Analysis of the VVER-440 fuel behavior in four-year cycle

The Dukovany NPP has implemented new fuel system, designed for four-year cycle,at the Unit-1 and Unit-2. This new fuel will be implemented at the Unit-4 and 3 during the years 1999 and 2000. This modernization of the Dukovany fuel cycle brings a significant improvement in the plant economy and reduction in the spent fuel storage capacity demand. Planned burn-up well-over 50 MWd/kgU (rod maximum) clearly belongs into high burn-up region, where thr specific phenomena are to be taken into account. Although the complete fuel system design analysis has been performed by the fuel vendor during the licensing process, it has been decided to check some design limits independently, using software tools available to Dukovany NPP and NRI Rez. Several fuel rods of new design had been selected from the Unit-1 and 2. Their detailed power history was reconstructed by MOBYDICK core simulator. Then, three independent codes RODQ2D, PIN and TRANSURANUS were employed for the fuel rods thermo-mechanical behavior modelling. Results of these calculations are used for additional proof of the fuel design limits fulfillment and the fuel codes cross-checking. Keeping of the fuel internal temperature and pressure limits was clearly demonstrated with a high level of reliability. Predictions of the all three codes used in fuel centre temperatures matched very well in most cases. Differences in the fuel rod internal pressures could be explained by different mechanical solution applied. (Authors)