Neutron-pulse propagation in multiplying assemblies

Axial neutron-pulse propagation experiments were conducted in cylindrical cores of the Cornell University Zero Power Reactor (ZPR). Energy-dependent neutron diffusion theory is found to provide a good prediction of the kinetic behavior of the assemblies. At short times the reactor response is that of an infinitely long reactor, and at long times exponential decay of Helmholtz spatial modes is observed. A space-independent pulse propagation velocity is not observed in most of the assemblies. Such a result is obtained only in infinitely long assemblies, and in most finite-length cores end-effect contamination cannot be neglected.In the Laplace transform domain the neutron density wave dispersion relations are obtained when the transform variable ξ is imaginary in the cores which would be prompt-subcritical if they were infinitely long. When ξ is real, the inverse attenuation length which would be measured in a static exponential experiment in an assembly uniformly poisoned by an absorber of strength ξ/υ is obtained. The agreement between the measured parameters and the predictions of diffusion theory improves as the neutron multiplication of the assembly decreases due to decreased end-effect contamination of the infinitely long assembly response. The effective multiplication of an assembly is seen to decrease due to spectral hardening as ξ increases when Re(ξ) is non-negative. This effect is reflected in the variation of the dispersion parameters with ξ.