Evolution of Technetium Speciation in Reducing Grout

Abstract

Cementitious waste forms (CWFs) are an important component of the strategy to immobilize high-level nuclear waste resulting from plutonium production by the U.S. Department of Energy (DOE).Technetium ( 9 9 Tc) is an abundant fission product of particular concern in CWFs due to the high solubility and mobility of pertechnetate, TcO 4 -, the stable form of technetium in aerobic environments. CWFs can more effectively immobilize 9 9 Tc if they contain additives that reduce mobile TcO 4 -to immobile Tc(IV) species. Leaching of 9 9 Tc from reducing CWFs that contain Tc(IV) is much slower than for CWFs containing TcO 4 -. Previous X-ray absorption fine structure (XAFS) studies showed that he Tc(IV) species were oxidized to TcO 4 -in reducing grout samples prepared on a laboratory scale. Whether the oxidizer was atmospheric O 2 or NO 3 -in the waste simulant was not determined. In actual CWFs, rapid oxidation of Tc(IV) by NO 3 -would be a concern, whereas oxidation by atmospheric O 2 would be of less concern due to the slow diffusion and reaction of O 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, the TcO 4 -was completely reduced using Na 2 S, and the samples were placed in containers that permitted O 2 diffusion. In these samples, all of the technetium was initially present as a Tc(IV) sulfide compound, TcS x , which was characterized using extended X-ray absorption fine structure (EXAFS) 2 spectroscopy, and is likely Tc 2 S 7 . The TcS x initially present in the grout samples was steadily oxidized over 4 years. In the second series of samples, all of the TcO 4 -was not initially reduced, and the grout samples were placed in airtight containers. In these samples, the remaining TcO 4 -continued to be reduced as the samples aged, presumably due to the presence of reducing blast furnace slag. When samples in the second series were exposed to atmosphere, the lower-valent technetium species were rapidly oxidized to TcO 4 -.KEYWORDS technetium, cement, grout, nuclear waste BRIEFS The speciation of technetium in reducing grout samples was followed using X-ray absorption fine structure spectroscopy. The reduced technetium species were susceptible to oxidation by oxygen, but nitrate had no observable effect over the duration of the experiment. MANUSCRIPT TEXTIntroduction Immobilization of the high-level nuclear waste created during plutonium production by

Experimental Study of Contaminant Release from Reducing Grout

“…In preparing the second batch, the following were added to 4 . The contaminant-bearing simulated saltstone was used in the column leaching experiment, whereas the uncontaminated simulated saltstone was used for grout material characterization.…”

“…Simulated SRS groundwater was prepared by diluting by a factor of 100 a stock solution made from reagent grade NaCl, KCl, CaSO 4 [12], the chemistry of the column effluent solution is expected to be dominated by its interaction with the grout material and contaminant release from the grout would not be expected to change significantly if other SRS groundwater compositions were used. The solution was not deaerated and was assumed to be O 2 -saturated.…”

“…Studies have shown that grouted systems containing blast furnace slag are effective in mitigating the release of Tc-99 [1][2]. Hydration of the blast furnace slag in the grout mixture releases sulfide species into the pore fluid, predominantly as S 2-, which can impose a strongly reducing redox potential (Eh) on the system [3] and can react with technetium to form relatively insoluble Tc 3 S 10 [4] or Tc 2 S 7 [5]. In cementitious systems, U(IV) is less soluble than U(VI) [6] and Se(IV) (as the selenite ion, SeO 3 2 ) is less mobile than Se(VI) (as the selenate ion, SeO 4 2 ) [7].…”

Technetium-99m Transport and Immobilisation in Porous Media: Development of a Novel Nuclear Imaging Technique

“…This work will aim to determine the effects of transport on 99 Tc sorption using a cement composition that is known to strongly sorb Tc(VII): an OPC Blast Furnace Slag (BFS) mixed cement (e.g. [9,10]). This cement produces reducing conditions (Eh of ~-300 mV [11]) as it contains high concentrations of sulphide, and has been shown to form insoluble Tc(IV)-S phases [9,10,12].…”

“…[9,10]). This cement produces reducing conditions (Eh of ~-300 mV [11]) as it contains high concentrations of sulphide, and has been shown to form insoluble Tc(IV)-S phases [9,10,12].…”