Hydride-Catalyzed Corrosion of Plutonium by Air: Initiation by Plutonium Monoxide Monohydride

Chemistry and kinetics of air reactions with plutonium monoxide monohydride (PuOH) and with mixtures of the oxide hydride and plutonium metal are defined by results of pressure-volumetemperature (PVT) measurements. Tests with specimens prepared by total and partial corrosion of plutonium in 0.05 M sodium chloride solution show that reaction of residual water continues to generate H, after liquid water is removed by evacuation. Rapid exposure of PuOH to air at room temperature does not produce a detectable reaction, but similar exposure of a partially corroded metal sample containing Pu and PuOH results in hydride (PuH,)catalyzed corrosion of the residual Pu. Kinetics of the first-order reaction resulting in formation of the PuH, catalyst and of the indiscriminate reaction of N, and 0, with plutonium metal are defined. The rate of the catalyzed Pu+air reaction is independent of temperature (E, = 0), varies as the square of air pressure, and equals 0.78 k 0.03 g Pu/cm2 min in air at one atmosphere. The absence of pyrophoric behavior for PuOH and differences in the reactivities of PuOH and PuOH+Pu mixtures are attributed to kinetic control by gaseous reaction products. Thermodynamic properties of the oxide hydride are estimated, particle size distributions of corrosion products are presented, and potential hazards associated with products formed by aqueous corrosion of plutonium are discussed. Introduction Interest in the corrosion chemistry of plutonium is promoted by the need to assess dispersal hazards associated with abnormal incidents involving metal and alloys. As noted in recent reports on the reaction kinetics of plutonium in air [ 1-41, metallic forms are not inherently dispersible. Corrosion in air is the most likely pathway by which metal is transformed into a dispersible material. The corrosion rate and the particle-size distribution of plutonium-containing product are essential parameters because they establish the mass of metal at risk during an incident and the fraction of that material with particle sizes in the respirable (<3 pm geometric diameter) and dispersible (<lo to 30 pm geometric diameter) ranges. Accurate risk assessment requires that conditions of the incident are specified and considered in defining the source term for dispersal. Kinetic data for oxidation of plutonium in air show that corrosion rates vary by factors of lo6-lo8, depending on temperature, humidity, and alloying [4]. Dispersible fractions vary by as much as 10'. The situation is further complicated because the temperature dependencies of corrosion rate and size distribution are such that their effects on the source term tend to cancel. The amount of material at risk after a given time period increases sharply with temperature, but the respirable and dispersible fractions decrease. Kinetic results presented in a recent report [4] show that a fire or other external heat source is not necessary for rapid corrosion of plutonium by air. Plutonium hydride (PuH,, 1.9 < x < 3.0) is a pyrophoric compound that reacts readily with oxygen at room temperature. Exposure of hydride-coated metal to air at room temperature often results in a rapid reaction that is more than 10" faster than normal corrosion in air. Hydrogen produced by that reaction is not released as H,, but reacts with available metal to form additional PuH,. The accompanying temperature increase is sufficient to initiate the reaction of hydride with nitrogen. Corrosion is catalyzed as hydride progressively advances into the metal ahead of the PuH,+air reaction. 0, and N, react indiscriminately during the process which eliminates all kinetic effects of alloying and humidity on the corrosion of plutonium in air. The inherent rate of the catalyzed reaction is four to five times faster than the constant rate for autothermic (self-sustained) reaction of oxygen at temperatures above the 500 k 25°C ignition point of plutonium metal and alloys [5] . Although surface hydride is essential for catalytic reaction, the hydrogen source may be unanticipated and includes radiolysis of plastics, elastomers, oils, and other organic compounds, as well as chemical reactions involving water [6] . Rapid corrosion of plutonium in air may also be initiated by reactive materials other than hydride. The pyrophoric PuzO, layer formed by auto-reduction of PuO, on the metal surface is capable of driving the temperature above the ignition point. Plutonium monoxide monohydride (PuOH), the hydridic compound formed by reaction of metal with near-neutral water or salt solution [7,8,9] is identified as a potential initiator of rapid reaction with air [ 101. Interest in the oxide hydride is increased because of the potential for its formation in process operations and in abnormal incidents. PuOH has recently been identified as the product formed by salt-catalyzed corrosion of plutonium in glovebox atmospheres containing hydrogen chloride and water vapors [ 1 13. The present study was initiated in an effort to define the chemical behavior of plutonium monoxide monohydride in air and to assess its potential as an initiator of rapid plutonium corrosion. Definition of kinetics for PuOH-coated plutonium in air is augmented by results of similar measurements for PuH,-coated metal. Thermochemical