The reaction of plutonium with water: phases in the Pu + O + H system

Abstract In early reports, the reaction of plutonium with water is described as ″very slow at room temperature″ [1]. The product is a finely divided black solid which prior workers have identified as hydrated oxide or hydroxide [2]. Results of recent experiments with synthetic sea water show that the Pu + H2O reaction is quite rapid and that the quantities of H2 produced are less than half those expected for the formation or Pu(OH)3 or Pu(OH)4 [3]. The primary measurements described in this report was designed to determine the reaction rate of Pu with 1.0 M CaCl2 solution, but kinetic measurements which continued for more than 450 days have provided valuable insight into the chemistry of complex Pu + U + H system. Kinetic measurements and the preparation of solid hydrolysis products for analysis were made with a stainless steel system fitted with a glass liner for containment of the reactants. Rates and extents of reaction at 23± 2°C were monitored by PVT methods. Both α-phase and δ-stabilized Pu were reacted with aqueous salt solutions having a wide range of concentrations. Samples of the initial hydrolysis product were isolated by removing residual water under dynamic vacuum or with molecular sieve. The reaction of Pu with H2O to form the black product is rapid when catalyzed by salt. The reaction rate R(mgPu cm−2 hr−1) is dependent on the anion concentration X (mol l−1) as follows: R = 1.2X0.7 for 10−7⩽ X ⩽ 1 M. Small variations are observed for different anions and cations and for α- and δ- phase Pu. When the quantity of H2 formed during the extended experiment (α-Pu + 1.0 M CaCl2 solution) is graphed against time, a series of linear curves with successively lower slopes is observed. Each linear (constant rate) section is attributed to the occurrence of a specific equilibrium hydrolysis reaction. During the first linear segment, Pu is completely converted to black product. During the second segment, that product reacts with water at a slower rate to produce additional H2 and a third product, etc. It is convenient to formulate the quantity of H2 as mol H2 produced per mol Pu because this ratio is directly correlated with the net chemical reaction; e.g., this ratio is 1.5 and 2.0 for formation of Pu(OH)3 and Pu(OH)4, respectively. Least squares analysis of the data for the extended test show that sharp slope changes occur at the following H2/Pu ratios: 0.507, 1.078, 1.455, 1.716, 1.763, 1.803, 1.831 and 1.993. Eight different hydrolysis products apparently form in succession. The breaks near 1.5 and 2.0 correspond to the formation of products containing trivalent and tetravalent Pu. The products existing at 0.50 and 1.07 must either have Pu in a valence state less than three or contain hydride, while those formed between the 1.50 and 2.00 ratios are mixed-valent. The black product formed at H2/Pu = 0.50 has been identified as plutonium monoxide monohydride Pu(O)H. This assignment is based on the results of chemical, TG, XRD, and XPS analyses. The data show that the product is not an hydroxide or hydrated oxide and that it chloride-free. XRD results for wet and dry products give identical fcc patterns with a = 540.1(2) pm. Although the parameter is similar to those of PuH2 (a = 535.9 pm) and PuO2 (a = 539.7 pm), thermal decomposition results show that the product is not a mixture of hydride and oxide. The observed parameter is consistent with that expected for a fluorite-related oxide hydride. Hydrolysis of Pu is described by the reaction: Pu + H2O→ Pu(o)H + 1 2 H2. XRD results are given in Table I. t001 . X-ray Diffraction Intensities for the Plutonium Monoxide Monohydride. hkl I/IO (%) 111 100 200 34 220 45 311 42 222 8 400 5 331 11 420 10 422 8 333 9 Our identification of the other hydrolysis products is based on results for the initial product and on the well-known phase relationships of Pr + O [4]. Particularly important observations are: (a) The product does not contain hydroxide, and this anion is not expected to exist in subsequent products. (b) The metals occupy fcc positions, and their rearrangement in subsequent phases at room temperature is unlikely. The observed H2/Pu ratios at which slope changes occur are in excellent agreement with the theoretical values for the following sequence of products: Pu(O)H (0.500), Pu7O9H3 (1.071), Pu2O3 (1.500), Pu7O12 (1.714), Pu9O16 (1.778),Pu10O18 (1.800) Pu12O22 (1.833) and PuO2 (2.000). All these phase are expected to have metal atoms on fcc positions with the anion positions of fluorite occupied at Pu(O)H and PuO2. During hydrolysis of the oxide hydride phases, the metal presumably remains trivalent while ordered defects are created on anion sites until the formation of C-type Pu2O3 is complete. As hydrolysis continues, ordered occupancy of anion sites occurs as Pu(III) is converted to Pu(IV). The mixed-valent oxides proposed for Pu are identical to those of Pr. Pu7O9H3 is considered to be an analog of Pu7O12. Our results suggest that six previously unknown phases exist in Pu + O + H. Although our characterization of the product formed by thermal decomposition of Pu(O)H in dynamic vacuum at 100-200°C is incomplete, all observations suggest that it is metastable plutonium monoxide, PuO. This work was performed under U.S. Department of Energy Contract DE-AC04-76DPO3533.