Validating CFD Models of Multiphase Mixing in the Waste Treatment Plant at the Hanford Site

Abstract

The Columbia River in Washington State is threatened by the radioactive legacy of the cold war. Two hundred thousand cubic meters (fifty-three million US gallons) of radioactive waste is stored in 177 underground tanks (60% of the Nation’s radioactive waste). A vast complex of waste treatment facilities is being built to convert this waste into stable glass (vitrification). The waste in these underground tanks is a combination of sludge, slurry, and liquid. The waste will be transported to a pre-treatment facility where it will be processed before vitrification. It is necessary to keep the solids in suspension during processing. The mixing devices selected for this task are known as pulse-jet mixers (PJMs). PJMs cyclically empty and refill with the contents of the vessel to keep it mixed. The transient operation of the PJMs has been proven successful in a number of applications, but needs additional evaluation to be proven effective for the slurries and requirements at the Waste Treatment Plant (WTP). Computational fluid dynamic (CFD) models of mixing vessels have been developed to demonstrate the ability of the PJMs to meet mixing criteria. Experimental studies have been performed to validate these models. These tests show good agreement with the transient multiphase CFD models developed for this engineering challenge.

CFD Evaluation of Mixing Processes for High-Level Nuclear Waste Tanks

“…Similarly, jet impingement is widely seen in environmental engineering processes such as rotorcraft brownout, soil erosion and sediment transport, pollutant discharge in rivers and thunderstorm microburst winds( [4], [5]). Another frequently seen practical application of orthogonally impinging jets is in the assessment of mixing performance of two miscible fluids in pulse jet mixing devices in the Waste Treatment and Immobilization Plant [6]. Furthermore, the problem of normal impinging jets is considered to be a standard test case for the development of turbulence models.…”

“…The mechanism of mixing process is shown schematically on the left of Figure 1, in-which, the mixing efficiency depends on the local radial wall jet velocity and spreading rate of jet. In practice, the controlling parameters in PJM vessel design rely on Poreh's correlations [6]. However, the PJM is operated at a "small" impingement height ratio, ≤ 1.5 ⁄ with slightly confined conditions and the impingement is not perfectly orthogonal.…”