Behavior Of Technetium Research Articles

Uranyl-Tc(VII)/Tc(V) hybrid clusters.

Pertechnetate (99TcVIIO4-), reduced Tc, and actinides co-exist in spent nuclear fuel and legacy wastes. They co-transport in fuel reprocessing and waste disposal scenarios, necessitating an understanding of co-speciation. Here, we report five new molecular cluster/framework structures with pentameric and tetrameric uranyl building units decorated by TcO4-/ReO4- oxoanions, or fused with the reduced technetyl cation Tc(V)O. The latter, obtained by Tc auto-reduction (without the intentional introduction of a reducing environment), broadens the basis for Tc-reduction and the burgeoning polyoxometalate-like behavior of technetium.

Hypolimnion behavior of technetium in freshwater at various stages of eutrophication

The article presents a study of various mechanisms of hypolimnion technetium behavior which occur at different stages of waterbodies eutrophication. Eutrophication stage was found to be positively correlated with the rate of 99Tc removal from water phase. The study identified a complex biogeochemical mechanism of technetium behavior. Planktonic phototrophic community plays a pivotal role in such a mechanism by ensuring fast oxygen consumption in hypolimnion. This creates favorable conditions for the active development of anaerobic bottom bacteria of sulfur and iron cycles given nutrients inflow. Sulfates and nitrates were discovered to have inhibitory effect on 99Tc biosorption by bottom sediments due to oxidizing conditions. Apart from the shift of redox potential of the medium to reducing values, the 99Tc removal and partial immobilization are also promoted by the presence of inorganic mineral phases of reduced sulfur and iron. These phases form a reducing barrier in the silt, thus preventing the oxidation and migration of technetium. We suggest the ways of stimulating in situ waterbody remediation by means of various additives, which will allow irreversible immobilization of technetium in silt once several growth periods have passed.

Modeling and Speciation Study of Uranium(VI) and Technetium(VII) Coextraction with DEHiBA.

The N,N-dialkylamide DEHiBA (N,N-di-2-ethylhexyl-isobutyramide) is a promising alternative extractant to TBP (tri-n-butylphosphate) to selectively extract uranium(VI) from plutonium(IV) and spent nuclear fuel fission products. Extraction of technetium, present as pertechnetic acid (HTcO4) in the spent fuel solution, by DEHiBA was studied for different nitric acid and uranium concentrations. The uranium(VI) and technetium(VII) coextraction mechanism with DEHiBA was investigated to better understand the behavior of technetium in the solvent extraction process. Uranium and technetium distribution ratios were first determined from batch experiments. On the basis of these data, a thermodynamic model was developed. This model takes into account deviations from ideality in the aqueous phase using the simple solution concept. A good representation of uranium and technetium distribution data was obtained when considering the formation of (DEHiBA)i(HNO3)j(HTcO4)k complexes, as well as mixed (DEHiBA)2(UO2)(NO3)(TcO4) and (DEHiBA)3(UO2)(NO3)(TcO4)(HNO3) complexes, where one pertechnetate anion replaces one nitrate in the uranium coordination sphere in the two complexes (DEHiBA)2(UO2)(NO3)2 and (DEHiBA)3(UO2)(NO3)2(HNO3). Combination of complementary spectroscopic techniques (FT-IR and X-ray absorption) supported by theoretical calculations (density functional theory) enabled full characterization of the formation of mixed uranium-technetium species (DEHiBA)2(UO2)(NO3)(TcO4) in the organic phase for the first time. The structural parameters of this complex are reported in the paper and lead to the conclusion that the pertechnetate group coordinates the uranyl cation in a monodentate fashion in the inner coordination sphere. This study shows how combining a macroscopic approach (distribution data acquisition and modeling) with molecular-scale investigations (FT-IR and X-ray absorption analysis supported by theoretical calculations) can provide a new insight into the description of a solvent extraction mechanism.

Retention of 99mTc at Ultra-trace Levels in Flowing Column Experiments – Insights into Bioreduction and Biomineralization for Remediation at Nuclear Facilities

