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

Abstract

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 0). 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).The synthesized gels, upon transfer, regained and retained their cohesiveness and had sufficient adhesive characteristics to stick to the walls of the inverted glass containers. The bulk densities and shear strengths of the gels ranged from 1.35 to 1.55 g/cm 3 and from ~25 to 80 kPa, respectively. The free water contents of these gels ranged from 31.5 to 38.7%. The gels remained amorphous after about 1 week of aging. Crystalline nitratine was present in the NAH and NAS gels but was not detected in the NAP gel.While both solvents, namely 2 molar (2 M) nitric acid (HNO 3 ) 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 effective at dissolving the NAP plug. In the 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 times) than 2 M sodium hydroxide. Unless there are operational benefits for the use of caustic verses nitric acid, 2 M nitric acid heated to 60°C should be the solvent of choice for dissolving these chemical plugs.For aged (~ 6 months) NAP and NAH plugs, no significant differences in either color or surface morphology were detected between the fresh and aged gels. Compared to the fresh gel, the aged NAS gel surface was entirely covered with small transparent glistening crystals likely to be nitratine.The aged NAS and NAP gel plugs required <150 psi pressure to initiate the extrusion from the pipe sections, whereas, the NAH plug required a minimal (<10 psi) pressure to start the extrusion process. The shear strengths of NAS plug sections extruded from the bottom of the pipe were greater than about 15 kPa, whereas the shear strengths of plug sections in the upper and middle part of the pipe ranged from approximately 9 to 13 kPa. The NAH plug sections had relatively lower shear strength values that ranged from ~2 to ~6 kPa, and the NAP plug sections were too pliable for UCS measurements with a pocket penetrometer.X-ray diffraction data indicated that the sections of the aged NAS plug contained nitratine as a crystalline component. The bulk of the aged gel mass consisted of amorphous material. The aged NAP plug consisted of amorphous...