Mobility Of Actinides Research Articles

Mobilization of actinides by dissolved organic compounds at the Nevada Test Site

The effect of dissolved organic matter (DOM) on Am(III), Pu(IV), Np(V), and U(VI) sorption was investigated with natural water (pH ∼8) and zeolitized tuff samples collected from the Rainier Mesa tunnel system, Nevada Test Site, where the USA detonated underground nuclear tests prior to 1992. Perched vadose zone water at Rainier Mesa has high levels of DOM as a result of microbial degradation of mining debris (diesel, wood, etc.). The Am and Pu sorption Kds were up to two orders of magnitude lower in water with high DOM (15–19 mg C/L) compared to the same water with DOM removed (<0.4 mg C/L) or in naturally low DOM (0.2 mg C/L) groundwater. In contrast, Kds of Np and U were less affected by DOM at these solution conditions. Uranium sorption decreased as a result of high dissolved inorganic C (DIC) resulting from microbial degradation of DOM. Thermodynamic model predictions, based on actinide–humic acid stability constants available in the literature, are in general agreement with measured Kd data, correctly predicting the effects of DIC and DOM on actinide retardation. This agreement is encouraging to future modeling efforts and suggests that effects of DOM and DIC can be incorporated into reactive transport modeling predictions. The Am and Pu transport rates in Rainier Mesa tunnel waters will be substantially faster as a result of the elevated DOM levels. Low diffusion rates of actinide–DOM macromolecular complexes may focus Pu and Am transport into fractures and minimize retardation via matrix diffusion. The resulting transport behavior will affect actinide distribution patterns and associated risk estimates.

Curium(III) Complexation with Desferrioxamine B (DFO) Investigated Using Fluorescence Spectroscopy

Abstract Hydroxamate-type siderophores like desferrioxamine B (DFO) are the most common siderophores ubiquitously found in the environment. These naturally occurring chelating substances have the potential to enhance the solubility and mobility of actinides by forming soluble complexes. The unknown interaction between curium(III) and aqueous DFO species is the subject of this paper. The reactions between soluble species of curium(III) and DFO were studied at trace curium(III) concentrations (3×10−7 M) in 0.1 M NaClO4 using time-resolved laser-induced fluorescence spectroscopy (TRLFS). Three Cm3+–DFO species, MpHqLr, could be identified from the luminescence spectra, CmH2DFO2+, CmHDFO+, and CmDFO, having emission maxima at 599, 611, and 614 nm, respectively. The large formation constants, logβ121=31.62±0.23, logβ111=25.70±0.17, and logβ101=16.80±0.40, compared to those of other chelating agents illustrate the unique complexation properties of hydroxamate-type siderophores. An indirect excitation mechanism for the curium(III) luminescence was observed in the presence of the DFO molecules.

Quantifying uranium complexation by groundwater dissolved organic carbon using asymmetrical flow field-flow fractionation

The long-term mobility of actinides in groundwaters is important for siting nuclear waste facilities and managing waste-rock piles at uranium mines. Dissolved organic carbon (DOC) may influence the mobility of uranium, but few field-based studies have been undertaken to examine this in typical groundwaters. In addition, few techniques are available to isolate DOC and directly quantify the metals complexed to it. Determination of U–organic matter association constants from analysis of field-collected samples compliments laboratory measurements, and these constants are needed for accurate transport calculations. The partitioning of U to DOC in a clay-rich aquitard was investigated in 10 groundwater samples collected between 2 and 30 m depths at one test site. A positive correlation was observed between the DOC (4–132 mg/L) and U concentrations (20–603 μg/L). The association of U and DOC was examined directly using on-line coupling of Asymmetrical Flow Field-Flow Fractionation (AsFlFFF) with UV absorbance (UVA) and inductively coupled plasma-mass spectrometer (ICP-MS) detectors. This method has the advantages of utilizing very small sample volumes (20–50 μL) as well as giving molecular weight information on U–organic matter complexes. AsFlFFF-UVA results showed that 47–98% of the DOC (4–136 mg C/L) was recovered in the AsFlFFF analysis, of which 25–64% occurred in the resolvable peak. This peak corresponded to a weight-average molecular weight of about 900–1400 Daltons (Da). In all cases, AsFlFFF-ICP-MS suggested that ≤ 2% of the U, likely present as U(VI), was complexed with the DOC. This result was in good agreement with the U speciation modeling performed on the sample taken from the 2.3 m depth, which predicted approximately 3% DOC-complexed U. This good agreement suggests that the AsFlFFF-ICP-MS method may be very useful for determining U–organic matter association in small volume samples. Because the pH (7.0–8.1) and carbonate concentrations of these waters are typical of many groundwaters, these data suggested that facilitated transport of U by DOC may be limited in its importance in many groundwater systems.

Thorium Complexation by Hydroxamate Siderophores in Perturbed Multicomponent Systems Using Flow Injection Electrospray Ionization Mass Spectrometry

Flow injection electrospray ionization mass spectrometry has been shown to produce simple, characteristic m/z signals for Th-hydroxamate siderophore (desferrioxamine and ferrichrome) complexes, with Th complexed as a simple 4+ ion in the environmentally relevant pH range investigated (pH 5-9). All species of interest for this study were identified optimally in the positive mode; thus, multiple species were analyzed concurrently in a single spectrum. Complexation of Th by the two siderophores was rapid in 1:1 molar aqueous solution, reaching equilibrium before the first measurement was possible at 2 min. However, a significant proportion of the equimolar siderophore remained uncomplexed. Both siderophores rapidly exchanged Th for Fe when equimolar Fe(III) was added to the Th complexes, and only a small proportion of each siderophore remained complexed with Th at equilibrium (7-30 min). The results show a difference in the affinities of the two siderophores for the metals; ferrichrome has a 5-fold higher affinity than desferrioxamine for Th and a 5-fold lower affinity than desferrioxamine for Fe. Also, siderophore-complexed Th interacted strongly with a cation-exchange resin suggesting that, even when complexed by trianionic siderophores, Th mobility will be impeded by interactions with negatively charged binding sites in subsurface environmental matrixes. These results have important implications regarding siderophore-enhanced actinide(IV) mobility in the terrestrial environment.

