A method to reduce space-time effects in the measurement of reactivities by the rod-drop technique

Abstract

A method to reduce the dependence on detector location is proposed for the measurement of reactivities of control rods with the rod-drop technique. Values are obtained from data adjustment for the delayed transient, using a function obtained by expansion of the neutronic density into the initial set of kinetic eigenfunctions. The principal hypotheses of the method here proposed, which were verified by experimentations and simulations, are: (1) the contribution of each mode “ q” is the same for every delayed neutron precursor group, (2) the contribution of the higher modes in the delayed population is small compared to that of the fundamental harmonic mode, and (3) for a real control rod insertion, the delayed transient in the adjustment region is similar to an instantaneous one having a delay time. In order to have reasonable experimental errors it is necessary to maintain the fluctuations of the signal within a small value, thus a model was developed to relate the fluctuations with parameters that can be modified in the measuring process. The method was applied in the RA-6 reactor to measure the worth of control rods. It was found that the method can be applied up to 15 dollars and that it does not depend on the relative location of detectors and control rods, as the method based on the point reactor model does.