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Surface and Corrosion Chemistry of PLUTONIUM

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Surface and Corrosion Chemistry of PLUTONIUM Surface and Corrosion Chemistry of PLUTONIUM John M. Haschke, Thomas H. Allen, and Luis A. Morales s 252 Los Alamos Science Number 26 2000 Surface and Corrosion Chemistry of Plutonium Elemental plutonium, the form in which most of the weapons-grade material exists, is a reactive metal. When exposed to air, moisture, and common elements such as oxygen and hydrogen, the metal surface readily corrodes and forms a powder of small plutonium-containing particles. Being easily airborne and inhaled, these particles pose a much greater risk of dispersal during an accident than the original metal. The present emphasis on enhancing nuclear security through safe mainte- nance of the nuclear stockpile and safe recovery, handling, and storage of surplus plutonium makes it more imperative than ever that the corrosion of plutonium be understood in all its manifestations. etallic plutonium was first a Los Alamos pioneer in plutonium initiated spontaneously at room temper- prepared at Los Alamos in corrosion, was prescient when he ature and advanced into the plutonium M1944, during the Manhattan wrote, “Most investigators are inclined metal at a rate of more than 1 centime- 10 Project. After samples became avail- to concentrate their attention on PuO2, ter per hour (cm/h), or a factor of 10 able, scientists studied the properties which can be well characterized, and faster than the corrosion rate in dry of the metal, including its reactions to ignore other compounds that may air. The reaction generated excessive with air, moisture, oxygen, and hydro- contribute to the overall corrosion temperatures and started under condi- gen. Extensive investigation of pluto- behavior” (Wick 1980). Indeed, we tions that are considered safe for the nium continued into the early 1970s, believe that plutonium oxides other routine handling of plutonium. and the results of those studies are than PuO2 and compounds other than Further studies suggest that the documented in the handbooks of oxides play a potent role in plutonium course of corrosion depends very chemistry, physics, and metallurgy. corrosion. These compounds appear heavily on the chemical condition of In dry air—less than 0.5 part per to be catalysts, causing anomalous the plutonium surface. A surface layer million (ppm) of water—at room tem- corrosion reactions to proceed at of sesquioxide (Pu2O3), which forms perature, plutonium behaves much like enormous rates. in the absence of oxygen, promotes other active metals, forming a protec- “Runaway” reactions can occur corrosion of the metal by hydrogen. tive layer of dioxide (PuO2) on the under fairly routine conditions. In one Conversely, a surface layer of hydride surface. The PuO2 layer limits corro- case, a failed storage package contain- (PuHx, where 1.9 < x < 3) increases sion of unalloyed plutonium to the al- ing a plutonium casting was examined the plutonium oxidation rate in oxygen most imperceptible rate of 20 picome- in a glove box, which had a by a factor of 1013 to a value near ters per hour. The corrosion chemistry nitrogen-rich atmosphere (less than 3 meters per hour (m/h). Finally, a sur- of plutonium is therefore assumed to 3 percent oxygen) normally used for face layer of the previously unknown be relatively simple and well under- handling plutonium metal. The pack- higher oxide PuO2+x, which forms on stood, but that assumption is far from age remained there for 3 hours after the PuO2 layer in the presence of true. Unexplained phenomena, such as disassembly and initial inspection. moisture, apparently enhances the bulk plutonium pyrophoricity (spontaneous Whenthe workers returned to continue corrosion of plutonium metal in moist ignition in air) and moisture-accelerated their evaluation, they found that the air. The fact that PuO2 reacts with corrosion, in air were observed in the inner container was hot to the touch water demonstrates that it is thermody- earliest plutonium studies (Cleveland and its diameter had increased by namically unstable in air and environ- 1979, Wick 1980, Katz et al. 1986). 50 percent in the region surrounding mental media. Evidently, its reaction Recent Los Alamos studies confirm the casting (Haschke and Martz with water is responsible for both and add to these examples of unex- 1998b). Our subsequent investigation the implosion and the pressurization of pectedly rapid corrosion. Moreover, showed that a corrosion reaction sealed containers of plutonium oxide our analysis suggests that J. T. Waber, involving both oxygen and nitrogen during their extended storage. Number 26 2000 Los Alamos Science 253 Surface and Corrosion Chemistry of Plutonium In this article, we present our obser- process broken down into a sequence of trolled by the rate at which oxygen dif- vations and analysis of the chemistry steps. A freshly burnished surface of fuses through the oxide layer.1 As the and kinetic behavior of important corro- plutonium is exposed to oxygen gas. oxide layer gets thicker, the time to sion reactions involving oxygen, nitro- Oxygen molecules (O2) adsorb on the diffuse