Terry C Wallace

Vacuum evaporation of KCl-NaCl salts : Part II. Vaporization-rate model and experimental results

A model based on the Hertz-Langmuir relation is used to describe how evaporation rates of the binary KCl-NaCl system change with time. The effective evaporation coefficient (α), which is a ratio of the actual evaporation rate to the theoretical maximum, was obtained for the KCl-NaCl system using this model. In the temperature range of 640 °C to 760 °C, the effective evaporation coefficient ranges from ∼0.4 to 0.1 for evaporation experiments conducted at 0.13 Pa. At temperatures below the melting point, the lower evaporation coefficients are suggested to result from the more complex path that a molecule needs to follow before escaping to the gas phase. At the higher liquid temperatures, the decreasing evaporation coefficients result from a combination of the increasing vaporflow resistances and the heat-transfer effects at the evaporation surface and the condensate layer. The microanalysis of the condensate verified that composition of the condensate changes with time, consistent with the model calculation. The microstructural examination revealed that the vaporate may have condensed as a single solution phase, which upon cooling forms fine lamellar structures of the equilibrium KCl and NaCl phases. In conclusion, the optimum design of the evaporation process and equipment must take the mass and heat transfer factors and equipment materials issues into consideration.

Review of Rover fuel element protective coating development at Los Alamos

The Los Alamos Scientific Laboratory (LASL) entered the nuclear propulsion field in 1955 and began work on all aspects of a nuclear propulsion program with a target exhaust temperature of ∼2750 K (Kirk 1990). A very extensive chemical vapor deposition coating technology for preventing catastrophic corrosion of reactor core components by the high temperature, high pressure hydrogen propellant gas was developed. Over the 17‐year term of the program, more than 50,000 fuel elements were coated and evaluated. Advances in performance were achieved only through closely coupled interaction between the developing fuel element fabrication and protective coating technologies. The endurance of fuel elements in high temperature, high pressure hydrogen environment increased from several minutes at 2000 K exit gas temperature to 2 hours at 2440 K exit gas temperature in a reactor test and 10 hours at 2350 K exit gas temperature in a Hot Gas test. The purpose of this paper is to highlight the rationale for selection of c...

Experimental and numerical optimization of the U-Zr-C ternary phase diagram

The U-Zr-C ternary-phase diagram is optimized using melting point and vapor pressure data recently measured at Los Alamos National Laboratory and by graphical and numerical curve fitting of self-consistent thermodynamic and phase diagram data reported in the literature. The system is described from 2473 K to 3693 K using isothermal ternary phase diagrams. The solidus and liquidus curves of the U{sub x}Zr{sub 1-x}C{sub y} solid solution were estimated from combined thermodynamic and adjusted phase diagram data. From the solidus and liquidus curves for the solid solution, the reversible heat transferred from the solid to the liquid phase for UC and ZrC were calculated to be 150 and 324 kJ/mol, respectively; and the maximum in the excess free energy for mixing was determined to be on the order of 1100 J/mol at 56% ZrC.

Vaporization Behavior of Non-Stoichiometric Refractory Carbide Materials and Direct Observations of the Vapor Phase Using Laser Diagnostics

Transition metal and actinide carbides, such as ZrC or NbC and UC or ThC, exhibit a wide range of stoichiometry. As a consequence, these materials vaporize incongruently. At relatively long times steady state vaporization can be achieved where the relative concentrations of atomic species on the solid surface equals that in the gas phase. The surface composition under these steady state conditions is termed the congruently vaporizing composition, or CVC. Modeling the vaporization or corrosion behavior of this dynamic process is complex and requires an understanding of how the surface composition changes with time and a knowledge of the CVC, which is both temperature and atmosphere dependent. This paper describes the vaporization and corrosion behavior of non-stoichiometric refractory carbide materials and, as an example, will describe a thermokinetic model that characterizes the vaporization behavior of the complex carbide UxZrl-xCy in hydrogen at 2500 to 3200 K. This model demonstrates that the steady state corrosion of UxZrl-xCy is rate limited by the gaseous transport of Zr where the partial pressure of Zr is determined by the CVC.