All aqueous radioactive solutions generate radiolytic hydrogen and Hanford underground waste tanks have a long history of dealing with this hazard. Currently there are a set of very conservative operational controls that have successfully maintained all tank headspaces at or below 25% of the lower flammability limit for hydrogen, or below 10,000 ppmv hydrogen. In the coming years, these waste liquids and solids will be retrieved and processed into stable waste forms and it is very likely that improved hazards analyses will increase operational flexibility with waste retrieval and treatment without jeopardizing safety.This paper will discuss three areas where more realistic hazard analyses will increase operational flexibility without compromising safe operations. For example, the passive ventilation rate measured in tank headspaces have half lives of 10-20 days or less. Yet the time-at-risk for off normal events uses the much more conservative barometric ventilation limit of about 130 days, one order of magnitude greater time at risk. More realistic passive ventilation rates will lower these time-at-risk scenarios for off normal events.Finally, retention of flammable gases in sediments and the subsequent sudden release into tank headspaces has limited tank operations significantly for certain high solids tanks. However, only precipitated solids with sodium nitrate and nitrite coupled with significant gas generation rates show sudden flammable gas releases.  In particular, simple oxide sludges do not show spontaneous gas release events even for tanks with quite large gas generation rates. Better defining the actual hazards of flammable gas generation will make it possible to increase operational flexibility without compromising safe operations in processing tank wastes.

Flammable gas hazard in Hanford waste tanks

248th National Meeting (2014)

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