Preparation and Properties of Hydroceramic Waste Forms Made with Simulated Hanford Low‐Activity Waste

Abstract

Approximately 85 million gallons of high‐level waste (HLW) is currently stored in underground tanks at the Hanford Reservation and the Savannah River Site (SRS). The waste consists of a hydroxide‐rich precipitate (sludge) and a sodium‐rich supernate. The supernate is a NaOH rich solution containing lesser amounts of NaNO3 and NaNO2 and small amounts of soluble fission products, cladding materials, and organics (volatile organics and semi‐volatile organics known as VOCs and SVOCs). The Department of Energy (DOE) has chosen glass as its waste form for both sludge and sodium‐rich supernate. However, because of the volume of the supernate, alternatives to vitrification are being sought for some of this waste. One alternative is to remove 137Cs and 90Sr from the supernate. Decontaminating the waste in this way allows the waste to be designated as low‐activity waste (LAW) and as such the waste now becomes eligible for solidification and disposal on site. SRS is solidifying its LAW with a blended Portland cement forming Saltstone. Hanford has been considering a bulk vitrification process in which the LAW will be mixed with Hanford soil and vitrified in place in a disposable carbon‐arc powered glass melter/waste container. Both waste forms can then be buried on site in appropriate vaults or low‐level waste land fills. A hydroceramic is an alternative waste form designed to solidify and stabilize LAW that is made from metakaolin plus NaOH and/or NaOH rich LAW supernate. In addition to NaOH, LAW can contain a wide range of sodium nitrate and sodium nitrite concentrations. Although a hydroceramic waste form can be made directly from some types of decontaminated waste, e.g., those that are highly alkaline (8–12M NaOH) and contain less than 25 mol% of NOx (NOx is used as the short‐note for nitrates and nitrites in this article.) relative to the total Na in the waste, by simply mixing the LAW with metakaolin and curing the resulting paste at 90°C, the remaining LAW, especially that stored at Hanford must be pretreated in some way before it can be similarly solidified; the relative molar proportion of NOx/Na must be reduced to 25% or less. In this paper calcination is evaluated as a potential pretreatment method for Hanford AN‐107 (AN‐107 is a waste storage tank on Hanford site) LAW, but in choosing this method it is necessary to divide the preparation of the hydroceramic waste form into two steps: denitration/denitrition of the liquid waste stream to produce a granular calcine followed by solidification using a metakaolin plus 4M NaOH binder. A simulated Hanford AN‐107 LAW was calcined at 375°, 450°, 525°, 600°, and 675°C in the presence of sucrose and metakaolin added as a calcination aid. It was shown that the leachability of the calcines decreased as calcination temperature increased, i.e., the waste form became more crystalline. In the second step, each of the granular calcines was mixed with additional metakaolin and just enough 4M NaOH to form a thick paste. The paste was precured at 40°C and then autoclaved at 90°C to form a monolith. X‐ray diffraction and scanning electron microscopy characterization showed that the calcines themselves contained an amorphous phase and crystalline hydroxysodalite, and that the hydroceramics made from these calcines plus additional metakaolin/NaOH binder consisted predominantly of zeolite A and hydroxysodalite. The temperature used to prepare the calcines not only affected the properties of the calcines, but those of the monolithic hydroceramics as well. Experimental results demonstrated that 525°C represented the optimal temperature for producing the most suitable calcine for subsequent solidification with metakaolin and 4M NaOH. The resulting hydroceramic nuclear waste form was strong and had the lowest overall leachability. The leachability of the hydroceramic is normally lower than that of the corresponding calcines up to ∼600°C. The product consistency test (PCT) determined normalized release rate NRNa for the hydroceramic (0.14 g/m2·day) was comparable to similar leach rates determined for Hanford's low‐activity waste reference material glass (0.08 g/m2·day) and a steam reformed calcine made with Hanford's AN‐107 tank waste (0.25 g/m2·day).