Space- and time-dependent neutron thermalization in a 230×190×190cm³ graphite pile has been studied by considering the reaction ratios for Gd/¹⁰B, Sm/¹⁰B, Cd/10B, Gd/Sm, Gd/Cd and Sm/Cd as spectral indicators. The reaction ratios were measured at r =10, 30, 50 and 70 cm from the fast neutron source placed at the center of the pile. The measurements were repeated at 2. 175 μsec intervals in the time range up to 600 μsec after the fast neutron burst. Theoretical time-dependent reaction ratios were obtained from the calculated space- and time-dependent thermal neutron energy spectra by solving numerically the neutron diffusion equation, using scattering kernels based on various phonon models for graphite. From comparisons of the measured and calculated time-dependent reaction ratios, the adequacy of the various phonon models for graphite is discussed. Also, proof is given of space-dependence shown by the time-dependent thermal neutron spectra ; and the relationship between the space-dependence of the time-dependent neutron spectra and that of the peaking times of the reaction ratios is made clear.

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Five benchmark problems were selected as standards for the evaluation of calculations on neutron penetration. For three of these problems there were available reliable experimental results obtained in absolute intensities of the neutron angular spectrum in graphite and in water and of the neutron reaction rate in a two-dimensional water shield. The two remaining problems were based on PALLAS transport calculations in graphite and in water in plane geometry. For the first three problems, comparisons were made between the results of PALLAS calculations using ENDF/B-I cross sections and those obtained by experiment. It is concluded that the PALLAS calculations result in fairly good agreement with experiment, except at certain spatial meshes in two-dimensional geometry, due to ray effect. Comparisons are also made of the angular flux spectrum in graphite between calculations using ENDF/B-I and B-III data, as well of the scalar flux spectrum in water between those by PALLAS and by ANISN codes.

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An example of optimization problem for the design of squared-off cascades to be in-corporated in uranium enrichment plants utilizing gaseous diffusion is solved from an economic point of view, whereby minimization is obtained of the plant annual expenditure expressed in terms of simple cost functions.
The equi-step squared-off cascade is studied. The salient features of such a cascade are illustrated by a comparison of the cascade performance between one aimed at minimum plant annual expenditure and one aimed at minimum total flow rate.
For the first-mentioned form of optimized cascade, the influence of the stage separation factor on plant economics is studied. The discussion covers the determination of optimum waste assay, and the exponential relationship between the enrichment coefficient and plant economics. The economic considerations affecting the stage separation factor are summarized in sensitivity curves relating this factor to changes in various economic indicators.

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Experiments have been made to determine the behavior of sodium aerosol under various atmospheric conditions, with the view to establishing an air monitoring technique to be used in sodium-cooled fast reactors.
Changes in both size and form of sodium aerosol particles released into the air were studied. At levels of humidity below 40%, the particles were observed to be in solid state, constituted of sodium oxides and sodium hydroxide. When sodium aerosol particles are exposed for more than 1 min in an atmosphere containing more than 60% humidity, they are mostly converted into droplets of sodium hydroxide. The size of the droplets increases with humidity, the diam-eter becoming at 95% humidity about 1. 4 times that at 40% (2. 7 times in volume). The final state of sodium aerosol suspended in air with a humidity exceeding 90% is in the form of droplets of sodium carbonate. When the CO₂ concentration in the air is as high as 1.0v/o the droplets solidify even at a humidity of 70%. The droplets can be made to solidify into grains by dehumidification through air heating. The size of the solid particles thus produced is nearly the same as that of the solid particles observed originally at low humidity, regardless of the level of humidity before heating.

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