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Abstract-The paper describes solving the problem of direct silicon determination at low levels in uranium materials, caused by the spectral interferences of polyatomic ions and the high value of blank levels, using inductively coupled plasma mass spectrometry (ICP MS). To overcome the interference problem, two pri mary techniques have been applied: double focusing high resolution ICP MS and dynamic reaction cell (DRC) filled with highly reactive ammonia gas. All measurements were performed at high resolution (m/Δm = 4000) on an Element 2 mass spectrometer and pressurized mode of a dynamic reaction cell on an Elan DRC II mass spectrometer. The ways to reducing background levels are investigated. The effects of oper ating conditions, such as plasma parameters, DRC system original parameters, and uranium matrix influence on the analytical signals of silicon at m/z = 28 have been observed for different mass spectrometer types. The detection limits and the random error characteristics (relative standard deviation) of silicon determination in uranium materials were estimated.

The results of investigations of the distribution of fluorocarbons in two-phase solid phase-gas and liquidvapor systems based on uranium hexafluoride are presented. The systems are studied under equilibrium conditions, under which a constant distribution of fluorocarbons is established between the phases in a closed volume, and under nonequilibrium conditions created with stationary removal of the gas phase from the volume with liquid or solid uranium hexafluoride. The investigations were performed on specially prepared standard mixtures of uranium hexafluoride with fluorocarbon fraction from 10 -4 to 10 -1 %. The special features of the evaporation processes in the two-phase systems studied are shown, the distribution laws of the fluorocarbons between phases of the main substance are established, and the regimes of evaporation of the mixtures with the smallest disruptions in the uniformity of the fluorocarbon inflow with the flow of the evaporated uranium hexafluoride are determined.Fluorocarbons are compounds with the structural formula C n F m and molecular mass from 700 to 1000 amu, much larger than that of uranium hexafluoride. Two-phase systems of uranium hexafluoride with fluorocarbons are of interest on the one hand because the content of fluorocarbons in uranium hexafluoride can be determined by obtaining small representative samples from filled shipment containers and technological tanks. On the other hand there is the desire to ensure that impurities flow uniformly into the gas phase when the uranium hexafluoride evaporates from the containers and tanks in the process systems in uranium separation plants. These requirements predetermined the problems investigated in the present investigation.Two-phase systems were studied in standard mixture of uranium hexafluoride with fluorocarbons, prepared by the mass and volume methods. After they were prepared, the mixtures were certified by following analytical methods developed in the Central Plant Laboratory of the Urals Electrochemical Combine: thermal destruction of the fluorocarbons to tetrafluoromethane [1], mass-spectrometric method with chemosorption of uranium hexafluoride on sodium fluoride [2] and chromato-mass-spectrometric method [3].

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