Calculation study of working conditions of DK-3 diagnostic rod cluster in WWER-440 reactor

The problem of stationary regime thermal-hydraulic reliability of DK-3 diagnostic rod cluster installed in WWER-440 reactor core is considered. Thermal-hydraulic characteristics of the rod cluster obtained in experiments with/without coolant boiling in DK-3 are presented. During the rod cluster operation in nominal regime within the entire period of the first fuel load campaign it was reliably cooled with no boiling if the coolant flow rate through the cluster was not less than 20 kg/s. In experimental regimes the coolant boiling within the cluster has been achieved. At the coolant flow rate of 10 kg/s the boiling took place at 1/3 of the rod height from the core top and the relative coolant enthalpy was 0.05-0.07. The maximum fuel and cladding temperature during the considered regimes did not exceed 1350 C and 335 C respectively