Assessment of the potential for ammonium nitrate formation and reaction in Tank 241-SY-101

Abstract

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.

Thermal and combined thermal and radiolytic reactions involving nitrous oxide, hydrogen, nitrogen, and ammonia in contact with tank 241-SY-101 simulated waste

“…Hanford Tank 241-SY-101 simulated waste SYl-SIM-92A, prepared using standard procedures (Bryan and Pederson 1994), was used in each experiment containing solids. This simulant contains major and minor inorganic components known to be present in actual wastes.…”

“…The gas pressure in each reaction system was atmospheric at the time the vessel was closed, immediately before the start of the experiment. The sampling methodology and the gas manifold system used to connect multiple reaction vessels were described in detail in Bryan and Pederson (1994).…”

“…Bryan and Pederson (in Strachan and Schulz 1993) also found that nitrous oxide decomposed in tests with simulated wastes conducted in stainless steel vessels, particularly under radiolytic conditions. More recently, Bryan and Pederson (1994) described reactions of nitrogen and hydrogen to yield ammonia and of nitrous oxide and hydrogen reactions to yield ammonia, nitrogen, oxygen, and nitrogen dioxide. Since gas products of radiolytic and thermal reactions in simulated wastes participate in further reactions, observed gas distributions in actual wastes may well vary as a function of the time the gases have remained trapped.…”

“…Unlike measurements reported previously (Bryan and Pederson 1994), gas reactions in this study were conducted in the presence of dried or wet simulated waste solids. These solids may serve as catalysts, thereby enhancing the rates of reactions and/or changing product distributions.…”

“…From studies using simulated wastes, the kinetics of formation and the stoichiometry of these gaseous products have been shown to be sensitive to the composition of the waste mixture (Delegard 1980; Jansky and Meissner, in Reynolds et al 1991;Ashby et al 1992Ashby et al , 1993Ashby et al , 1994aAshby et al ,1994bMeisel et al 1991Meisel et al , 1992Meisel et al , 1993Bryan et al , 1993Bryan and Pederson 1994). Nitrite, hydroxide, aluminate, transition metal and chloride ion concentrations, the identity and concentration of organic waste components, temperature, and radiation dose/dose rate all affect gas generation behavior.…”