Reactor noise in accelerator driven systems – II

Reactor Noise in accelerator driven systems (ADS) is expected to show differences from critical or radioactive source driven sub-critical systems due to the periodically pulsed source and its non-Poisson character. A theory of ADS Reactor Noise, incorporating these features was worked out by us and has been published earlier. The present paper is a continuation of the earlier work and constitutes an extension and generalization of the theory. We present theoretical arguments as well as empirical evidence to elucidate the non-Poisson character of the ADS neutron source. We solve the delta-function pulse train problem by treating the source as an exponentially correlated process as against the uncorrelated non-Poisson process considered earlier. In yet another direction, the theory is extended to treat pulses having finite width by describing the source as a doubly stochastic Poisson point process. It is shown that the results of the extended theory reduce to those of our earlier formulation (or to other published results) under appropriate conditions. Evaluation of various descriptors gets simplified by going to the Laplace domain but results in infinite series. Calculations in the time domain are more difficult but yield closed form expressions. We describe both approaches in the course of this paper. We use the Rossi alpha and Feynman alpha formulae, which are evaluated for all the cases considered, for illustrating our approach. In addition to these, we also obtain the auto correlation function, cross-correlation function and the corresponding power spectral in one of the cases. A brief discussion on the various noise techniques vis-a-vis other methods for sub-critical monitoring is included.