Examination of control rod ejection in WWER-440 type reactors at different circumstances using the code DYN3D

For nuclear reactors it is very important to examine the reactivity initiated transients caused by the ejection of a control rod. The event is found to be dependent on different thermal and neutronic parameters. In this paper the emphasis is laid on the effect of the power level at which the transient began and on the effect of the heat transfer coefficient measured in the gap between the fuel and the cladding. The most significant transients can be established by the ejection of the most effective control rod. So the first step is to determine the position of this rod. It was done by steady state calculations A calculation was carried out with all the rods inserted to the half level of the core, criticality was reached by adjusting the power level. Seven other calculations were made for each control rod at withdrawn position while the other six rods were inserted to the half plane of the core. From the results the most effective control rod could be determined.(authors)