Solidification of Sodium Bearing Waste Using Hydroceramic and Portland Cement Binders

AM-S19-39 A hydroceramic is a monolithic solid produced by a hydration driven reaction that occurs after mixing metakaolin with concentrated NaOH solution. The chemistry of the starting materials is optimized to encourage the formation of tectosilicates (zeolites, feldspathoids) during curing. Furthermore, the process is forgiving enough so that a variety of caustic waste streams can be accommodated without sacrificing performance. This characteristic makes a hydroceramic perfectly suited to solidify low activity sodium bearing waste (SBW) now in storage at Hanford, Savannah River and INEEL. SBW normally contains three main ingredients, NaOH, NaNO3 and NaNO2. Depending upon the level of nitrate and nitrite salts in the SBW the SBW can be solidified by direct mixing and curing (low concentrations) or alternately must first be heat treated in some manner to decompose its nitrate/nitrite inventory. Two procedures, both using approximately the same blend of aluminosilicate, carbonaceous materials and SBW are being evaluated by the DOE. The first is a calcine formed by heating the blended materials in air at 525°C. The second is a granular product produced during steam reforming in a Studsvik reactor at 725°C. In both cases, nitrate and nitrite are reduced and the nacent Na ions that form react with the metakaolinite forming X-ray amorphous precursors and crystalline zeolites (zeolite A, hydroxysodalite) and feldspathoids (cancrinite) rather than Na2CO3. Although the products have low leachabilities in their own right, they can be further solidified and made into a hydroceramic using the approach outlined above for liquid SBW. Performance data for hydroceramics made from a simulated low nitrate/nitrite Savannah River SBW solidified by direct solidification and also via intermediate calcination and then solidification suggest that leachabilities are nearly the same. Thus direct solidification is recommended for this type of SBW. Performance data for a heavily nitrated and nitrited Hanford SBW that was calcined at 525°C with metakaolin and sucrose and then solidified using a hydroceramic binder and a Portland cement binder are compared. The hydroceramic exhibited superior performance vis à vis a more convention grout formulation. INTRODUCTION Radioactive waste produced during DOE’s active weapons production program has been stored in tanks at the Hanford, Savannah River and INEEL sites for more than 50 years. In all cases nitric acid was used to dissolve nuclear fuel rods during reprocessing. After extracting fissile materials and other elements of interest the remaining waste liquid was placed into storage. Of the three sites only INEEL decided to calcine its acid waste directly using a fluidized bed calciner. Their calcines are now in storage on site in bins (bin sets) 1 . Savannah River and Hanford chose to neutralize their waste streams with an excess of NaOH to facilitate their storage in underground steel tanks. Once the pH was changed, a significant amount of insoluble