Calculation and experimental substantiation of the WWER fuel operability at high fuel burnups

In the design three-year fuel cycles of WWER-440 and two-year fuel cycles of WWER-1000 the fuel assemblies with steel spacer grids (and guiding channels in WWER-1000 FA) were used. The average burnup of spent fuel is ∼ 29 MW*day/kg. The experience of fuel operation, data of post-reactor investigations of spent fuel, results of special experiments with fresh and spent fuel, development of thermal-mechanical codes and codes of accident analysis, made it possible to substantiate in complex the increase in WWER fuel burnup, eliminate the excess conservatism in designing and operation and begin to develop new fuels cycles with advanced fuel assemblies. The paper presents the main principles, criteria and results of substantiation of WWER fuel rod availability at high burnups under normal operation conditions and of the analysis of fuel behavior in design accidents. (Authors)