Investigation of variable compositions on the removal of technetium from Hanford Waste Treatment Plant low activity waste melter off-gas condensate simulant

Abstract

The Low Activity Waste (LAW) vitrification facility at the Hanford Waste Treatment and Immobilization Plant (WTP) will generate an aqueous condensate recycle stream (LAW Off-Gas Condensate) from the offgas system. The plan for disposition of this stream during baseline operations is to send it to the WTP Pretreatment Facility, where it will be blended with LAW, concentrated by evaporation and recycled to the LAW vitrification facility again. The primary reason to recycle this stream is so that the semi-volatile 99 Tc isotope eventually becomes incorporated into the glass. This stream also contains non-radioactive salt components that are problematic in the melter, so diversion of this stream to another process would eliminate recycling of these salts and would enable simplified operation of the LAW melter and the Pretreatment Facilities. This diversion from recycling this stream within WTP would have the effect of decreasing the LAW vitrification mission duration and quantity of glass waste. The concept being tested here involves removing the 99 Tc so that the decontaminated aqueous stream, with the problematic salts, can be disposed elsewhere. Technetium will not be removed from the aqueous tank waste during pretreatment in the Hanford WTP and will be sent to the LAW melter. It is intended that 99 Tc will be immobilized in the LAW glass. Because it is semi-volatile at melter temperatures and roughly 70% vaporizes, the only way to get it to stay in the glass is by repeated recycle into the LAW melter. Although other radionuclides are expected to be present in low concentration in the LAW Off-Gas Condensate, such as 129 I, 90 Sr, 137 Cs, 241 Pu, and 241 Am, it is the longlived and environmentally mobile 99 Tc that is the primary component of concern. This LAW Off-Gas Condensate stream originates from the Submerged Bed Scrubber (SBS) and the Wet Electrostatic Precipitator (WESP) from the LAW melter off-gas system. Pilot simulant tests indicate that this stream is expected to be a dilute salt solution with near neutral pH, and will likely contain some insoluble solids from melter carryover. The soluble salt components are expected to be mostly sodium and ammonium salts of nitrate, chloride, and fluoride. Although this stream has not yet been generated and will not be available until the WTP begins operation, a simulant has been produced based on models, calculations, and comparison with pilot-scale tests. SRNL-STI-2017-00087 Revision 0 vi that Hg(II) is reduced and removed from the aqueous phase upon addition of Sn(II). Conversely, no reaction was observed with nitrite ion. It was also observed that only trace amounts of hydrogen were generated in the reaction, even with a large excess of SnCl 2 , which will avoid flammable gas operational issues. Additionally, tests showed that perrhenate ion (ReO 4-) is not a suitable surrogate for pertechnetate, as it is not reduced by Sn(II) in this simulant. Another component of this program is to begin to mature the technology readiness of this process. A key component of that...

Bench scale experiments for the remediation of Hanford Waste Treatment Plant low activity waste melter off-gas condensate

“…8 A total of 5.0 L of non-radioactive simulant was prepared in three batches (two 2-L batches and one 1-L batch) for this bench scale testing and was also combined with approximately 500 mL of simulant of the same composition prepared previously. 18 The simulant was prepared from dissolution of laboratory chemicals in deionized water, in the order shown in Table 2-1. Because the HTWOS model is not constrained to generate a charge-balanced composition, no formulation can match all component concentrations simultaneously, and the chemical formulation must balance between cations and anions to create a mixture that can actually be synthesized.…”

“…After pH adjustment, all three batches of simulant were filtered through a 0.45-micron Nylon filter to remove any insoluble material and were then combined. The remaining ~500 mL of simulant from a previous preparation 18 was also added to the freshly prepared 5 L prior to removing samples for analysis. Duplicate samples were analyzed for elemental composition by Inductively Coupled Plasma -Emission Spectroscopy (ICP-ES), anions and ammonium by Ion Chromatography (IC).…”

Evaluation of Immobilizing Secondary Waste from a Proposed Treatment Process for Hanford WTP LAW Melter Condensate

“…To enable diversion, a process 4 is being developed by the Savannah River National Laboratory (SRNL) to remove the 99 Tc so that the decontaminated aqueous stream, with the problematic halide and sulfate salts, can be disposed elsewhere (Figure 1-2). Previous lab-scale testing [5][6][7][8][9][10] has shown SnCl 2 to be an effective agent for the 99 Tc removal through reductive precipitation. The removal is believed to work by reducing the Tc(VII) ion in the soluble pertechnetate (TcO 4 -) to Tc(IV), leading to precipitation of technetium dioxide (TcO 2 ).…”