through the layer becomes gen, and hydrogen. We observe that oxide surface at a concentration deter- longer, and thus the oxidation rate PuHx, Pu2O3, and PuO2+x on the metal mined by temperature and the partial decreases with time. Figure 1(d) shows surface catalyze anomalous corrosion pressure of oxygen in the gas phase. the decreasing slope of the curve reactions and that the Pu(III) and The adsorbed molecules then dissociate describing thickness versus time during Pu(VI) oxidation states are important in on the oxide surface to form atomic parabolic growth. addition to the predominant Pu(IV) oxygen, a species that either recom- Parabolic growth tapers off to a state. We also outline the kinetics that bines or associates with electrons to steady-state regime when the low- relates specific surface compounds and form oxide ions (O2–). Both adsorption density oxide—11.45 grams per cubic conditions to anomalous reactions. Our and dissociation depend strongly on the centimeter (g/cm3)—on high-density analysis provides explanations for many electronic properties of the oxide layer plutonium (19.86 g/cm3) begins to puzzling phenomena, but it also leaves and its ability to transport electrons induce stresses that lead to the localized a multitude of unanswered questions. from the metal to the gas-solid inter- spallation of oxide particles from the We conclude that the present under- face. After entering the oxide lattice, surface. The thickness of the oxide standing of plutonium chemistry is O2– diffuses through the oxide layer layer then varies from point to point inadequate and that the new evidence and ultimately reacts with plutonium to with regions of thin oxide in recently presents an immediate technical chal- produce oxide, electrons, and heat at spalled areas, regions of thick oxide in lenge to the scientific community. the oxide-metal interface. The slowest unspalled areas, and regions of interme- step in this sequence is called rate lim- diate oxide thickness in between, as iting because the overall reaction can illustrated in Figure 1(c). During this Plutonium Oxides and proceed no faster. stage, the average thickness of the Atmospheric Oxidation Kinetic data on the thickness τ of oxide layer and the isothermal oxida- the adherent oxide layer as a function tion rate reach constant values as Plutonium corrosion and oxidation of time t demonstrate that classic oxy- diffusion-controlled oxidation contin- are often treated as equivalent topics gen diffusion is the rate-limiting step in ues. The corrosion rate becomes con- because plutonium oxides are the only the Pu + O2 reaction. On a freshly bur- stant because the continuous spallation products normally observed during nished plutonium surface, the thickness of oxide particles and the reoxidation atmospheric corrosion. The metal does τ reflects the extent of the reaction. of the surface maintain a steady-state not react appreciably with elemental This thickness exhibits parabolic diffusion barrier of constant average nitrogen even at elevated temperatures, growth, increasing linearly with the thickness. As shown in Haschke et al. although plutonium mononitride (PuN) square root of t at a fixed temperature (1996), the corrosion rate of unalloyed is a stable compound. Therefore, we T. The rate of reaction, which is the plutonium in dry air at 25°C is approxi- begin with a review of oxide chemistry time derivative of the thickness, must mately 0.5 nanogram of plutonium and oxidation kinetics. therefore be inversely proportional per centimeter squared a minute to the thickness, exactly the behavior (ng Pu/cm2 min), and the steady-state Diffusion-Controlled Oxidation in expected if the rate of reaction is con- oxide thickness is 4 to 5 micrometers Dry Air. Like aluminum and other (Martz et al. 1994). reactive metals, plutonium is As expected, the diffusion rate passivated, or rendered unreactive, upon 1 For a diffusion-controlled reaction, the rate at through the oxide layer increases exposure to air because a coherent which the thickness of the product layer strongly with increasing temperature (continuous, uncracked) oxide layer increases is inversely proportional to that thick- and produces a corresponding increase ness, as described by the differential equation rapidly forms over the entire surface. dτ/dt = k/τ. The proportionality constant k is in the oxidation rate of plutonium. Dur- Although oxidation continues despite characteristic of the reaction. We derive the ing both the parabolic and constant-rate the protective oxide coating, its rate parabolic rate law by rewriting this equation as stages of oxidation, the reaction rate R τdτ = kdt, integrating it, and applying the in dry air at room temperature is boundary condition τ = 0 at t = 0. In the result- obeys the classical Arrhenius relation- extremely low. Evidence from kinetic ing expression of the parabolic rate law, ship R = exp(–E /R*T), where E is the τ2 a a = kpt, kp is the parabolic rate constant that data demonstrates that the oxidation includes the temperature-dependent coefficient activation energy for the reaction and rate is limited by the rate at which oxy- for diffusion of the reactant through the product.
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