MITIGATING VOLCANIC ASHFALL AT A HIGH-LEVEL WASTE TREATMENT FACILITY

摘要

Since 2002, the U.S. Department of Energy has been constructing the Waste Treatment and Immobilization Plant (WTP) at the Hanford Site in Washington State. This facility will vitrify high-level liquid waste currently held in underground storage tanks. WTP safety systems must perform their functions under the effects of natural phenomena hazards (NPH), such as ashfall from Mt. St. Helens, the recently active volcano 200 km away. Volcanic ash poses two challenges for WTP. In addition to structural loads from ash deposits, the facility's air handling and other equipment must withstand the hazards posed by airborne particles. WTP safety systems have a performance goal to fail with an annual frequency less than 10~(-4). The initial design value for WTP called for ashfall structural loading of 61 kg/m~2, and an airborne concentration of 220 mg/m~3. The Hanford site ashfall hazard analysis was reviewed in 2010. A new analysis of the hazard completed in 2012 derived an ashfall structural load of approximately 112 kg/m~2, with a mean annual frequency of exceedance of 10~(-4), nearly twice the previous value. A new airborne concentration analysis calculated 1500 mg/m~3 during a 10~(-4) annual frequency ashfall event. Considering possible ash re-suspension during the ashfall event, the concentration could rise as high as 2600 mg/m~3. In the current facility design, WTP processes require considerable clean air to drive tank mixing, and removing such a high concentration of ash from large volumes of air is challenging. Several options for resolving this technical challenge are discussed.