Fluorination of Uranium Metal to UF 3 and UF 4 by Nitrogen Trifluoride: Evidence for Elusive UF 2
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Fluorination of Uranium Metal to UF3 and UF4 by Nitrogen Trifluoride: Evidence for Elusive UF2 Bruce K. McNamara1, Randall D. Scheele, Andrew M. Casella, Anne E. Kozelisky, Doinita Neiner Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA, U.S.A. ABSTRACT We have recently found that uranium and plutonium metals will react with nitrogen trifluoride (NF3) at temperatures below 120°C. These are the first reported instances of such low temperature fluorination reactions using NF3 and implicate metal catalyzed dissociation of the NF3 bond. We additionally present preliminary evidence for a surface mediated product distribution. Reaction of uranium metal with NF3 promotes products that are apparently determined by the concentration of the fluorinating reagent between 60 and 120°C. INTRODUCTION The reactions of NF3 with uranium and plutonium oxides typically require temperatures in excess of 400°C for any detectable fluorination reaction to occur in thermogravimetric (TG) and differential thermal analysis (DTA) experiments [1]. In general these temperatures are required to promote thermal dissociation of the NF3 bond according to the reaction: [2, 3] .
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NF3(g) = NF2 (g) + F (g).
(1)
A lowered temperature reactivity of NF3 on uranium metal is described here. A similar reactivity was observed for plutonium metal and will be reported elsewhere. Metal-catalyzed cleavage of NF3 bonds has been proposed on elemental Si and Ru surfaces to occur through dissociative adsorption processes. For instance when adsorbed on the Si(100) surface, the NF3 molecule dissociated with initial formation of Si–N and Si–F bonds[4, 5, 6]. Adsorption on the Ru(0001) surface led to formation of F, NF, and NF2 adsorbates [7]. It is proposed here that fluorination of uranium metal is initiated by similar dissociative adsorption processes since purely thermal cleavage of the NF3 bond should not occur at temperatures lower than 300-400°C. Previous to the experiments described below the product UF3 directly from uranium metal had not been reported in the fluorination literature. The observation of UF3 formation from uranium metal prompted us to measure the NF3 concentration and temperature dependences for each product UF3 and UF4. An apparent bifurcation in the product distribution with change in the concentration of fluorinating reagent has not been previously reported for surface reactions of NF3 or for any reaction on uranium and plutonium metal surfaces. Data herein suggest that a site preference or NF3 site coverage can promote a preference towards a given product channel. EXPERIMENT
TG data for the reaction of NF3 on uranium metal were acquired using Seiko (EXSTAR6000) in a fume hood. Off gases were scrubbed in a double water trap before release to the plenum of a hood. NF3 was purchased from Advanced Specialty Gases (Reno, NV). NF3/UHP argon mixtures were premixed prior to their entry into the furnace chamber of the TG apparatus. The premixed gas was routed through a 1/16″ nickel tube to an area about 1″ from the s
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