• Energy Research
• Journal of Nuclear Energy close

Abstract Using a pulsed source of 14 MeV neutrons, the slowing down time to an energy of 0·555 eV was measured, by a cadmium transmission technique, in 6 blocks of graphite in the buckling range 2·990–22·186 x 10 −3 cm −2 . The results indicated that diffusion time and diffusion cooling effects were small. A value of 37 ± 2 μsec was obtained for the slowing down time in an infinite block of graphite of density 1685 g cm −3 . The result is compared with theory and indicates the marginal superiority of the crystalline model for describing slowing down to these energies in graphite.

Abstract The singular eigenfunction expansion method has been used to obtain the solution for the one-speed disadvantage factor in slab cells of fuel and moderator which may scatter neutrons in a highly an isotropic manner. New two-media cross-product integral relations, together with previously known one-medium orthogonality relations, have been utilized to obtain the solution. Sample calculations on slab cells of natural uranium and water show that, for cases in which the anisotropy of the scattering is quadratic or more, the disadvantage factor differs from the isotropic result by 7–27 per cent depending upon the order of the anisotropy, the effective mass of the scatterer, and the slab cell width. In all calculations, scattering in the fuel has been assumed to be isotropic.

Abstract The behaviour has been studied of the Universities Research Reactor (URR) at Risley, Lancs., when subjected to pulses of fast neutrons. The URR is an enriched-uranium, water-cooled and moderated, graphite-reflected system, the core consisting of two coupled fuel slabs spaced 1 ft apart and containing approximately 3300 g of 235 U. The work is reported in two parts. In the work reported in this first section the pulsed source was located symmetrically relative to the core slabs (so that the coupled-core nature of the reactor was suppressed) and the results analysed using the more conventional point-reactor models. A reflector decay mode, as predicted by a three-region Cohn model was not observed. The delayed critical prompt decay constant of the URR was found to be 33·5 ± 0·6 sec −1 . The experiments showed that, for sub-critical core loadings in a highly reflected system such as the URR, the data is analysed most readily and reliably by the Simmons-King method supplemented by calculated changes in the generation time. For this reactor, the Gozani method corrected for kinetic distortion was found to be the most suitable of the modified pulse techniques. METHUSELAH II—GOG multigroup-diffusion poison-removal calculations gave prompt and delayed flux distributions which accounted well for the observed spatial dependence of the modified-pulse-technique reactivities. The experimental and calculated decay constants, although substantially in agreement at critical, diverged by as much as 15–25 per cent for the subcritical core loadings. Part 11 of this work will examine the coupled, two-slab, nature of the URR.

Abstract The thermalization of a burst of fast neutrons after injection into a graphite stack of dimensions 101 × 101 × 81 cm 3 has been studied by the reaction rates method. The Cd/Hg reaction ratio was measured in the centre and near one face of the stack over the period 90–1290 μus after neutron injection. Characteristic prompt gamma rays from both indicator materials were detected simultaneously using a small Ge(Li) detector placed within the stack. The results compare favourably with previous theoretical determinations of the time-dependent Cd/Hg ratio and with ratios calculated from time-dependent differential neutron spectra.

Abstract The cross section of the 235 U(n f) reaction has been measured with a high-energy resolution for neutrons between 6 eV and 3 keV. A fission fragment and a fission neutron detector were used simultaneously. Resonance parameters for 78 levels between 6 and 150 eV are given. The statistical distributions of Γ f , Γ γ and α have been studied.