The thermodynamics of vaporization of neptunium and plutonium

Abstract

The vapor pressure of liquid neptunium was determined over the temperature range 1540 to 2140 K by a combination of mass-effusion and mass-spectrometric measurements. The measured pressures are given by log 10 “p o(Np)/atm ’ = −(22370 ± 95)K T + (5.196 ± 0.056) and the resulting enthalpy and entropy of vaporization and the standard deviations therein are ΔH v o(Np, 1800 K) = (102.4 ± 0.4) kcal th mol −1 and ΔS v o(Np, 1800 K = (23.8 ± 0.3) cal th K −1 mol −1. The enthalpy of sublimation at 298 K is estimated as ΔH s o(Np, 298 K) = (110 ± 2) kcal th mol −1. The vapor pressure of plutonium was redetermined over the temperature range 1210 to 1620 K. The results yield log“p o(Pu)/atm’ = −(17120 ± 114)K T + (4.592 ± 0.082) , ΔH v o(Pu, 1400 K) = (78.3 ± 0.5) kcal th mol −1, and ΔS v o(Pu, 1400 K) = (21.0 ± 0.4) cal th K −1 mol −1. The measured pressures are 20 to 30 per cent less than those of two previously reported absolute measurements. Based on a recent assessment of the thermal functions for solid and liquid plutonium, the results of the present study show the best second-third law agreement, i.e., ΔH s o(II, Pu, 298.15 K) = (82.8 ± 0.5) kcal th mol −1 and ΔH s o(III, Pu, 298.15 K) = (83.3 ± 0.4) kcal th mol −1.