codes

This thesis analyzes the behavior of the nuclear fuel of a WWER-1000 reactor during a Rod Ejection Accident (REA). The aim of the work was to establish also a complete methodology for execute calculations in the future on high burnup fuels using the state-of-the-art of the nuclear codes. The tools used for the analysis were a 3D neutronics / thermal-hydraulics coupled codes and a 2D neutronics transport code for the cross section libraries generation. The study performed several calculations analyzing the effect of different cross section modelling and of different status of the plant on the evolution of the accident. Sensitivity analyses were also carried on modifying various physical parameters of the nuclear fuel. The results show that the Hot Zero Power transient, with the fuel at the End of Cycle and with the plant having only two pumps in operation, is the more dangerous situation for the REA. Massive ebullition of the channels and DNB on the clad surfaces were discovered. The Energy Release to the Fuel calculated is below the safety limits, but some concerns remain, especially thinking to a fuel with an increased burn-up