Theory of neutron noise induced by source fluctuations in accelerator-driven subcritical reactors

The space-dependent neutron noise in a source-driven subcritical system is investigated. The noise is induced by the fluctuations of the source (inten sity or position). Besides of investigating the linearly exact solutions in a 1-D system, the validity of the point kinetic and adiabatic approximatio ns is also investigated. These reactor physical approximations have actuall y not been defined in a consistent manner before for non-critical systems, thus the present study also supplies the theory of such approximations in s ource-driven subcritical systems. It is found that the applicability of the se approximations depends, in addition to frequency of the perturbation and system size, also on the space-time structure of the perturbation. Dependi ng on whether or not the source can be factorized what regards dependence o n space and time (frequency), the domain of applicability of the reactor ph ysics approximations can be either wider or more restricted than in the tra ditional case. A general trend is however, that for a given perturbation ty pe the applicability of reactor kinetic approximations increases with incre asing system subcriticality. This is shown through both developing the gene ral theory and also through quantitative evaluation of concrete cases. (C) 1999 Elsevier Science Ltd. All rights reserved.