Measurement of sub-critical reactivity in a heavy water reactor by neutron noise methods using a time stamping data acquisition system

Abstract Experiments were performed to measure the sub critical reactivity in a zero power heavy water moderated reactor by neutron noise methods. The measurements were carried out using a time stamping data acquisition system developed for this purpose. This system has the advantage that it gives the complete time history of the detected neutron pulse train and a single set of data so acquired can be used for analysis by any of the noise techniques. The data obtained in the course of the experiments were analyzed using two commonly used neutron noise techniques namely the Feynman alpha and the auto correlation function methods. Noise analysis in heavy water reactors is more cumbersome than in light water reactors due to an overlap of the prompt neutron and delayed neutron time scales and consequently an unavoidable contribution from delayed neutrons and photo neutrons. Measurements were done at various levels of sub criticality by varying the moderator level height. The measured sub-critical reactivities were found to be in good agreement with theoretical estimates.