Operation of the Kozloduj NPP units using the power and temperature reactivity effects at the end of the burnup cycle

The excess reactivity released by the power and coolant temperature reactivity effects has been used to prolong the burnup cycle and to improve the fuel burnup of WWER-440 reactors. The effect of this mode of operation was 206 additional effective days, and 2x109 KWh electric energy till the end of 1983. Excess reactivity is released in two stages: a) The turbine valves are fully open. Pressuire in the main stream collector is lowered to about 40 ata, resulting in a coolant temperatire decrease at the core inlet. The released reactivity is compensated by control rod insertion. Fuel burnup is compensated by control rods withdrawn so that pressure in the steam collector is kept constant. The reactor power output is nominal. 2) After the regulating rod bank is fully withdrawn, secondary pressure is kept constant by partly closing the turbine valve. Reactor thermal power is decreasing. Analysis of the results and the constraints shows that the effect is significant and can be optimized by taking into account the amount of planned maintenance works and the change in the energy cost