Chemistry and kinetics of the pyrophoric plutonium hydride-air reaction

The chemistry and kinetics of the pyrophoric reaction of the plutonium hydride solid solution (PuHx, 1.9 ≤ x ≤ 3) are derived from pressure-time and gas analysis data obtained after exposure of PuH2.7 to air in a closed system. The reaction is described by two sequential steps that result in reaction of all O2, partial reaction of N2, and formation of H2. Hydrogen formed by indiscriminate reaction of N2 and O2 at their 3.71:1 M ratio in air during the initial step is accommodated as PuH3 inside a product layer of Pu2O3 and PuN. H2 is formed by reaction of O2 and partial reaction of N2 with PuH3 during the second step. Both steps of reaction are described by general equations for all values of x. The rate of the first step is proportional to the square of the O2 pressure, but independent of temperature, x, and N2 pressure. The second step is a factor of ten slower than step one with its rate controlled by diffusion of O2 through a boundary layer of product H2 and unreacted N2. Rates and enthalpies of reaction are presented and anticipated effects of reactant configuration on the heat flux are discussed.