Analysis of the Leaching Efficiency of Inhibited Water and Tank Simulant in Removing Residues on Thermowell Pipes

Abstract

A cleaning protocol was determined and modeled, using the shrinking core model, for the removal of a solid residue coating thermo well pipes contained in the riser of a Savannah River Site (SRS) waste tank (Tank 48H). The solid residues on two sets of thermo well pipe samples removed from the D2 riser in SRS Tank 48H were characterized by high performance liquid chromatography (HPLC), ion chromatography (IC) and gamma spectroscopy. The residue thickness was determined using the ASTM standard D 3483-05 and was found to be three orders of magnitudes below the 1 mm thickness estimated from an earlier video of the tank cooling coil inspection. The actual estimated thickness ranged from 4 to 20.4 microns. The mass per unit area ranged from 0.15 to 0.82 milligrams per square centimeter. The residues appear to consist primarily of potassium tetraphenylborate (39.8 wt% KTPB) and dried salt solution (33.5 wt% total of nitrates, nitrites, and oxalate salts), although ∼30% of the solid mass was not accounted for in the mass balance. No evidence of residue buildup was found inside the pipe, as expected. Two aqueous solutions representing waste processing streams were chosen for the cleaning trials. The residue leaching characteristics were measured by placing one pipe in inhibited water (0.01 M NaNO2 and 0.01 M NaOH) and one pipe in Defense Waste Processing Facility (DWPF) Recycle simulant (primarily 0.20 M NaNO2 and 0.18 M NaOH). After soaking for less than 4 weeks, the inhibited water was 95.4% effective at removing the residue and the DWPF Recycle simulant was 93.5% effective. The surface appearance of the pipes after leaching tests appeared close to the clean shiny appearance of a new pipe. Total gamma counts of leachates averaged 48.1 dpm/mL, or an equivalent of 2.35E-11 Ci/gm Cs-137 (dry solids basis), which is much lower than the 1.4 E-03 Ci/gm expected for Tank 48H dry slurry solids.

Keywords:

• shrinking model
• tetraphenylborate

Notes

*This calculation assumes a density of 0.38 g/ml (7). D. D. Walker, “Combustibility of Tetraphenylborate,” WSRC-RP-89-261, May 1989. The density of salt crystals is somewhat higher for sodium nitrate is around 2.26 g/mL. Depending on porosity and salt fraction, the density of the residue may be higher than 0.38 g/mL For total mass calculation, mass/area is a better parameter.

^#Ranges were calculated per Ref. 10 methodology.