FAST-NEUTRON FLUX DISTRIBUTION IN DUCTED LIGHT AND HEAVY CONCRETE SHIELDS

This paper reports on a study of the effect of cylindrical air-filled ducts of different lengths and diameters on the distribution of fast neutrons in ordinary concrete (rho = 2.3 g/cm3) and ilmenite concrete (rho = 4.6 g/cm3). The source of neutrons was a collimated beam of reactor neutrons emitted from one of the horizontal channels of the ET-RR-1 reactor. The measurements were performed using phosphorus activation detectors. The data show that the presence of air-filled ducts tends to increase the neutron flux in the medium surrounding the duct, as compared with the corresponding values for solid concrete. A marked peak was observed at about 10 cm after the end of the duct. The value of the flux at these peaks is dependent on the duct length and diameter. Also, it was shown that the neutron flux at any point along the duct axis passing through ilmenite concrete is much lower than the corresponding value in ordinary concrete. In addition, solid ilmenite concrete is much better for fast-neutron attenuation than solid ordinary concrete, especially for large thicknesses. However, for small thicknesses (< 30 cm) ordinary concrete is the best. Semiempirical formulae have been derived to calculate the fast-neutron fluxes at different thicknesses along the axis of air-filled ducts of different diameters inserted in ordinary and ilmenite concretes and to calculate the fast-neutron flux distribution in solid concrete media. The derived formulae give results which are in good agreement with the measured values.