ABSTRACTThe behavior of technetium at ultra-trace (<10−10 mol l−1) concentrations in bioreducing sediment column experiments was investigated using 99mTc γ-camera imaging. Four flowing sediment columns were biostimulated for varying periods of time, using acetate as an electron donor, such that on the day of imaging they represented oxic, early metal-reducing, Fe(III)-reducing and sulfate-reducing conditions. Prior to imaging, columns were spiked with 9.6 MBq of 99mTc(VII)O4−, which is relevant to the 99Tc mass concentrations observed at nuclear facilities, run under site relevant flow conditions, and imaged using γ-camera imaging at 20 min intervals over 12h. In the oxic column 99mTc behaved as a conservative tracer and did not interact significantly with the sediment. In all of the biostimulated columns 99mTc associated with the sediment although the spike was least mobile in the order sulfate < Fe(III)-reducing < early metal-reducing columns, confirming higher reactivity for 99mTc under increasingly reducing conditions. Columns were destructively sampled after 99mTc decay (5 days storage), and 0.5 N HCl extractable Fe as Fe(II) was measured at 2 cm intervals along the column length. The Fe(II) measured in all electron donor amended columns was present in stoichiometric excess to the 99mTc. Geochemical modelling and aqueous geochemical data from the different biostimulation treatments suggested that the elevated pH observed in the more reduced columns lead to increased sorbed and mineral associated Fe(II), both stronger reductants for Tc(VII) than aqueous Fe(II). In the sulfate reduction column the presence of FeS lead to the fastest rates of 99mTc immobilization. The results are the first to show the variable but significant retention of 99mTc at ultra-trace levels relevant to conditions at many nuclear sites in a range of biostimulated sediment columns and present a positive outlook for the treatment of 99Tc contaminated groundwater through in-situ biostimulation.

A biokinetic model for systemic technetium in adult humans

This paper reviews biokinetic data for technetium and proposes a biokinetic model for systemic technetium in adult humans. The development of parameter values focuses on data for pertechnetate the most commonly encountered form of technetium and the form expected to be present in body fluids. The model is intended as a default model for occupational or environmental intake of technetium, i.e. applicable in the absence of form- or site-specific information. Tissues depicted explicitly in the model include thyroid, salivary glands, stomach wall, right colon wall, liver, kidneys, and bone. Compared with the ICRP’s current biokinetic model for occupational or environmental intake of technetium (ICRP , ), the proposed model provides a more detailed and biologically realistic description of the systemic behaviour of technetium and is based on a broader set of experimental and medical data. For acute input of 99mTc (T1/2 = 6.02 h) to blood, the ratios of cumulative (time-integrated) activity predicted by the current ICRP model to that predicted by the proposed model range from 0.4–7 for systemic regions addressed explicitly in both models. For acute input of 99Tc (T1/2 = 2.1 × 105 year) to blood, the corresponding ratios range from 0.2–30.

Behavior of technetium in nuclear waste vitrification processes

Nearly 100 tests were performed with prototypical melters and off-gas system components to investigate the extents to which technetium is incorporated into the glass melt, partitioned to the off-gas stream, and captured by the off-gas treatment system components during waste vitrification. The tests employed several simulants, spiked with 99mTc and Re (a potential surrogate), of the low activity waste separated from nuclear wastes in storage in the Hanford tanks, which is planned for immobilization in borosilicate glass. Single-pass technetium retention averaged about 35 % and increased significantly with recycle of the off-gas treatment fluids. The fraction escaping the recycle loop was very small.

Methyl-hydrazine Deoxidize Tc(VII) in Nitric Acid in Presence of U(VI) and Behavior of Technetium in the U/Pu Splitting Stage of APOR Process

The reaction between methyl-hydrazine and Tc(VII) in nitric acid in present of U(VI) was studied in this paper, the influence of acidity, methyl-hydrazine concentration, and temperature on the reaction was investigated meanwhile. Results showed that U(VI) can accelerate this reaction distinctly. When U(VI) concentration is 80g/l, Tc(VII) can be deoxidized in 1.5hours, on the contrary, the reaction will last 6hours if there is no U(VI) in the system. The behavior of technetium in the U/Pu splitting stage was investigated using a multistage mixer-setter. In this experiment about 90% Tc(VII) was deoxidized within 3 stages when contacting with the reducing regent and uranium. Finally nearly 99.9% of technetium went to plutonium stream.

Technetium Incorporation into C14 and C15 Laves Intermetallic Phases

ABSTRACTLaves-type intermetallic phases have been observed to be the dominant phases in a series of alloy compositions being designed for the immobilization of technetium in a metallic waste form. The dominant metals in the alloy compositions were Fe-Mo and Fe-Mo-Zr. The alloy composition, Fe-Mo-Zr, also contained Pd, Zr, Cr, and Ni. Both non-radioactive rhenium-containing and radioactive technetium-bearing alloy compositions were investigated. In the Fe-Mo series, the phases observed were Fe2Mo (C14 Laves phase) and ferrite in agreement with predictions. Both Tc and Re resided predominantly in the Laves phases. In the Fe-Mo-Zr system, the phases included hexagonal C14 with the composition (Fe,Cr)2Mo, cubic C15 phase with a (Fe,Ni)2Zr composition, and the hcp phase Pd2Zr. The observation of these phases was in agreement with predictions. Re was found in the C14 intermetallic, (Fe,Cr)2Mo. Technetium was also observed to be partitioned preferentially into the (Fe,Cr)2Mo phase; however, this phase exhibited a cubic structure consistent with the C15 structural type. The composition of Laves phases is influenced by both the atomic size and electro-negativity of the constituent elements. The long-term release behavior of technetium under nuclear waste disposal conditions may be more dependent on the corrosion characteristics of these individual Laves phases containing Tc than the other metallic phases.

