Equilibria in the niobium-hydrogen system were determined in the range 100 to 900 C, 0.1 to 1000 mm of mercury hydrogen pressure, and hydrogen/niobium atomic ratios of 0.01 to 0.85. X-ray measurements were obtained at 25 to 400 C at hydrogen/niobium ratios up to 0.54. The studies showed that a solid solution of hydrogen in niobium is produced throughout most of the system. A miscibility gap was foundat low temperatures and pressures, with a critical point at about a temperature of 140 C, a hydrogen pressure of 0.01 mm of mercury, and a hydrogen/niobium ratio of 0.3. Sorption rates at 300 to 550 C were initially linear. At higher temperatures, sorption rates were controlled by diffusion in the metal matrix. Diffusion coefficients at 600 to 700 C can be expressed by D '^-0.0215 exp [(-9370 ± 600)/RT \. Desorption rates were lower than those predicted by diffusion.

The reaction of hydrogen with uranium to produce uranium hydride was studied in the temperature range 96~176 at pressure levels of p -P0 equal to 430, 150, and 70 mm of mercury (where p is system pressure and p0 is the plateau dissociation pressure of the uranium hydride product). Reaction rates followed the linear law. At a given p --p0, the linear rate increased with increasing temperature in the range 96 ~ to about 250~ and decreased with increasing temperature from about 250 ~ to 400~ Variation of the linear rate with temperature and pressure in the range 96 ~ to about 250~ is given by the equation: r = 4.11 X 10-~p ~/4 exp (-1820/RT) where r is in units of ml/cm~/sec and p is in units of mm Hg. In the range from about 250 ~ to 400~ an empirical relationship between the reaction rate and pressure is proposed where the rate is a function of p -p~/po. However, since none of the reaction mechanisms was determined, the significance of the pressure dependencies is not known. Two different initial reactions were obtained. One in the range 96 ~ to about 250~ at p -p0 of 70 and 150 mm Hg and 96~176 at p -p0 of 430 mm Hg showed a gradual increase in the rate of hydrogen consumption until the rate became linear. The other initial reaction followed the parabolic rate law in the range oi about 250~176 at p --p0 of 70 and 150 mm Hg.