Existence of short and long range relaxation lengths in heterogeneous media

Experimental data related to boiling water reactors show that the phase of the cross power spectra density between the detector response at two points in the system is described by a pure delay process above certain values of the frequency. For this to occur the adjoint flux (detector field of view) must be sharply peaked around each one of the detectors, which in turn implies the existence of a short range relaxation length. It is shown that the existence of this type of relaxation length is a direct consequence of the physical properties of heterogeneous systems and not a consequence of the number of groups used to describe the neutron field in the equivalent homogeneous system as it has been done traditionally. To show this point a one-group neutron diffusion model which accounts explicitly for the heterogeneities of the system is presented