In diffuse reflectance spectroscopy the Kubelka-Munk equations have been used extensively. These equations provide simple solutions to the inverse problem of obtaining information on the scattering and absorption cross sections from reflected light. Proof is provided that the basic Kubelka-Munk equation [Formula: see text] should be replaced by the equation [Formula: see text] and that the Kubelka-Munk function [Formula: see text] should be replaced by the function [Formula: see text] Here r( x) is the reflectance; s is the scattering cross section (cm−1); a = ( k + s)/ s, where k is the absorption cross section (cm−1); and R∞ is the reflection coefficient of an infinitely thick sample. We note, however, that because of a redefinition of a carried out by Kubelka and Munk in the process of their calculations, the scattering cross section s calculated from their expression [Formula: see text] is correct. But the Kubelka-Munk theory still overestimates k by a factor of two.

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Graphite functions as a structural material and as a barrier to fission product release in HTGR/VHTR designs, and elucidation of transport parameters for fission products in reactor-grade graphite is thus required for reactor source terms calculations. We measured iodine diffusion in spheres of IG-110 graphite using a release method based on Fickain diffusion kinetics. Two sources of iodine were loaded into the graphite spheres; molecular iodine (I 2) and cesium iodide (CsI).Measurements of the diffusion coefficient were made over a temperature range of 873-1293 K. We have obtained the following Arrhenius expressions for iodine diffusion: ,

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The motion of a sphere in a rarefied gas is considered with the use of the BGK model and a boundary condition of diffuse reflection. The role of temperature variation in the Knudsen layer is studied; results show that velocity and pressure in the layer and the drag on the sphere are not sensitive to this variation.