Slurry growth and gas retention in synthetic Hanford waste

Of 177 high-level waste storage tanks on the Hanford Site, 23 have been placed on a safety watch list because they may produce flammable gases. One tank in particular, Tank 241-SY-101 (Tank 101-SY), has exhibited slow increases in waste volume followed by a rip decrease accompanied by venting of large quantities of gases. Such cycles occur every 8-15 weeks beginning in the early 1980's an continuing to the present time. The concentration of hydrogen in the space above the waste slurry has approached it lower flammability limit during some of the gas release episodes. Other tanks have also been observed to periodically vent flammable gases, but to a lesser degree (Tank 241-SY-103 and 241-An-104, in particular). This paper summarizes the results of studies conducted at Pacific Northwest Laboratory (PNL) to help establish the causes of generation, retention, and episodic release of flammable gases from Tank 241-SY-101 (Tank 101-SY). Physical and chemical properties of crusts formed in laboratory tests using synthetic Tank 101-SY wastes are summarized, as are the results of gas generation measurements using these synthetic wastes. Microscopic-scale phenomena responsible for the retention of gas within the wastes are also discussed