Executive Summary Radioactive waste that is currently stored in large underground tanks at the Hanford Site will be staged in selected double-shell tanks (DSTs) and then transferred to the Waste Treatment and Immobilization Plant (WTP). Before being transferred, the waste will be mixed, sampled, and characterized to determine if the waste composition meets the waste feed specifications. Washington River Protection Solutions (WRPS) is conducting a Tank Mixing and Sampling Demonstration Program

more »

... determine the mixing effectiveness of the current baseline mixing system that uses two jet mixer pumps to mobilize and mix waste in DSTs and to determine the adequacy of the planned sampling method. The overall purpose of the demonstration program is to mitigate the technical risk associated with the mixing and sampling systems meeting the feed certification requirements for transferring waste to the WTP. A critical aspect of meeting the feed certification requirements is ensuring that the collected samples will adequately represent the contents of the tank and that all batches of transferred waste are sufficiently uniform so that they all are equivalent to the characterized samples. However, the tank waste contains slurries of solid particles that settle and may become progressively more concentrated towards the bottom of the tank. Obtaining a perfectly uniform distribution of waste in the DSTs with a mixing system may not be possible. To determine how close this ideal can be approached, the expected performance of the DST mixing and sampling systems must be quantified. There have been a number of laboratory studies and full-scale tank farm studies that evaluated mixing behavior; these studies are discussed later in this report. However, these studies do not provide adequate quantitative information to evaluate the performance of the planned sampling and the batch uniformity. Accordingly, WRPS' demonstration program is focusing specifically on quantifying the uniformity of samples and batch transferred material to reduce the technical risk associated with these systems. The first phase of the demonstration program is to conduct scaled tests that appropriately match fullscale behavior and to evaluate sampling methodologies and the uniformity of batch transfers. The initial scaled testing will use non-cohesive particles where the particles do not have cohesive interactions that would cause the particles to stick together. The purpose of this report is to analyze existing data and evaluate whether scaled mixing tests with cohesive simulants are needed to meet the overall objectives of the small-scale mixing demonstration program. This evaluation will focus on estimating the role of cohesive particle interactions on various physical phenomena that occur in parts of the mixing process. A specific focus of the evaluation will be on the uniformity of suspended solids in the mixed region. The evaluation shows that cohesive particle interaction will have multiple effects through a number of different mechanisms. Some of the effects improve solids uniformity while others will likely degrade uniformity. An overall assessment was made that estimated the combined behavior. A very striking and consistent observation from all the tank farm and scaled test data is that during jet mixing operations, the waste particles segregate into an upper mixed region of an essentially constant concentration that is independent of elevation and a region at the bottom of the tank that contains a higher concentration of particles. Tank farm and scaled-test data suggest that a substantial fraction of the waste solids is not lifted above the bottom region of the tank but remains towards the bottom as a stratified layer. The evaluation suggests that the solids concentration in this layer is sufficiently high that the slurry will be non-Newtonian with a small but still significant yield stress. The non-Newtonian behavior and yield stress are iv thought to be caused primarily by cohesive particle interactions. Studies have shown that even a small yield stress reduces the jet momentum in the fluid at a distance from the jet and thus reduces the ability of the jet to mobilize solids. It is expected that any significant shear thinning behavior in a non-Newtonian fluid will have the same effect as a fluid having a yield stress, which is a specific type of shear thinning fluid. Accordingly, in the expected stratified condition of a jet-mixed tank, cohesive particle interactions will reduce the ability of the jet to lift particles into the upper region of the tank. The conclusion of this overall assessment is that stratification will likely occur, and cohesive particle interactions will reduce the fraction of particles lifted into the upper region of the tank. The cohesive particle interactions do not, on their own, cause the stratified layer and reduced particle lifting to occur, but the cohesive interactions if present will accentuate this behavior although the magnitude of the impact is difficult to estimate. Hence, the conclusion is that testing with only non-cohesive particles will create technical uncertainty in meeting the objectives of the Tank Mixing and Sampling Demonstration Program. The evaluation also shows that for the particles that are lifted into the upper region of the tank, the total particle concentration and the relative amounts of different size particles are extremely uniform. However, in comparison to the overall concentration in the tank and the initial relative amounts of particles of different size, the material in the upper region is clearly different. Finally, this study conducted an extensive review of the literature on the uniformity of suspended solids in jet mixed tanks. There are no existing studies or data that specifically quantify the role of cohesive interactions on uniformity. The absence of definitive data further suggests that at least some testing with cohesive particles as part of the Tank Mixing and Sampling Demonstration Program will be needed to determine the impact of the cohesive particle interactions. Based on the evaluation presented in this report and the absence of definitive studies, the recommendation is to conduct scoping tests to determine the magnitude of the impact caused by cohesive particle interactions on mixing. If the impact is determined to be significant, the recommendation is to conduct quantitative mixing tests at multiple scales with cohesive particles to augment the initial testing with non-cohesive particles. If initial testing can demonstrate that the impact of cohesive interactions is small over the range of important test conditions, then extensive scaled testing should not be necessary. v Acronyms and Abbreviations ADMP advanced design mixer pump CFD computational fluid dynamics DOE