Evaluation of Technetium Getters to Improve the Performance of Cast Stone

Technical Report · Sun Nov 01 00:00:00 EDT 2015

DOI:https://doi.org/10.2172/1228352· OSTI ID:1228352

Neeway, James J. ^[1]; Qafoku, Nikolla P. ^[1]; Serne, R. Jeffrey ^[1]; Lawter, Amanda R. ^[1]; Stephenson, John R. ^[1]; Lukens, Wayne W. ^[1]; Westsik, Joseph H. ^[1]

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1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

Cast Stone has been selected as the preferred waste form for solidification of aqueous secondary liquid effluents from the Hanford Tank Waste Treatment and Immobilization Plant (WTP) process condensates and low-activity waste (LAW) melter off-gas caustic scrubber effluents. Cast Stone is also being evaluated as a supplemental immobilization technology to provide the necessary LAW treatment capacity to complete the Hanford tank waste cleanup mission in a timely and cost effective manner. One of the major radionuclides that Cast Stone has the potential to immobilize is technetium (Tc). The mechanism for immobilization is through the reduction of the highly mobile Tc(VII) species to the less mobile Tc(IV) species by the blast furnace slag (BFS) used in the Cast Stone formulation. Technetium immobilization through this method would be beneficial because Tc is one of the most difficult contaminants to address at the U.S. Department of Energy (DOE) Hanford Site due to its complex chemical behavior in tank waste, limited incorporation in mid- to high-temperature immobilization processes (vitrification, steam reformation, etc.), and high mobility in subsurface environments. In fact, the Tank Closure and Waste Management Environmental Impact Statement for the Hanford Site, Richland, Washington (TC&WM EIS) identifies technetium-99 (⁹⁹Tc) as one of the radioactive tank waste components contributing the most to the environmental impact associated with the cleanup of the Hanford Site. The TC&WM EIS, along with an earlier supplemental waste-form risk assessment, used a diffusion-limited release model to estimate the release of different contaminants from the WTP process waste forms. In both of these predictive modeling exercises, where effective diffusivities based on grout performance data available at the time, groundwater at the 100-m down-gradient well exceeded the allowable maximum permissible concentrations for ⁹⁹Tc. (900 pCi/L). Recent relatively short-term (63 day) leach tests conducted on both LAW and secondary waste Cast Stone monoliths indicated that ⁹⁹Tc diffusivities were at or near diffusivities where the groundwater at the 100-m down-gradient well would exceed the allowable maximum permissible ⁹⁹Tc concentrations. There is, therefore, a need and an opportunity to improve the retention of Tc in the Cast Stone waste form. One method to improve the performance of the Cast Stone waste form is through the addition of “getters” that selectively sequester Tc inside Cast Stone.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1228352
• Report Number(s): PNNL-23667 Rev. 1; EMSP-RPT-026 Rev1; EY4049110; TRN: US1600027
• Country of Publication: United States
• Language: English

Related Subjects

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
TECHNETIUM 99
WASTE FORMS
LIQUID WASTES
HANFORD RESERVATION
WASTE PROCESSING
TANKS
GROUND WATER
MAXIMUM PERMISSIBLE CONCENTRATION
PERFORMANCE
TEMPERATURE RANGE 0400-1000 K
GETTERS
EVALUATION
LEACHING
REDUCTION
SOLIDIFICATION
UNDERGROUND
ENVIRONMENTAL IMPACTS
ABUNDANCE
DIFFUSION
GROUTING
MOBILITY
RETENTION
SIMULATION
SLAGS
RADIONUCLIDE MIGRATION
VITRIFICATION
GASEOUS WASTES
LOW-LEVEL RADIOACTIVE WASTES
Cast stone