Joseph C. Martz

A mechanism for plutonium pyrophoricity

A proposed mechanism for plutonium pyrophoricity quantitatively predicts the ignition temperature of plutonium as a function of surface : mass ratio and particle size. Plutonium must exceed 475°C before self-ignition occurs. External heating of massive samples is necessary to achieve this condition, while finely divided materials can reach the ignition point by an alternative, two-step mechanism. First, the thin layer of surface PuO2 on the metal undergoes kinetically controlled reduction to Pu2O3 near 150°C. Second, the trivalent Pu2O3 reacts with gas-phase oxygen to reform PuO2. Heat generated from the second reaction is sufficient to raise the temperature of small particles or thin foils above the 475°C ignition point. Details of this mechanism are given, including a discussion of plutonium oxidation and a calculation of adiabatic temperature increase due to oxidation of the Pu2O3 surface layer. Plutonium pyrophoricity data are summarized and compared to model results.

Oxidation kinetics of plutonium in air : consequences for environmental dispersal

Abstract Kinetic studies show that plutonium corrosion in air is catalyzed by plutonium hydride on the metal surface and causes storage containers to fail. The reaction initiates at 25°C, indiscriminately consumes both O 2 and N 2 , and transforms Pu into a dispersible product at a 10 7 –10 10 faster rate (0.6±0.1 g Pu/cm 2 min) than normal air oxidation. The catalyzed reaction of O 2 advances into the metal at a linear rate of 2.9 m/h. Rate equations and particle size data for atmospheric and catalyzed corrosion at temperatures up to 3500°C provide a technical basis for assessing the dispersal hazard posed by plutonium metal.

A mechanism for combustive heating and explosive dispersal of plutonium

Abstract A first-principles model is developed for defining the combustive explosion of ignited plutonium droplets during free-fall in air and for quantifying fractions of plutonium aerosolization. The model, which is based on literature data and a conceptual description of the combustion and explosion processes, determines heat accumulation and temperature of a droplet over time using the rates of oxidation and heat loss, as well as physical and thermochemical properties of plutonium metal and oxide. Self-heating during combustion leads to fusion of the oxide layer and pressurization of the droplet by plutonium vapor that ultimately vents through the molten oxide to produce fume aerosol and a hollow spherule of residual oxide. Observed dependencies of time-to-explosion and aerosolized fraction on droplet size are accurately calculated by the model. All ignited droplets with sizes greater than a threshold diameter (approximately 0.01 cm) explode with aerosolization of residual plutonium (68–75%), if the free-fall period or distance is sufficient for the droplet temperature to exceed 3300°C. A practical size window for explosion is bounded by the threshold value and by the diameter (about 0.13 cm) of a droplet that attains the explosion temperature during a free-fall distance of ten meters. Calculations show that a 1.0 cm-diameter drop heated initially to 1000°C is 0.3% oxidized after free-falling for this distance in 1000°C air and reaches a maximum temperature of 1560°C. Since minimum diameters of one centimeter are observed for naturally occurring plutonium drops during containment failures at 1000°C and the amount of dispersible plutonium is limited by the extent of oxidation, the reliability of large aerosol release fractions recommended in the literature for fuel-fire scenarios are questionable. Effects of independent variables on combustion and explosion are discussed and application of the model in predicting aerosol fractions released by plutonium fragments formed during highly energetic events is described.

Plutonium Aging: From Mystery to Enigma

The unusual behavior of plutonium in the solid state is a result of plutonium sitting at a transition in the actinide series between 5f electrons participating in bonding vs. being localized or inert in the core of the atom. The metallurgical consequences of this peculiar electronic structure are examined. We then review the consequences of aging on the properties of plutonium and some of its alloys. The principal mechanisms of aging are through surface corrosion (especially with oxygen and hydrogen) and through self-irradiation damage.

A generalized model of heat effects in surface reactions. II. Application to plasma etching reactions

Part I of this paper presented a generalized model of heat effects operative during surface reactions. The enhancement in reaction rate due to autothermic effects was analyzed as a function of activation energy, bulk temperature, and a parameter termed the characteristic temperature. Application of this model to experimental plasma etching data is presented in part II of this work. Characteristic temperatures calculated from experimental data in numerous plasma etching systems agree closely with the critical characteristic temperature predicted by the heat of the reaction model. Possible reasons for this consistency are given. Further, the autothermic enhancement in the Ta‐CF4/O2 etching system is accurately predicted as a function of reactant concentration by a heat of reaction model.

A generalized model of heat effects in surface reactions. I. Model development

Many important classes of surface reactions exhibit both high heats of reaction and large, positive activation energies. In addition, many surface reactions often occur in thermally isolated environments. As a result, significant autothermic effects are possible. In part I of this article, a generalized model of these effects is presented which describes the enhancement in reaction rate as a function of activation energy, bulk temperature, and a parameter termed the characteristic temperature. Reactant concentration and reaction order effects are also considered. Part II of this work presents the application of this model to numerous experimental plasma etching data.