Thermal and combined thermal and radiolytic reactions involving nitrous oxide, hydrogen, and nitrogen in the gas phase; comparison of gas generation rates in supernate and solid fractions of Tank 241-SY-101 simulated waste

Abstract

DISCLAIMERPortions of this document may be illegible in electronic image products. lrnages are produced from the best available original document. Executive Summary. This report summarizes progress made in evaluating mechanisms by which flammable gases are generated in Hanford double-shell tank wastes, based on the results of laboratory tests using simulated waste mixtures. Work described in this report was conducted at Pacific Northwest Laboratory (PNL)Ca) for the Flammable Gas Safety Project, the purpose of which is to develop information needed to support Westinghouse Hanford Company (WHC) in their efforts to ensure the safe interim storage of wastes at the Hanford Site. This work is related to gas generation studies being performed at Georgia Institute of Technology (GIT), under subcontract to PNL, using simulated wastes, and to studies being performed at WHC using actual wastes.Described in this report are 1) the results of tests to evaluate the rates of thermal and combined thermal and radiolytic reactions involving flammable gases, and 2) the results of experiments intended to compare gas generation rates in a homogeneous liquid with those in a heterogeneous slurry. Flammable gases generated by the radiolysis of water and by the the&al 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 therefore may change continuously as a function of the time that the gas bubbles are retained. It has often been assumed, although not demonstrated, that the rates of flammable gas generation in the convecting liquid layer and nonconvecting settled solids layer in W o r d double-shell waste tanks are approximately equal. Factors that could lead to different gas generation rates in the two layers include different water content, different concentrations of organic waste components, and catalysis by solid phases. Gas Reactions under T h e m l and Combined T h e m 1 and Radiolytic Conditions:The decomposition of nitrous oxide, reactions between nitrous oxide, and reactions between nitrogen and hydrogen 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 100 Mrad (60Co), while reaction times extended to 500 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. Simulated waste mixtures were excluded from these tests.The primary radiolytic/thermal products of nitrous oxide decomposition were nitrogen, oxygen, and nitrogen dioxide. Under thermal-only conditions, the extent of nitrous oxide decomposition was too small to be observable. Under combined the...

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

“…These solid phase reactants have been shown in earlier tests to produce nitrogen under similar temperature and irradiation conditions in simulated wastes (Bryan and Pederson 1994; Bryan et al 1992a). In contrast to the work reported for the gas phase reactions of nitrous oxide and hydrogen (Bryan and Pederson 1995), when these gases are in contact with simulated waste no measurable amount of ammonia is produced.Ammonia was the principal product of the thermal and combined thermal and radiolytic reactions of nitrogen and hydrogen. Radiolytic ammonia yields showed a strong inverse relation with radiation dose.…”

“…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.…”

Gas generation from Tank 241-SY-103 waste

“…The material examined was provided by the WHC Characterization program, which obtained the core samples from Tank 241-SY-103. G a s generation measurements are being made using reaction vessels and a g a s manifold system similar to that used in earlier studies with simulated waste (Bryan and Pederson 1995).…”

Thermal and Radiolytic Gas Generation Tests on Material from Tanks 241-U-103, 241-AW-101, 241-S-106, and 241-S-102: Status Report

“…Gas generationmeasurementswere made using reaction vessels and a gas manifoldsystem similarto those used in earlier studies with simulatedwaste (Bryan and Pederson 1995) and describedin earlier reports detailingwork with actual waste (Bryanet aL 1996;King et al 1997). Each vesselhas a separate pressuretransducer on the gas manifoldline.…”