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The vertical ammonia concentration distributions determined by the retained gas sampler (RGS) apparatus were modeled for double-shell tanks (DSTs) AW-101, AN-103, AN-104, and AN-105 and single-shell tanks (SSTs) A-101, S-106, and U-103.'") One-dimensional models of the vertical transport of ammonia in the tanks were used for the modeling. Transport in the nonconvective settled solids and floating solids layers is assumed to occur primarily via some type of diffusion process, while transport in the convective liquid layers is incorporated into the model via mass transfer coefficients based on empirical correlations. Mass transfer between the tup of the waste and the tank headspace and the effects of ventilation of the headspace are also included in the models. The resulting models contain a large number of parameters, but many of them can be determined from known properties of the waste configuration or can be estimated within reasonable bounds from data on the waste samples themselves. The models are used to extract effective diffusion coefficients for transport in the nonconvective layers based on the measured values of ammonia from the RGS apparatus.The modeling indicates that the higher concentrations of ammonia seen in bubbles trapped inside the waste relative to the ammonia concentrations in the tank headspace can be explained by a combination of slow transport of ammonia via diffusion in the nonconvective layers and ventilation of the tank headspace by either passive or active means. Slow transport by diffusion causes a higher concentration of ammonia to build up deep within the waste until the concentration gradients between the interior and top of the waste are sufficient to allow ammonia to escape at the same rate at which it is being generated in the waste.The results for the DSTs present a fairly consistent picture of ammonia transport. The DSTs all consist of a bottom layer of nonconvective settled solids over which lies a convective layer. A layer of floating solids lies on top of the convective liquid layer and acts as a transport barrier to the release of ammonia from the underlying liquid and settled solids layers. Values for the effective diffusion coefficient in the floating solids layer are 6-20 times that of ammonia in water, while the values in the settled solids nonconvective layer are 2 times that of ammonia in water. The only exception is Tank AN-105, where the value of the effective diffusion coefficient in the settled solids layer is 16 times that of ammonia in water. Based on the Stokes-Einstein relation, the value for the molecular diffusion coefficient for ammonia in a typical liquid waste is expected to be 20-30 times lower than the diffusion coefficient of ammonia in water, so the effective diffusion coefficients for ammonia in the DSTs appear to be high. This suggests other transport mechanisms may be present in the nonconvective layers, thus enhancing the rate of ammonia transport. Possible mechanisms are slow creeping flows within the nonconvective layers due to the presence of ...

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A Y‐Ba‐Cu‐O superconductor was irradiated at 360 K in the NRX reactor, with a well‐thermalized flux of 1.82 × 1017 n/(m2·s) (E < 1 MeV). Superconducting characteristics at 90 K disappeared by a fluence of 6.6 × 1021 n/m2 (10 h) but were recovered after a postirradiation heat treatment at 760 K in flowing oxygen. The results suggest that suitable shielding will be required for any application which exposes the superconductor to neutron irradiation.

is performed at 360 K with a well‐thermalized flux of 1.82·1017 n/(m2·s).