Electrochemical reduction removal of technetium-99 from Hanford tank wastes

Abstract

DISCLAIMER DISCLAIMERPortions of this document may be illegible electronic image products. Images are produced from the best available original document. summaryThe removal of technetium (99Tc) from Hanford tank waste supernatant liquids has been demonstrated using an electrochemical-based separation process. A potential cleanup strategy is to retrieve the waste and separate components into high-level and low-level waste fractions (Orme et al. 1996). However, some of the tanks contain technetium-99 (99Tc) at concentrations deemed to be unacceptable for ultimate processing and disposal. Conventional extraction processes have been shown to be inefficient at removal of 99Tc due to the presence of nonpertechnetate species (Blanchard et al. 1997). Electrochemical processing has been shown to oxidize the nonextractable species and subsequently separate the 99Tc by electrodeposition.The data obtained were used to support a comparison of ion exchange and electrochemical processing as removal methods. The electrochemical process has the flexibility to serve as a standalone process or to support conventional processes as a pretreatment step for the oxidation of nonextractable 99Tc and/or organic decomplexation.A separation procedure developed by AEA Technologies (AEAT) for simulated Hanford tank supernatant liquids was adapted for the actual waste studies conducted at Pacific Northwest National Laboratory (PNNL).W Prior to electroreduction separation of 99Tc from the supernatant liquid, an electrochemical oxidation was carried out in which nonpertechnetate or nonextractable 99Tc was oxidized to more readily extractable species such as pertechnetate, and the organic content was decreased. After oxidation, an electroreduction was performed to remove the 99Tc from the supernatant liquid as Tc or Tc02 deposited on the cathode.Removal of 99Tc was demonstrated with two supernatant liquids from Tanks 24 1 -AW-101 (AW-101) and 241-AN-107 (AN-107). Tank AW-101 waste has lower nonpertechnetate and lower organic contents than AN-107. During supernatant liquid oxidation, more than 40% of the nonpertechnetate or nonextractable 99Tc present in the AW-101 waste was oxidized to an extractable form. Up to 75% of the nonextractable 99Tc was'oxidized in the AN-107 waste.During the oxidation of the nonextractable 99Tc, other species such as organic compounds are also oxidized. During two of the pre-oxidation experiments, the total organic carbon content was reduced by S O % . In addition, a portion of the 9oSr present in the high-organic AN-I07 supernatant liquid was decomplexed, resulting in a 21 9% decrease of 9oSr present as a complex in the supernatant liquid. iii This work has demonstrated successful removal of 99Tc using a combination of electrochemical oxidation and reductioddeposition. The amount of decontamination meets anticipated removal targets for AW-101; the AN-107 results are less conclusive, although they show that electrochemical processing was able to remove 99Tc. Simulant testing was shown to provide a basis for procedure...

Preliminary Tests For Development Of A Non-Pertechnetate Analysis Method

“…Additionally, no work was done to examine if a small amount of a species at mass 99 is present that is not Tc, and further testing and analyses are needed to resolve this. This is needed to finally establish if some of the "non-pertechnetate" that was not oxidized in the electrochemical oxidation tests [12] was actually 99 Ru, and if the small amount of early breakthrough of mass 99 species during ion exchange column tests was actually 99 Ru. If the concentration of pertechnetate and nonpertechnetate currently in the tanks can be accurately quantified, then that information can be used in future decision about a low temperature waste form and tank closure.…”

“…Further, if more than one nonpertechnetate species exists, it is important to quantify the entire amount of Tc that is not pertechnetate. As described above, a few tank samples that were rigorously oxidized and then had pertechnetate removed still exhibited a small amount of a species at mass 99 as identified by ICP-MS measurements [12]. It is speculated that this is not Tc, but may be non-radioactive 99 Ru.…”

“…Initial testing at SRNL involved identifying the oxidation method expected to oxidize all of the non-pertechnetate to pertechnetate. Literature reports [1,4,12] indicated that the complete oxidation of the non-pertechnetate is challenging. Oxidation with hydrogen peroxide, sodium hypochlorite/hydrogen peroxide, sodium bromate, sodium bismuthate, lead(IV) oxide and ammonium vanadate were unsuccessful.…”

“…An automated Tc analyzer was developed and showed that peroxydisulfate was an effective oxidizer [13]. Electrochemical oxidation was also found to be mostly successful, and destruction of the soluble organic carbon species trended with oxidation of the non-pertechnetateTc [12]. But some mass 99 species remained non-extractable with an ion exchange resin even after complete electrochemical oxidation of organics.…”

Incorporating technetium in minerals and other solids: A review

“…Additionally, direct electrolytic reduction of TcO 4 from solutions to Tc metal such as off-gas scrubber streams has been shown to be unsatisfactory, yielding incomplete reduction and Tc 4+ species [139], but has been demonstrated with some success on Hanford tank waste [140].…”