Technetium Inventory, Distribution, and Speciation in Hanford Tanks (Rev.1)

Abstract

This report was revised in September 2014 to add detail and correct inaccuracies in Section 5.0 on the fate of technetium (Tc) recycle from the off-gas systems downstream of the low-activity waste (LAW) melters back to the melters, based on several reports that were not found in the original literature search on the topic. The newly provided reports, from experts active in the Hanford Tank Waste Treatment and Immobilization Plant (WTP) glass studies, the Vitreous State Laboratory at The Catholic University of America (VSL) melter and off-gas system demonstrations, and overall WTP systems analysis, were not originally found on electronic databases commonly searched. The major revisions to Section 5.0 also required changes to Section 7.0 (Summary and Conclusions) and this executive summary. The purpose of this report is threefold: 1) assemble the available information regarding Tc inventory, distribution between phases, and speciation in Hanford's 177 storage tanks into a single, detailed, comprehensive assessment; 2) discuss the fate (distribution/speciation) of Tc once retrieved from the storage tanks and processed into final waste forms; and 3) discuss/document in less detail the available data on the inventory of Tc in other "pools" such as the vadose zone below inactive cribs and trenches, below single-shell tanks (SSTs) that have leaked, and in the groundwater below the Hanford Site. A thorough understanding of the inventory for mobile contaminants is key to any performance or risk assessment for Hanford Site facilities because potential groundwater and river contamination levels are proportional to the amount of contaminants disposed at the Hanford Site. Because the majority of the total 99 Tc produced at Hanford (~3.26 × 10 4 × 10 4 Ci) is currently stored in Hanford's 177 tanks (~2.65 × 10 4 Ci), there is a need to understand the fate of this 99 Tc as it is removed from the tanks and processed into final solid waste forms. Current flowsheets for the WTP process show most of the 99 Tc will be immobilized as LAW form(s), with glass being the current baseline LAW waste form. The LAW that cannot be accommodated in the current LAW Facility may be vitrified in a second LAW vitrification facility that would be built, or a supplemental solidification process may be chosen. A third alternative being investigated is to increase the throughput of the WTP LAW Facility such that it can process the entire LAW inventory. Whatever approach is taken, the current baseline is that solid LAW waste forms will remain on the Hanford Site and be disposed at the Integrated Disposal Facility (IDF). Only a small fraction of Hanford's 99 Tc will be shipped to a geologic repository within the immobilized high-level waste. Past performance assessment (PA) studies, which focused on groundwater protection, have shown that 99 Tc within various waste forms disposed in the IDF would be the primary dose contributor to the IDF performance. It also should be noted-based on the 2001 IDF PA (Mann et al. 2001), the Supplemental ILAW risk assessme...

Spectroscopic Properties of Tc(I) Tricarbonyl Species Relevant to the Hanford Tank Waste

“…It can potentially provide useful structural information to compliment NMR technique. There is no precedent of IR measurements of the actual tank waste, however it remains to be attractive techniques due to its simplicity and availability (Serne et al 2014).…”

Options for the Separation and Immobilization of Technetium

“…One obvious advantage of the disposition of Tc in the metal alloy waste form is the significant reduction of the generated waste form volume, which leads to a reduction of the required storage facility footprint. This advantage is explained by the fact that the total Tc currently stored in Hanford's tanks is approximately 26,500 Ci (Serne et al, 2014) or 1560 kg. This total Tc inventory can be stabilized in the estimated metal alloy waste form volume of just 1 to 4 m 3 , depending on the Tc loading.…”

Synthesis and Characterization of Tc(I) Carbonyl Nitrosyl Species Relevant to the Hanford Tank Waste: FY 2016 Status Report

“…Soluble Tc in the saltcake and supernatant fractions of both single-shell tanks (SSTs) and double-shell tanks (DSTs) constitutes most of the Tc at the Hanford Site. The DSTs contain 57% (1.5110 4 Ci) of the total Tc inventory (2.65 10 4 Ci) in Hanford tanks, with the bulk (1.33 10 4 Ci) being in four DST farms (AP, AN, AW, and AZ) in the 200-E Area (Serne et al 2014). In strongly alkaline environments, Tc exists as pertechnetate, TcO 4 -(oxidation state +7), and in the reduced forms (oxidation state < +7) collectively known as non-pertechnetate species.…”

“…This makes the generation of NO + coordinated Tc(I) dicarbonyl nitrosyl species [Tc(CO) 2 (NO)] 2+ and formation of their congeners a distinct possibility. To this regard, a literature review report "Technetium Inventory, Distribution, and Speciation in Hanford Tanks" (Serne et al 2014) states that based on all investigations and detailed measurements, non-pertechnetate species in the Hanford DST supernates are tentatively identified as Tc(I) carbonyl complexes derived from either [Tc(CO) 3 ] + or [Tc(CO) 2 (NO)] 2+ precursor molecules that may lead to final species such as [Tc(CO) 3 ] + •gluconate as a result of the radiolytic decomposition of organics and nitrite in the DSTs.…”