PNNL

Abstract

Introduction The Hanford Site has 177 underground storage tanks containing radioactive wastes that are complex mixes of radioactive and chemical products. Some of these wastes are known to generate and retain large quantities of flammable gases consisting of hydrogen, nitrous oxide, nitrogen, and ammonia. Because these gases are flammable and have the potential for rapid release, the gas generation rate for each tank must be determined to establish the flammability hazard (Johnson et al. 1997). An understanding of gas generation is important to operation of the waste tanks for several reasons. First, knowledge of the overall rate of generation is needed to verify that any given tank has sufficient ventilation to ensure that flammable gases are main-tained at a safe level within the dome space. Under-stand-ing the mechanisms for production of the various gases is important so that future waste opera-tions do not create conditions that promote the production of hydrogen, ammonia, and nitrous oxide. Studying the generation of gases also provides important data for the compo-si-tion of the gas mixture, which in turn is needed to assess the flammability characteristics. Finally, information about generation of gases, including the influence of various chemical constituents, temperature, and dose, would aid in assessing the future behavior of the waste during interim storage, implementation of controls, and final waste treatment. This paper summarizes the current knowledge of gas generation pathways and discusses models used in predicting gas generation rates from actual Hanford radioactive wastes. A comparison is made between measured gas generation rates and rates by the predictive models.