Experimental Study of Contaminant Release from Reducing Grout

A column experiment was conducted to study the release behavior of technetium, uranium, and selenium initially sequestered in reducing grout similar in composition to Savannah River Site (SRS) saltstone, a cementitious waste form made by mixing salt solution from SRS liquid waste storage tanks with a dry mix containing blast furnace slag, fly ash, and Portland cement. The data suggest that uranium was retained in the grout possibly as a CaUO4 phase, whereas most of the selenium was released. Technetium release initially was relatively constant, and then increased significantly after 26 pore volumes. The increase in technetium release was slightly delayed relative to the observed Eh increase. The system Eh-pH started under conditions in which technetium solubility is low, constrained by Tc3O4 solubility, but eventually transitioned into the stability field of the pertechnetate ion. The delay in technetium release relative to the Eh increase was possibly due to slow oxidation of technetium at depth within the grout particles, which in turn was likely controlled by O2 diffusion into the particles. In contrast to technetium and uranium, selenium release was not solubility limited and selenium likely was present in the pore solution initially as a HSe− species.

The behavior of technetium in technetium-scrubbing stage of PUREX process

Technetium decontamination factor as a function of the acidity, flow ratio, scrubbing stage number was investigated by counter-current cascade experiments or mixer-settler batch tests. Results showed that the acidity of the scrubbing acid has little influence on the decontamination factor of technetium when the acidity was kept in the range of 4.5–6 M. The most effective method to increase the decontamination factor is to lower the feed-to-acid flow ratio. Keeping other condition same the factor increased to 10.3 from 3.1 when feed-to-acid ratio changed to 4 from 5.6. The loss of uranium and plutonium can be recovered through a re-extraction technology.

Methodological studies of the determination of 99Tc in matrix-rich environmental samples

A few experiments testing technetium behavior at different conditions for complex matrixes (soils, sediments) were performed. It was confirmed that the incineration of samples soaked with ammonia can be done without technetium losses. Next, for the precipitation of iron(III) hydroxide at different pH levels, the degree of co-precipitation of technetium was studied. It can be helpful during purification of very matrix-rich and massive samples. Finally the technetium and rhenium fractionation during separation of technetium from the environmental samples was studied. This test clearly showed limitations for using Re as yield tracer during 99Tc analyses in case of mineral matrixes.

Sorption of Tc(IV) to some geological materials with reference to radioactive waste disposal

AbstractOne of the most important isotopes to be considered for disposal in the proposed UK Geological Disposal Facility (GDF) for higher-activity radioactive wastes will be99Tc, due to its long half-life, high fission yield and ability to migrate through the geosphere as the pertechnetate ion. Much of the Tc is likely to be in the lower Tc(IV) oxidation state due to the low Eh in the near field. Batch Tc(IV) sorption experiments have been performed from pH 3–13, using95mTc as a spike, in the presence of quartz, hematite, goethite, plagioclase feldspar, sand and shale. Solutions containing Tc(IV) were prepared at trace concentrations to avoid precipitation of TcO2. Values for the partition coefficient (Rd) were found to range from ∽4 up to ∽2 × 104cm3g–1. Rdwas heavily dependent on pH in all cases, with the highest values being found in the circumneutral area. These data will inform the performance assessment for the near-field behaviour of technetium in the UK's planned higher-activity wastes GDF. Surface complexation modelling has been performed, fitting the data using both monodentate and bidentate binding models.

Effect of some reducing and complexing agents on the extraction behavior of technetium in the TBP-HNO3 system

The effect of a series of complexing and reducing agents on the extraction-chemical behavior of technetium as applied to extraction splitting of uranium and plutonium in the Purex process was examined. Kinetic parameters of the catalytic decomposition of N2H5NO3 under the action of Tc in the presence of these agents were evaluated. Variation of the ratio of the oxidized and reduced Tc species in the course of the process and in the hydrazine-free systems was determined. Reagents preventing oxidation of the reduced technetium and decomposition of N2H5NO3 in nitric acid solutions (acetohydroxamic acid, hydroxylamine, ascorbic acid, etc.), inhibiting the reduction of Tc with hydrazine nitrate (H2O2, HN3, etc.), and known as complexing agents toward quadrivalent actinides but indifferent to Tc were considered. The possibilities of using these reagents for modifying the Tc behavior in the course of SNF extraction reprocessing with the aim to direct this element to different process streams were discussed.

