Pretreatment of neutralized cladding removal waste (NCRW) sludge - results of FY 1991 studies

Neutralized cladding removal waste (NCRW) sludge is a unique waste material that is stored in two underground double-shell tanks at the U.S. Department of Energy's Hanford Site. The NCRW sludge was formed by neutralization of the solution resulting from the chemical decladding of Zircaloy-clad metallic uranium fuel by the Zirflex process. The sludge consists of zirconium and sodium hydroxides and fluorides, with small amounts of potassium, nitrite, and other nonradioactive materials. The sludge also contains uranium, transuranic (TRU) elements, and mixed fission products typical of the nonvolatiles present in irradiated fuel. The NCRW sludge is considered a TRU waste, which must be vitrified for ultimate disposal in a geologic repository. The TRU portion of the waste may be separated from the larger amount of bulk waste material so only the TRU portion would require vitrification and geologic disposal. Separation would significantly reduce waste disposal costs. Work is underway to develop the transuranic extraction (TRUEX) process. This solvent extraction process has been demonstrated to separate a large percentage of the TRU elements from the bulk components of NCRW sludge. Earlier studies identified potential problems in the TRUEX processing of NCRW sludge: potential corrosion of imbedded piping in the facility initially plannedmore » for the process, instability of dissolved NCRW solutions towards precipitation, formation of interfacial crud during the TRUEX solvent extraction step, and the amount of phosphorus in the TRU product stream. These four problems were studied in FY 1991 and the results indicate that: a solution of 2 M HNO[sub 3] at a F/(Zr + Al) ratio of about 2 adequately dissolves washed NCRW sludge; such solutions should not be corrosive towards stainless steel materials; dissolved NCRW sludge solutions obtained by dissolution of washed sludge at low F/(Zr + Al) ratios (about 2) are much more stable with respect to precipitation.« less