(327e) Solids in Hanford’s Low Activity Waste Pretreatment System Feed Tanks and Implications for Design Requirements

The Hanford Site has approximately 56 million gallons of nuclear waste generated from plutonium production. Much of this waste is highly concentrated electrolyte solutions of sodium-bearing salts that contain cesium-137 as the dominant radionuclide. The Hanford site is currently pursuing a Direct-Feed Low-Activity Waste treatment process (DFLAW). In the DFLAW process, any suspended solids are removed by filtration and the cesium is removed by ion-exchange in the Low-Activity Waste Pretreatment System (LAWPS), prior to being transferred to the Low Activity Waste Vitrification facility for immobilization. The vitrified glass product will be disposed of as low-activity waste.

A number of larger tanks (1.25 million gallon capacity) are used to prepare feed to the LAWPS. These large tanks contain accumulated solids in the bottom. The presence of solids in the tanks leads to a number of questions that must be answered in order to most efficiently feed LAWPS. The questions answered here are: What is the quantity of solids? What is the identity of the solids? How does the identity of the solids impact the filter cleaning strategy and the strategy of using temperature control to prevent salt precipitation? How do you minimize the amount of solids sent to LAWPS in order to maximize the throughput through the filtration system?

The quantity of solids in each tank was recorded using sludge weight measurements in each of the tanks, with some tanks having up to 112,000 gallons of solids. Even the feed tank (Tank 241-AP-107) had solids in the bottom. The identity of the solids was determined in waste samples by X-ray diffraction, polarized light microscopy, and scanning electron microscopy with energy dispersive spectroscopy. All tanks were found to have thermonatrite (Na₂CO₃*H₂O), sodium oxalate (Na₂C₂O₄), kogarkoite (Na₃FSO₄) and natrophosphate (Na₇F(PO₄)₂*19H₂O). All of these phases were salts, so it can be concluded the waste will be saturated with these salts prior to dilution. The Hanford Tank Farm Contractor intends to dilute these wastes prior to feeding them to the LAWPS, but if care is not taken during dilution, additional salt can dissolve, re-saturating the dilute supernate with these salts. Thus, the mixing process should be designed to minimize the mixing of the dilute supernate with the solids layer. The LAWPS feed tank, which is continuously recirculated during operation, should maintain the pump inlets and outlets as far away from the solids layer as possible while still maintaining sufficient volume in the tank for processing so that currents within the tank do not interact with the solids layer.

The solubility of three of the four salts increases with temperature, so their precipitation can be prevented in the LAWPS facility by slightly increasing the temperature of the incoming feed by 1 to 2 °C. The solubility of thermonatrite, however, decreases with increasing temperature, which means this salt could precipitate if it is near saturation and the temperature is increased. Nonetheless, a small temperature increase should not cause thermonatrite precipitation as long as the sodium molarity of the feed to LAWPS is maintained below 6 M; a value determined by evaluating the solubility of thermonatrite in aqueous solutions of the three most prevalent electrolytes in Hanford waste (NaOH, NaNO₂, and NaNO₃).