Effect of complexing agents (DTPA and oxalic acid) on the extraction behavior of technetium in the TBP-N2H5NO3-HNO3 system

The effect of complexing agents DTPA and H2C2O4 on the extraction-chemical behavior of Tc, as applied to extractive splitting of U and Pu in the Purex process, was studied. The kinetic relationships of catalytic decomposition of N2H5NO3 under the action of Tc in the presence of these agents were determined. Variation of the ratio of the oxidized and reduced Tc species in the course of this process at various HNO3 concentrations was examined. In the final step of the reaction of N2H5NO3 with Tc in a nitric acid solution, when the reductant content is stabilized, the oxidation of reduced Tc also stops or continues at a considerably lower rate than before stabilization. The presence of DTPA decreases the rate of Tc(IV) oxidation owing to its binding in a complex and decreases the fraction of Tc(VII) in the system by the end of the reaction, with no decrease in the rate of N2H5NO3 decomposition noted under these conditions. The presence of H2C2O4 in solutions decelerates the Tc reduction and the N2H5NO3 decomposition; the oxidation of the reduced Tc either is insignificant or is not observed at all, depending on the conditions and H2C2O4 content.

Role of Acetohydroxamic Acid in Selective Extraction of Technetium and Uranium Employing N,N-Dihexyloctanamide as Extractant

Straight chain N,N-dihexyloctanamide (DHOA) has been identified as a promising alternate extractant to tributyl phosphate (TBP) for the reprocessing of uranium based spent fuels. The present work compares extraction behavior of technetium using DHOA and TBP solutions in n-dodecane, under varying experimental conditions such as acidity (0.5–6 M HNO3); extractant concentration (1.1 and 1.5 M), and uranium loading (50 g/L, relevant for Pu rich spent fuel feed solutions). The effect of acetohydroxamic acid concentration on U, Pu, Np, and Tc extraction behavior has also been investigated. Pu(IV)-AHA interaction and its influence on extraction using TBP and DHOA extractants has been studied spectrophotometrically. The experimental data suggest that 1.1 M DHOA is better than 1.1 M TBP with respect to co-extraction of Tc and U, and U decontamination with respect to Np/Pu.

Redox-active ligands interrogate the complex oxidation state behavior of technetium

Redox-active ligands interrogate the complex oxidation state behavior of technetium

Comparison of the Behavior of Technetium and Rhenium in Low Activity Waste Glass Formulations Subjected to the Vapor Hydration Test

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Adsorption behavior of VII group elements on tertiary pyridine resin in hydrochloric acid solution

We investigated the adsorption behaviors of technetium and rhenium on tertiary pyridine resin in hydrochloric acid solution. Technetium has a little larger distribution coefficient in higher concentration of hydrochloric acid region than rhenium. However, the tendency of adsorption behavior of technetium and rhenium is similar. We concluded that the rhenium can be used as the substitute for technetium. Adsorption behaviors of platinum group elements were also investigated. The separation of technetium from platinum group elements is discussed.

Distribution of technetium in PUREX process streams

Extraction of technetium has been carried out from the aqueous medium containing nitric acid under different experimental conditions to investigate its extraction behaviour in 30% tri-n-butyl phosphate in n-dodecane. In order to study the distribution behaviour of technetium in different streams of PUREX process, experiments were carried out under process conditions. The distribution of technetium was also studied using anion-exchange resin. Based on these results, the path of technetium in the PUREX process streams has been established which will be useful in the development of an advanced PUREX flow-sheet for containment and isolation of technetium in an environmental friendly fuel cycle.

Physicochemical behavior of uranium and technetium in some new stages of the nuclear fuel cycle

Recently published results of the cooperative studies executed by the Institute of Physical Chemistry and Electrochemistry of Russian Academy of Sciences (IPCE RAS), Research Center Vallee du Rhone (CEA, France), Institute of Nuclear Physics Orsay and University Bordeaux I [Centre Nationale de Recherches Scientifiques (CNRS), France] are reviewed. The review includes data obtained for the first time on the electrochemical properties and anodic dissolution of uranium monocarbide considered among possible bases of the fuel for the reactors of the IVth generation. The results of the investigations in the field of the synthesis of tetraalkyl ammonium pertechnetates, their physicochemical properties and thermal decomposition with formation of Tc metal and Tc monocarbide are summarized. The electrochemical reactions with participation of Tc ions in various oxidation states and their application in the nuclear fuel cycle are described. The behavior of technetium in natural waters near the locations of the radwastes long-term storage and in the areas surrounding nuclear power plants in RF and France was studied.