High temperature vapor pressure measurements have been made on pure plutonium metal by the Knudsen effusion technique. The reported experimental results extend into the transition region between molecular and viscous or hydrodynamic flow. Under the conditions used, linearity was observed up to temperatures in excess of 2200 K where pressures approaching 100 Pa were measured. The results over the temperature range 1724ndash;2219 K yield log10PPu(Pa) = (9.735plusmn;0.105) minus;17066plusmn;208/Tand the enthalpy and entropy of vaporization and the standard deviations therein are Dgr;H0v(Pu,1975 K) =326.78plusmn;3.97 kJ molminus;1, Dgr;S0v(Pu,1975 K) =90.54plusmn;2.01 Jthinsp;Kminus;1molminus;1. Based on the most recently available free energy functions for plutonium liquid and gas, the values of the standard enthalpy of vaporization calculated via secondhyphen; and thirdhyphen;law methods are Dgr;H0v(II, Pu,298 K) =344.14plusmn;3.97 kJ molminus;1, Dgr;H0v(III, Pu,298 K) =341.67plusmn;1.26 kJ molminus;1. Single crystal tungsten containers were used to hold the charge of plutonium and proved to be very satisfactory in alleviating problems of liquid metal creep and liquid/cell interactions normally encountered with actinides held at high temperatures for long periods.
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