Saltstone Clean Cap Formulation

The current operation strategy for using Saltstone Vault 4 to receive 0.2 Ci/gallon salt solution waste involves pouring a clean grout layer over the radioactive grout prior to initiating pour into another cell. This will minimize the radiating surface area and reduce the dose rate at the vault and surrounding area. The Clean Cap will be used to shield about four feet of Saltstone poured into a Z-Area vault cell prior to moving to another cell. The minimum thickness of the Clean Cap layer will be determined by the cesium concentration and resulting dose levels and it is expected to be about one foot thick based on current calculations for 0.1 Ci Saltstone that is produced in the Saltstone process by stabilization of 0.2 Ci salt solution. This report documents experiments performed to identify a formulation for the Clean Cap. Thermal transient calculations, adiabatic temperature rise measurements, pour height, time between pour calculations and shielding calculations were beyond the scope and time limitations of this study. However, data required for shielding calculations (composition and specific gravity) are provided for shielding calculations. The approach used to design a Clean Cap formulation was to produce a slurry from the reference premix (10/45/45 weight percent cement/slag/fly ash) and domestic water that resembled as closely as possible the properties of the Saltstone slurry. In addition, options were investigated that may offer advantages such as less bleed water and less heat generation. The options with less bleed water required addition of dispersants. The options with lower heat contained more fly ash and less slag. A mix containing 10/45/45 weight percent cement/slag/fly ash with a water to premix ratio of 0.60 is recommended for the Clean Cap. Although this mix may generate more than 3 volume percent standing water (bleed water), it has rheological, mixing and flow properties that are similar to previously processed Saltstone. The recommended Clean Cap mix generates more bleed water than the reference Saltstone formulation because the specific gravity of water, the carrier fluid, is less than that of the carrier fluid in Saltstone, 1 versus 1.1 to 1.2, respectively. In addition, the development of slurry structure as a result of hydration reactions is slightly slower than in the salt solution slurry. In other words, the Clean Cap mix has a slightly longer gel time. The lower density of the carrier fluid and the slower development of slurry structure, enable more settling to occur (more standing water) in the Clean Cap slurry. Consequently, for the same rheological properties, the Clean Cap slurry will have more bleed water. In an attempt to reduce the bleed water, the water to premix ratio was lowered and dispersants (high range water reducers) were added. Below water to premix ratios of 0.35, little bleed water and settling was observed. However, a low water to premix Clean Cap mix is not recommended because processing has not been demonstrated in the Saltstone facility. The lowest water to premix ratio processed in Z-Area was 0.478 in the last attempt to produce a clean cap. Although this option may provide significant advantages (less bleed water and potentially better flow) process testing in the Saltstone Facility or in a pilot scale facility in conjunction with laboratory testing will be required to demonstrate mixing, pumping and flow properties. Other additives were tested to minimize bleed water. These additives were found to be unsatisfactory in one or more ways and therefore, were not recommended at this time. An air entraining agent and a thickener had some benefit in reducing bleed water but were found too difficult to implement as an additive in the Saltstone facility. Surfactants (air entrainers) added to the mixing water in the hold tank could generate foam as the result of agitation to mix the tank, and the thickener increased the apparent viscosity and yield stress