A Method for Determining Reactivity-Time Function of Safety Rods

For accidental analysis of HERBE fast-thermal core, an accurate reactivity-time function for reactor safety rods is necessary. The HERBE core was designed with four safety rods: two of them are the actual safety rods, and the other two are additional safety rods which include holds during motion. The reactivity-time function is determined in two steps: (1) safety rods reactivity-position function is measured using inverse method; (2) rod drop position-time function is measured using a new method. In previously proposed method, it was determined by measurement of rod drop times and assuming constant acceleration during any particular interval of a rod motion. The complex dependence of the reactivity-time function for the HERBE safety rods during reactor shutdown is determined by combining both previously obtained reactivity worth data and measurements of safety rods trajectory. Integral reactivity-time function of the safety rods, including rods interference reactivity effects, is shown. In this new method an improvement for accurate safety rod position measurement, compared to previously proposed method, is obtained. At the same time, the assumption of the constant acceleration of the safety rods in the motion intervals is validated