Characterization of radionuclide-chelating agent complexes found in low-level radioactive decontamination waste. Literature review

Abstract

DISCLAIMERNUREGER-6124 is not a substitute for NRC regulations and compliance is not required. The approaches andor methods described in this NUREG/CR are provided for information only. Publication of this report does not necessarily constitute NRC approval or agreement with the information contained herein. AbstractUnder the regulations outlined in 10 CFR 61, the U.S. Nuclear Regulatory Commission is responsible for regulating the safe land disposal of low-level radioactive wastes that may contain organic chelating agents. Such agents include ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), . picolinic acid, oxalic acid, and citric acid, and can form radionuclide-chelate complexes that may enhance the migration of radionuclides from disposal sites.Data from the available literature indicate that chelates, most notably picolinate and EDTA, can leach from solidified decontamination wastes in moderate concentration (i.e., 1-100 ppm) and can potentially complex certain radionuclides in the leachates. The effects of the formation of such radionuclide-chelate complexes on the migration of radionuclides in groundwater systems is still difficult to quantitatively predict owing to the dependence of such migration on several factors including the chemical composition of the groundwater, the specific adsorbing surfaces present in the sods, and the ability of microorganisms to biodegrade the chelates. However, in general it appears that both EDTA and DTPA have the potential to mobilize radionuclides from waste disposal sites because such chelates can leach in moderate concentration, form strong radionuclide-chelate complexes, and can be recalcitrant to biodegradation. It also appears that oxalic acid and citric acid will not greatly enhance the mobility of radionuclides from waste disposal sites because these chelates do not appear to leach in high concentration, tend to form relatively weak radionuclide-chelate complexes, and can be readily biodegraded. In the case of picolinic acid, insufficient data are available on adsorption, complexation of key radionuclides (such as the actinides), and biodegradation to make definitive predictions, although the available data indicate that picolinic acid can chelate certain radionuclides in the leachates. Contents 3.12Mass spectra of (a) Cu(EDTA) and (b) Cu(HEDTA I Executive SummaryA variety of chemical decontamination processes are used to remove the build-up of radioactive-activated metals and corrosion products from the cooling systems of nuclear power plants. AU of these decontamination processes use chelating agents, such as ethylenediaminetetraaceec acid (EDTA), picolinic acid, oxalic acid, citric acid, and less frequently, diethylenetriaminepentaacetic acid (DTPA), to complex the released radionuclides. The complexed radionuclides and any excess uncomplexed chelates are then removed onto cation-or anion-exchange resins. The U.S. Nuclear Regulatory Commission (NRC), as defined in 10 CFR 61, is responsible for regulating the disposal of suc...

Radionuclide-Chelating Agent Complexes in Low-Level Radioactive Decontamination Waste; Stability, Adsorption and Transport Potential

“…The following summary was mainly extracted from Serne et al (1996). Less work has been done on DTPA, and only a few studies have been conducted with picolinic acid see Serne et al 1996).…”

“…The following summary was mainly extracted from Serne et al (1996). Less work has been done on DTPA, and only a few studies have been conducted with picolinic acid see Serne et al 1996). The adsorption of these chelates and their metal complexes, especially on oxides, can be highly pH-dependent and all of the chelates can undergo what is termed "ligand-like" adsorption (i.e., adsorption of the chelates is strongest at low pH, decreases rapidly in the neutral pH region, and can be negligible at high pH [>8]).…”

Study of radionuclide leaching from the residues of K Basin sludge dissolution

“…If necessary to meet TWRS acceptance, the citric and oxalic acids could be destroyed by hydrogen peroxide in strong nitric acid solution before neutralization. The CITROX process has additional appeal because there is local commercial support for both this process (Serne 1996) (Newton 1998) and support for general process application to Hanford fuel storage basin desludging (Schneidmiller 1998). …”

“…For a less corrosive form of iron oxide dissolution than the use of HC1, this is a commercially available reactor decontamination technology that is compatible with TWRS acceptance --a variation of the CITROX process (Serne 1996). Citric and oxalic acids totaling 6% of both reagents are stirred with the residue.…”

Applicability of the Linear Sorption Isotherm Model to Represent Contaminant Transport Processes in Site Wide Performance Assessments

“…Such non polar and/or hydrophobic organic compounds if disposed in large quantities and high concentration could potentially affect radionuclide and metal migration by creating a reducing zone, however, field evidence suggests that this did not occur to any significant extent at the Hanford Site (see Serne and Wood 1990 and references therein). There is also no evidence of such organic compound impacts at other nuclear waste sites across North America 1995).…”