Effects of control rod movement parameters on the simulation of VVER-1200 rod ejection accidents

The article examines the effects of duration and movement mode of the control rod of the VVER-1200 control and protection system during its fast ejection from the core on the results of accident simulation. Two modes of movement are considered including uniform and equi-accelerated modes, in which the duration of the bottom-stop ejection varies from 0.01 to 5 s. The ejection of the control rod is simulated for reactor states of a full and minimum-controlled power, as well as with and without leakage for the first six 18-month VVER-1200 fuel campaigns. Calculations were carried out using the ATHLET/BIPR-VVER code (version 1.0). The performed studies demonstrated insignificant sensitivity of extreme fuel temperatures and departure-from-nucleate-boiling margin to ejection parameters. The approximated uniform movement of the control rod during 0.1 s of ejection is sufficiently conservative for use in the deterministic safety analysis of VVER nuclear power plants.