Analysis of rod-drop measurements at a VVER-440 reactor

During start-up of the fifth unit of Nord nuclear power plant (NPP) several different rod-drop measurements were performed. The reactivity values obtained experimentally by a reactimeter on the basis of inverse point-kinetics differed, in some cases considerably, from values obtained by stationary keff-calculations. In order to analyse these differences, calculations with a three-dimensional kinetic code were performed, which clearly showed that the local flux changes caused by dropped rods are responsible for the differences between calculated and measured rod worths. Furthermore, it is possible to use these calculations for the evaluation of correction coefficients. These coefficients, applied to the measured reactivity values, lead to corrected reactivity values, which are in fairly good agreement with calculated values. In our opinion this combination of calculation and measurement seems to be a suitable approach to the determination of rod worth under conditions typical for a NPP with VVER-440-type reactors.