Thermal Neutron Diffusion and Rethermalization in Hydrogenous Materials

Using the thermal columns of a reactor as neutron sources two experiments have been performed, the aim of the first being to test the ability of models for the scattering of neutrons by hydrogen bound in water to predict thermal neutron diffusion lengths in solutions of 1/v and non-1/v absorbers. In the second experiment rethermalization in Santowax-R (terphenyl) near a temperature discontinuity has been studied. Diffusion length measurements in both pure water and boric acid solutions have been carried out at 22.3 and 65°C, and in solutions of cadmium and gadolinium at the lower temperature only. In each case measurements were made over a wide range of concentrations. All measured inverse diffusion lengths lie within the (ΣT)min limit of Corngold with the exception of those from the two most strongly absorbing cadmium solutions. Representation of each set of results as a power series in absorber concentration has enabled the coefficients up to the spectral distortion term to be determined and it is shown that considerable diffusion heating occurs in gadolinium solutions in contrast to the slight diffusion cooling observed in cadmium solutions. In a theoretical analysis four scattering models for bound hydrogen were used. Diffusion lengths were calculated by solving the transport equation for the lowest space eigenvalue in the BL approximation with L = 2, using the method described by Honeck. The differentially heated experiment combined cold water and hot Santowax-R in slab geometry. Fission rates of 235U and 239Pu were measured with fission chamber detectors as a function of position across the tanks containing the liquids. Two diameters of chamber, 0.6 cm and 0.3 cm, were used to identify finite detector size effects. The general features of the fission rate distributions in the water and Santowax are discussed. In view of current interest concerning the existence of a rethermalization length when the principal scattering is by bound protons, the form of the transient flux in the hot tank is examined in detail. For comparison with the experimental results the fission rates of 23U and 239Pu have been calculated using the Winfrith DSN transport theory code. Scattering by hydrogen in water has been calculated from the Haywood model and in Santowax from a model fitted to the phased chopper velocity selector measurements of Brugger. (author)