Evolution of Technetium Speciation in Reducing Grout

摘要

Cementitious waste forms (CWFs) are an important component of the strategy to stabilize nuclear waste resulting from plutonium production by the U.S.Department of Energy.Technetium (~(99)Tc) is an abundant fission product of particular concern in CWFs because of the high solubility and mobility of Tc(VII),pertechnetate (TcO_4~-),the stable form of technetium in aerobic environments.CWFs can more effectively stabilize "Tc if they contain additives that chemically reduce mobile TcO_4~- to immobile Tc(IV) species.The "Tc leach rate of reducing CWFs that contain Tc(IV) is much lower than that for CWFs that contain Tc0)4~-.Previous X-ray absorption fine structure studies showed that Tc(IV) species were oxidized to TcO_4~-in reducing grout samples prepared on a laboratory scale.Whether the oxidizer was atmospheric 0_2 or N0_3~-in the waste simulant was not determined.In actual CWFs,rapid oxidation of Tc(IV) by NO_3~- would be of concern,whereas oxidation by atmospheric 0_2 would be of less concern due to the slow diffusion and reaction of 0_2 with the reducing CWF.To address this uncertainty,two series of reducing grouts were prepared using TcO_4~- containing waste simulants with and without NO_3~-.In the first series of samples,referred to as "permeable samples",the TcO_4~-was completely reduced using Na_2S,and the samples were sealed in cuvettes made of polystyrene,which has a relatively large 0_2 diffusion coefficient.In these samples,all of the technetium was initially present as a Tc(IV) sulfide compound,TcS_x,which was characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy.The EXAFS data is consistent with a structure consisting of triangular clusters of Tc(IV) centers linked together through a combination of disulfide and sulfide bridges as in MoS_3.From the EXAFS model,the stoichiometry of TcS_x is Tc_3S_(10),which is presumably the compound generally referred to as "Tc_2S_7".The TcS_x initially present in the permeable samples was steadily oxidized over 4 years.In the second series of samples,called "impermeable samples",the Tc0_4~- was not initially completely reduced,and the grout samples were sealed in cuvettes made of poly-(methyl methacrylate),which has a small 0_2 diffusion coefficient.In the impermeable samples,the remaining TcO_4~- continued to be reduced,presumably by blastfurnace slag in the grout,as the samples aged.When the impermeable samples were opened and exposed to atmosphere,the lower-valent technetium species were rapidly oxidized to TcO_4~-.