Development of Iron Phosphate Waste Forms for Fission Products Waste Streams

Most liquid high-level nuclear waste is currently being immobilized to a solid form as a borosilicate glass by vitrification. However, many nuclear wastes have complex and diverse chemical compositions that reduce their compatibility with borosilicate glass. The current baseline spent nuclear fuel reprocess generates secondary waste streams that generally include large amounts of MoO3 and noble metals that are poorly soluble in borosilicate glasses and thereby limit the waste loading. New waste forms as alternatives to borosilicate glass are being sought to increase the waste loading while retaining acceptable chemical durability and thus to decrease the total nuclear waste volume required for storage and disposal. The main goal of the SBIR Phase I and Phase II projects is to develop suitable iron phosphate-based compositions for vitrifying the MoO3-rich waste (Collins-CLT) generated from reprocessed spent nuclear fuel with greater waste loadings than can presently be achieved with borosilicate glass and while retaining chemical durability that meets or exceeds appropriate Department of Energy (DOE) standards. The Phase I research successfully produced an iron phosphate waste form containing 40 wt% of simulated Collins-CLT waste, and this waste form can be prepared at 1300C. The product consistency test (PCT) response and the vapor hydration test (VHT) corrosion rate of this waste form (as-cast) meet current DOE chemical durability requirements. Structure-composition-property relationships in simplified Na-Fe-Mo-phosphate glasses were also studied to better understand the more complex waste-loaded glasses. Phase II activities will build on the Phase I accomplishments by optimizing the iron phosphate compositions and measuring useful thermal, electrical, and chemical properties of the iron phosphate waste forms for commercial-scale production. The Phase II work will also address the unanswered questions identified in Phase I, including the preferential molybdenum leach rate, the nature of the Mo-Fe redox reactions in glass melts, and the effects of melt redox on the waste form structure and chemical stability. Commercial-scale (60 kg) melting of a selected iron phosphate waste form in a cold crucible induction melter will be demonstrated. The proposed research will produce chemically stable iron phosphate waste forms with increased radioactivity concentrations that will reduce by up to 50% the total nuclear waste volume from spent nuclear fuel needed for long-term storage and disposal. This would lead to considerable savings of time and money for the Nations effort to remediate nuclear waste especially that associated with the advanced domestic fuel cycle program.