Hydrofluoric acid dissolution of spent radioactive and hazardous HEPA filters

This document discusses a process that treats High Efficiency Particulate Air (HEPA) filters contaminated with radioactive particles containing transuranic and hazardous elements which has been studied at the Idaho Chemical Processing Plant (ICPP). This process uses three to four steps to spent HEPA filter so that they meet regulatory requirements for disposal. The first step of the process is to bake the spent HEPA filters at 550 degrees C for 3 hours to volatilize organic binder which is used to provide strength in the filter's silicon glass media. In the second step, the filters are first leached in nitric acid, then are dissolved in hydrofluoric acid. The dissolution is conducted using an optimum fluoride to silicone molar ratio of 7/1 to prevent: (1) excess unreacted HF (free HF), (2) the formation of insoluble fluoride salts, and (3) the need of excessive aluminum nitrate to complex the free HF. Finally, the free HF is complexed with aluminum nitrate to an [HF] between 0.020 and 0.04 molar. The resulting solution from this process is minimally corrosive to stainless steel and is a stable high-level liquid waste that can be converted to a high-level solid waste in a fluidized bed operated at 500 degrees C. The stainless steel HEPA filter housing, which is not dissolved during the dissolution step, is compacted and disposed of as a nonhazardous, low-level radioactive waste