This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi-S. A. Bryan bility for the accpracy, completeness, or usefulness of any information, apparatus, product, or K.H. POOl process disclosed, or represents that its use would not infringe privately owned rights. Refer-J. D.Matheson ence herein to any specific commercial preduct, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

This document assesses the likelihood of ammonium nitrate formation in Tank 241-SY-101 and considers any associated explosion hazards. Two principal scenarios by which ammonium nitrate may be formed were considered: (a) precipitation of ammonium nitrate in the waste, and (b) ammonium nitrate formation via the gas phase reaction of ammonia and nitrogen dioxide. The first of these can be dismissed because ammonium ions, which are necessary for ammonium nitrate " precipitation, can exist only in negligibly small concentrations in strongly alkaline solutions. Also, ammonium nitrate cannot form in dried wastes. Not only will ammonium ion concentrations be negligibly small, but the aqueous ammonia/ammonia vapor equilibrium will ensure that ammonia will be lost as a vapor to ventilation air. Gas phase reactions between ammonia, nitrogen dioxide, and water vapor in the gas phase represent the most likely means by which ammonium nitrate aerosols could be formed in Tank 241-SY-101. If produced, these particles would probably collect on High-Efficiency Paniculate Air (HEPA) filters, risers, and other piping in the ventilation system. Other oxides of nitrogen, such as nitrous oxide, will not react with ammonia to yield ammonium nitrate. Predicted ammonium nitrate formation rates are largely controlled by the concentration of nitrogen dioxide. This gas has not been detected among those gases vented from the wastes using Fourier Transform Infrared Spectrometry (FTIR) or mass spectrometry. While detection limits for nitrogen dioxide have not been established experimentally, the maximum concentration of nitrogen dioxide in the gas phase in Tank 241-SY-101 was estimated at 0.1 ppm based on calculations using the HITRAN data base and on FTIR spectra of gases vented from the wastes. At 50°C and with 100 ppm ammonia also present, less than one gram of ammonium nitrate per year is estimated to be formed in the tank. To date, ammonium nitrate has not been detected on HEPA filters in the ventilation system, so any quantity that has been formed in the tank must be quite small, in good agreement with rate calculations. The potential for runaway exothermic reactions involving _immonium nitrate in Tank 241-SY-101 is minimal, limited most obviously by a lack of sufficient quantities of ammonium nitrate. Dilution by non-reacting waste components, particularly water, would prevent hazardous exothermic reactions from occurring within the waste slurry, even if ammonium nitrate were present. As for relatively pure ammonium nitrate possibly collecting on the filters-the quantity, temperature, configuration, and confining pressure are inappropriate to lead to detonation of this shock-insensitive material.

The solution chemistry of Pu in nitric acid is explored via electrochemistry and spectroelectrochemistry. By utilizing and comparing these techniques, an improved understanding of Pu behavior and its dependence on nitric acid concentration can be achieved. Here the Pu (III/IV) couple is characterized using cyclic voltammetry, square wave voltammetry, and a spectroelectrochemical Nernst step. Results indicate the formal reduction potential of the couple shifts negative with increasing acid concentration and reversible electrochemistry is no longer attainable above 6 M HNO3. Spectroelectrochemistry is also used to explore the irreversible oxidation of Pu(IV) to Pu(VI) and shine light on the mechanism and acid dependence of the redox reaction.

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