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

Abstract

Tank 241-SY-101 simulated waste. Flammable gases generated by the radiolysis of water and by the thermal and radiolytic decomposition of organic waste constituents may themselves participate in further reactions. Examples include the decomposition of nitrous oxide to yield nitrogen and oxygen, the reaction of nitrous oxide and hydrogen to produce nitrogen and water, and the reaction of nitrogen and hydrogen to produce ammonia. The composition of the gases trapped in bubbles in the wastes might therefore change continuously as a function of the time that the gas bubbles are retained.The decomposition of nitrous oxide, reactions between nitrous oxide and hydrogen, reactions between nitrogen and hydrogen, and decomposition reactions of ammonia in contact with Tank 241-SY-101 simulated waste under thermal and combined thermal and radiolytic conditions were investigated in the temperature range of 60 to 150°C. Radiation doses ranged from 0 to more than 10 Mrad ( @ ' C O ) , while reaction times extended to 70 hours. Reactions were performed in stainless steel vessels that allowed continuous monitoring of gas pressures. Gas compositions were evaluated using mass spectrometry and infrared spectrometry.The primary radiolytic and thermal products of nitrous oxide decomposition were nitrogen, oxygen, and nitrogen dioxide. Under thermal-only conditions, the extent of nitrous oxide decomposition was less than that for the companion experiment under combined thermal and radiolytic conditions. However, the thermal decomposition in the presence of simulated waste was significant compared with that observed in earlier tests in the absence of simulated waste (Bryan and Pederson 1995). G-values measured for the decomposition of nitrous oxide in the presence of simulated waste showed a significant temperature dependence, consistent with a surface reaction with nitrous oxide contributing to the decomposition reaction. G(N20) ranged in value from -7 to -17 in the systems containing wet and dried simulated waste.In contrast, G(N20) for the gas phase decomposition of nitrous oxide was determined to be constant at -12 molecules/100 eV, in good agreement with literature values. No temperature dependence of G(-N20) was apparent from 60 to 150°C. Nitrogen dioxide yields decreased with increasing temperature, while nitrogen yields increased with increasing temperature. Assuming a gamma dose rate of lo00 R/h in the actual wastes, and assuming that nitrous oxide composes 530% of the gases trapped in the nonconvecting layer, it is estimated that = 1 % of the retained nitrous oxide could be consumed by these gas-phase radiolytic reactions per year. However, the thermal decomposition of nitrous oxide contributes significantly to the total decomposition of nitrous oxide when in contact with simulated waste. iii Catalysis of nitrous oxide decomposition by solid phases is a significant possibility. At least some of the gases will be retained as bubbles attached to solid particles, in response to surface tension forces. Thus the gases will be in in...

The Chemistry of Flammable Gas Generation

“…This value indicates that 0.9% nitrous oxide would be consumed in 1 year at a dose rate of 1,000 R/h. Bryan and Pederson (1995) conclude that the gas phase decomposition of nitrous oxide is of minor importance in waste.…”

“…The G values for the decomposition of nitrous oxide and the formation of nitrogen, oxygen, and other nitrogen oxides obtained by Harteck and Dondes (1956) at 25 "C and by Bryan and Pederson (1995) at between 60 "C and 150 "C for the pure substance are shown in Figure 4-4. The radiolysis reaction also was investigated in the presence of dry and moist waste simulants .…”

“…Consequently, radiolytic and thermal reactions of the gases with each other and with the other constituents of the waste have been investigated to determine whether those chemical reactions alter the composition of the mixtures. Bryan and Pederson (1995) investigated the decomposition of nitrous oxide to produce nitrogen and oxygen; the reaction between nitrogen and hydrogen to form ammonia as the principal product; the reaction between nitrous oxide and hydrogen to form ammonia, nitrogen, and nitrogen dioxide; and the decomposition of ammonia. They conducted tests with the gases alone and with the gases in contact with dried and moist simulated waste.…”

“…The observations presented in this figure are in good agreement with previous findings indicating that ammonia is formed radiolytically from the elements but also decomposes to them even more rapidly (Spinks and Woods 1990). Bryan and Pederson (1995) investigated the thermal and radiolytic decomposition of ammonia in the presence of waste simulants. The amount of nitrogen found in the experiments implied that about 10% of the original ammonia decomposed at 60 "C in 150 hours.…”

“…The radiolytic decomposition reaction of ammonia is temperature dependent (Spinks and Woods 1990;Bryan and Pederson 1995) and is more rapid in the presence of moist simulant. At 60 "C, G(-NH3) is 6.4 molecules/100 eV for dry simulant and 15.2 molecules/100 eV for moist simulant.…”