Evaluation of Technetium Getters to Improve the Performance of Cast Stone

Neeway, James J.¹  Qafoku, Nikolla P.¹  Serne, R. Jeffrey¹  Lawter, Amanda R.¹  Stephenson, John R.¹  Lukens, Wayne W.¹  Westsik, Joseph H.¹ 

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

关键词: 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;   

DOI  :  10.2172/1228352
RP-ID  :  PNNL--23667 Rev. 1
RP-ID  :  EMSP-RPT--026 Rev1
PID  :  OSTI ID: 1228352
Others  :  Other: EY4049110
Others  :  TRN: US1600027

学科分类：核能源与工程

美国|英语

来源: SciTech Connect

【 摘 要 】

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.