The Study of Thermal Neutrons in Shields Containing Ducts by Source-Separation Techniques

Abstract

The behavior of thermal neutrons in a water shield containing ducts was studied experimentally and compared with theoretical models. In addition to measuring the total flux, a source separation technique was used to isolate the various flux components, which are: 1) direct flux, uncollided neutrons which enter the duct mouth2) scattered flux, collided neutrons which enter the duct mouth3) penetration flux, collided neutrons which originally enter the shield. Duct diameters from 1 to 4 in. were considered. The shield absorption properties were altered by dissolving various amounts of boric acid in the water. The duct cross-sectional shape was changed by partially flooding the interior of the duct.The experimental results indicated that the direct flux is inversely proportional to distance squared. For sufficiently long ducts, the direct flux is nearly the total flux. For shorter ducts, either the scattered flux or penetration flux may produce the largest contribution to the total flux. Each of these components peak near the duct mouth and then attenuate more rapidly than the direct flux.Successful calculational models were developed for each of the flux components. These permitted determination of the total flux to within a factor of 1.3 at distances greater than two or three duct diameters from the mouth.