Neutron leakage through a reactor shield composed primarily of iron is discussed. This is of interest whenever the hydrogen content of a shield is reduced either by design requirements or thermal deterioration. Work done at several sites on individual aspects of the problem is combined to present an over-all description of the neutron streaming. In general there are two different phenomena involved, each determined by the geometry. In the case of a long thin streaming path, such as a structural member penetrating the shield, the leakage consists of neutrons which have suffered no collisions. These neutrons will have energies corresponding to energies at which the iron total cross section is small. Iron has several antiresonances in the interval 25 to 100 kev, with the largest dip apparently at 25 kev, so most of the neutron leakage will be at these energies. The other case involves the attenuation of neutrons by large slabs of iron with little or no hydrogen (or other good moderator) present. The 25 kev neutrons are still present, but they are augmented by a large number of neutrons of energy between thermal and 1 Mev. These neutrons may have collided elastically many times but with only a small energy loss each time. Above 1 Mev, inelastic scattering suppresses the leakage, and below a few volts, absorption removes the neutrons.
