Preparation and Characterization of Chemical Plugs Based on Selected Hanford Waste Simulants

This report presents the results of preparation and characterization of chemical plugs based on selected Hanford Site waste simulants. Included are the results of chemical plug bench testing conducted in support of the M1/M6 Flow Loop Chemical Plugging/Unplugging Test (TP-RPP-WTP-495 Rev A). These results support the proposed plug simulants for the chemical plugging/ unplugging tests. Based on the available simulant data, a set of simulants was identified that would likely result in chemical plugs. The three types of chemical plugs that were generated and tested in this task consisted of: 1. Aluminum hydroxide (NAH), 2. Sodium aluminosilicate (NAS), and 3. Sodium aluminum phosphate (NAP). While both solvents, namely 2 molar (2 M) nitric acid (HNO3) and 2 M sodium hydroxide (NaOH) at 60°C, used in these tests were effective in dissolving the chemical plugs, the 2 M nitric acid was significantly more effective in dissolving the NAH and NAS plugs. The caustic was only slightly more effecting at dissolving the NAP plug. In the bench-scale dissolution tests, hot (60°C) 2 M nitric acid was the most effective solvent in that it completely dissolved both NAH and NAS chemical plugs much faster (1.5 – 2 x) than 2 M sodium hydroxide. So unless there are operational benefits for the use of caustic verses nitric acid, 2 M nitric acid heated to 60°C C should be the solvent of choice for dissolving these chemical plugs. Flow-loop testing was planned to identify a combination of parameters such as pressure, flush solution, composition, and temperature that would effectively dissolve and flush each type of chemical plug from preformed chemical plugs in 3-inch-diameter and 4-feet-long pipe sections. However, based on a review of the results of the bench-top tests and technical discussions, the Waste Treatment Plant (WTP) Research and Technology (R&T), Engineering and Mechanical Systems (EMS), and Operations concluded that flow-loop testing of the chemically plugged pipe sections would not provide any additional information or useful data. The decision was communicated through a Sub Contract Change Notice (SCN-070) that included a revised scope as follows: • Photographing the chemical plugs in the pipes before extrusion to compare the morphology of aged gels with that of fresh gels. • Setting up an extrusion apparatus and extruding the chemical plugs. • Documenting the qualitative observations on the efforts to remove the chemical plug materials from the pipe sections. • Performing X-ray diffraction (XRD) analysis of extruded gel samples to detect any crystallization of gel during storage. • Disposing of the extruded gel as a waste. • Documenting the analytical results in a test report. There were no significant morphological differences between the fresh and aged plugs except for an overgrowth of small transparent crystals on the surface of the aged NAS gel plug. An initial pressure of <150 psi was required to start extruding the aged NAS and NAP plugs, whereas the NAH plug began to extrude with the application of minimal pressure. The shear strength of extruded samples ranged from ~9 to >15 KPa for the NAS plug and from ~2 to 6 KPa for the NAH plug. Following extrusion, the NAP plug sections were thixotropic. The bulk of all the aged gel plugs consisted of amorphous material with nitratine constituting the crystalline phase. A separate question about the whether the current in-tank waste conditions will bound the future multi-tank blended feed conditions for the Waste Treatment Plant is outside the scope of this study