PNNL

Abstract

A sample of high-level radioactive tank waste was characterized to provide a basis for developing a waste simulant. The simulant is required for engineered-scaled testing of pretreatment processes in a non-radiological facility. The waste material examined was derived from the bismuth phosphate process, which was the first industrial process implemented to separate plutonium from irradiated nuclear fuel. The bismuth phosphate sludge is a complex mixture rich in bismuth, iron, sodium, phosphorus, silicon, and uranium. The form of phosphorus in this particular tank waste material is of specific importance because that is the primary component (other than water-soluble sodium salts) that must be removed from the high-level waste solids by pretreatment. This work shows unequivocally that the phosphorus present in this waste material is not present as bismuth phosphate. Rather, the phosphorus appears to be incorporated mostly into an amorphous iron(III) phosphate species. The bismuth in the sludge solids is best described as bismuth ferrite, BiFeO3. Infrared spectral data, microscopy, and thermal analysis data are presented to support these conclusions. The behavior of phosphorus during caustic leaching of the bismuth phosphate sludge solids is also discussed.