Reactivity Monitoring in ADS with Neutron Fluctuation Analysis

This paper gives an overview of the statistical methods with which the reactivity in a source-driven subcritical system can be determined as well as giving several new results. These methods are based on measurement of the variance-to-mean (Feynman-alpha) or the correlations (Rossi-alpha) of the detector counts. In the traditional method a simple radioactive source is used which emits one neutron at a time, and whose source intensity is constant. In a future ADS the source will be either a spallation source, producing several correlated source neutrons for each incoming proton, or a neutron generator with a time-varying source intensity due to pulsed mode of operation. We give here the Feynman-alpha formula for both traditional and spallation sources as well as the Feynman-alpha formula for pulsed neutron generator sources. The formal solution for a randomly pulsed system is also given but is not evaluated. The results concerning the pulsed system are new. (authors)