Investigation of (n, 2n) reaction and fission rates in iron-shielded uranium samples bombarded by 14.9 MeV neutrons

The effect of the thickness of iron shielding on the (n, 2n) reaction rate in a fusion reactor (hybrid) blanket is investigated. The results are compared with the fission rate-dependence. Samples of natural uranium where irradiated with 14 MeV neutrons with iron slabs of various thickness between the neutron generator target and the samples. Both reactions are threshold reactions but the fact that the ²³⁸U (n, 2n) reaction threshold is at 6 MeV and that of fission is at 2 MeV makes the ratio between the two very much geometry-dependent. Two geometrical effects take place, the 1 r² and the build-up. While the build-up affects the (n, 2n) reaction rate, the fission rate is affected more by the 1 r² effect. The reason is that both elastic and inelastic scattering end up with neutrons with energy above fission threshold, while only elastic scattering brings high energy neutrons to the sample and cause (n, 2n) reaction. A comparison was made with calculated results where the geometrical effects do not exist.