Implementation of Uncertainty Analysis for Evaluation of Nuclear Reactors VVER-1000 Fuel Safety Margins during Normal Operation by FEMAXI-6 Computer Code Calculations

The mathematical modelling and evaluation of the nuclear fuel while burning in nuclear reactors is of significant importance if one wants to predict the reactor core behavior and to get assured that the necessary criteria for its safe operation are met. These criteria reflect the maximum fuel temperature, cladding temperature, coolant heating, etc. The computer code that we chose for performance of such analysis is FEMAXI-6 (Japan Atomic Energy Agency). We prepared input-data deck for fuel rods of the type used in Kozloduy NPP and performed predictive calculations for the evaluation of the safety margins. However, for the proper evaluation it is important to know the initial geometrical and material data. These data are available only through their nominal values and uncertainty ranges, given by the fuel vendor as the fuel itself is sealed in tubes, made of zirconium alloy. Therefore, the lack of knowledge about the exact values of these important characteristics requires the implementation of an uncertainty analysis in order to evaluate properly the safety criteria.
As a rule, the uncertainty analyses are applied for evaluation of margins in safety analyses, performed for transients and accidents. There are several known methods for performance of such analyses. One of them is the method, developed by GRS (Gesellschaft für Anlagen- und Reaktorsicherheit) and implemented in SUSA (Software for Uncertainty and Sensitivity Analyses). We tested successfully the Wolfram Mathematica’s capability to implement the same approach by analysis of the two, most heavily loaded fuel rods in the reactor core (containing correspondingly UO2 and UO2  Gd2O3 fuel) during normal reactor operation. Finally, the following conclusion about this fuel is made: its operation in thermo-mechanical and thermo-hydraulic aspects is safe. The safety criterion: critical heat flux ratio (CHFR) is met.