Numerical Modeling of Mixing of Chemically Reacting, Non- Newtonian Slurry for Tank Waste Retrieval

Abstract

(7) Number of Graduate Students Actively Involved in the Project: 1 (8) Research Objectives: Many highly radioactive wastes will be retrieved by installing mixer pumps that inject high-speed jets to stir up the sludge, saltcake, and supernatant liquid in the tank, blending them into a slurry. This slurry will then be pumped out of the tank into a waste treatment facility. Our objectives are to investigate interactions-chemical reactions, waste rheology, and slurry mixing-occurring during the retrieval operation and to provide a scientific basis for the waste retrieval decision-making process. Specific objectives are to• Evaluate numerical modeling of chemically active, non-Newtonian tank waste mixing, coupled with chemical reactions and realistic rheology • Conduct numerical modeling analysis of local and global mixing of non-Newtonian and Newtonian slurries • Provide the bases to develop a scientifically justifiable, decision-making support tool for the tank waste retrieval operation. (9) Research Progress and Implications: This report summarizes work done in 1.5 years of 3-year project. (9.1) Yield strength of the waste: The chemically reactive, computational hydrodynamic code, TEMPEST, was modified to account for the yield strength of non-Newtonian tank waste sludge and saltcake that resist mobilization by mixer pump jets. The modified code was tested to model mobilization of sludge with and without yield strength. Test results showed that the 18-m/s slurry pump jets burrow into the sludge bank and erode only portions of the sludge where the shear and normal stresses acting on the sludge are greater than or equal to the sludge's yield strength. This modeling capability was used to assess pump jet mixing for Hanford Tank 401-AZ-102 and is now assisting in tank waste mixing test plans at Hanford. Once validated with the results of the Tank 401-AZ-101 mixer pump test, this added modeling capability could be used to determine how many pumps are needed to mobilize the tank waste, and, with a given number of pumps, how much and how fast tank waste will be mixed. (9.2) Examine 2-D reactive transport simulation of tank waste: During mixing, the waste can undergo aqueous and solid dissolution/precipitation reactions. This phenomenon is being examined under four twodimensional pump jet mixing setups: Newtonian and non-Newtonian wastes with and without chemical reactions having NaNO 3 (s) and NaNO 2 (s) dissolution during mixing. As shown in Figure 1, the study indicated that in addition to Newtonian and non-Newtonian rheology differences, dissolution of these solids changed the amount of the solids in the tank and the waste's physical properties and rheology, further affecting waste mixing.