Effects of resonance absorption in fusion device heterogeneous media

For high energy neutrons, absorption cross-sections are typically low and vary slowly. The neutron transport problem is then simple to solve. However, in the resonance region, in particular in heavy elements such as tungsten, the problem is more complex. The cross-sections tend to have a great deal of structure; their variations are substantial, sudden and irregular. The neutron flux then has two components: a smooth slowing-down function and a fine structure that presents sharp dips at the resonance edges. These dips lead to a substantially reduced absorption reaction rate. This effect is called self-shielding and its magnitude is extremely dependent upon the geometry (heterogeneity), the isotopes involved and the surrounding materials (moderator, coolant).