Bacterial Siderophores Promote Dissolution of UO2 under Reducing Conditions

Tetravalent actinides are often considered environmentally immobile due to their strong hydrolysis and formation of sparingly soluble oxide phases. However, biogenic ligands commonly found in the soil environment may increase their solubility and mobility. We studied the adsorption and dissolution kinetics of UO2 in the presence of a microbial siderophore, desferrioxamine-B (DFO-B), under reducing conditions. Using batch and continuous flow stirred tank reactors (CFSTR),we found that DFO-B increases the solubility of UIV and accelerates UO2 dissolution rates through a ligand-promoted dissolution mechanism. DFO-B adsorption to UO2 followed a Langmuir-type isotherm. The maximum adsorbed DFO-B concentrations were 3.3 micromol m(-2) between pH 3 and 8 and declined above pH 8. DFO-B dissolved UO2 at a DFO-B surface-saturated net rate of 64 nmol h(-1) m(-2) (pH 7.5, l = 0.01 M) according to the first-order rate equation R = kL[Lads], with a rate coefficient kL of 0.019 h(-1). Even at very low siderophore concentrations (e.g. 1 microM), net dissolution rates (16 nmol h(-1) m(-2), pH 7.5, l = 0.01 M) were substantially greater than net proton-promoted dissolution rates (3 nmol h(-1) m(-2), pH 7-7.5, l = 0.01 M). Interestingly, adding dissolved FeIII had negligible effects on DFO-B-promoted UO2 dissolution rates, despite its potential as a competitor for DFO-B and as an oxidant of UIV. Our results suggest that strong organic ligands could influence the environmental mobility of tetravalent actinides and should be considered in predictions for nuclear waste storage and remediation strategies.

Effect of humic acid on the pH-dependent adsorption of terbium (III) onto geological materials

Mobilization of actinides by interaction with humic colloids in aquifers is essentially determined by the geochemical conditions. In this study, the pH dependence of the influence of humic acid on metal adsorption on a variety of geological solids (kaolinite, phyllite, diabase, granite, sand) was investigated for Tb(III) as an analogue of trivalent actinides, using 160Tb as a radiotracer. Humic material was radiolabelled with 131I to allow experiments at low DOC concentrations, as encountered in subsurface systems in the far-field of a nuclear waste repository. For all solids, a changeover from mobilization to demobilization is observed on acidification. Except for phyllite, the reversal occurs at slightly acidic pH values, and is thus relevant in respect of risk assessments. A composite distribution model was employed to reproduce the changeover on the basis of the underlying constituent processes. For this purpose, humate complexation of Tb(III) and adsorption of humic acid as a function of pH were investigated as well. Although the ternary systems cannot be constructed quantitatively by combining the binary subsystems, the relevant interdependences are adequately described by the composite approach. For a more general discussion in view of the diversity of natural organic colloids, adsorption isotherms of various humic and fulvic acids on sand were compared.

Colloid influence on the radionuclide migration from a nuclear waste repository

Abstract Colloid formation is discussed as a possible pathway for the radionuclide release from a nuclear waste repository. An assessment of the colloid relevance on radionuclide migration requires insight into the possible colloid generation mechanisms, their stability and mobility under given groundwater conditions. In various experiments dedicated to the investigation of nuclear waste form behaviour in contact with groundwater, colloidal species are observed mainly for the tri- and tetravalent actinides even in rather high ionic strength solutions where destabilization of colloids is expected. Experimental evidence of laboratory and field studies suggests colloid instability in saline groundwater with time. Groundwater of low ionic strength and high pH enhances colloid stability, as demonstrated in various laboratory and field experiments. The results of an in situ colloid characterization study at the Äspö hard rock laboratory in Sweden are discussed as an example. The mechanism of radionuclide interaction with colloids and notably the reversibility of the radionuclide-colloid binding are other key issues of colloid studies. Kinetic stabilization of the radionuclide binding to colloids may lead to a considerable enhancement of the colloid-mediated radionuclide migration. Substantial kinetic inhibition of actinide dissociation from humic colloids has been established by studying the behaviour of natural humic colloid borne U, Th and rare earth elements. Such behaviour might be explained by the incorporation of these elements into inorganic nanoparticles stabilized by humic coating. Spectroscopic evidence for the occurrence of actinide incorporation into colloidal structures is briefly discussed and the importance of considering the kinetics involved in all kinds of colloidal processes is emphasized. The enhanced migration of tri- and tetravalent actinide ions has been observed recently in various in situ dipole tests at the Grimsel hard rock laboratory in Switzerland. Such experimental findings underline the necessity of further studies on the colloid influence on actinide mobility. Moreover, an improved understanding of colloid-rock interaction mechanisms is required, which is essential for the description and prediction of colloid filtration processes.

Colloid-borne americium migration in Gorleben groundwater: significance of iron secondary phase transformation.

The mobility of actinides in natural water may be enhanced by colloid-mediated transport. In this context the reversibility of actinide colloid interaction is a key factor. Iron is an element that can generate colloids under conditions found in natural waters. In this paper, the impact of hematite and the low-crystalline precursor 2-line ferrihydrite on colloid-mediated transport of americium(III) is investigated. Am(III)-containing iron colloids are generated from two different approaches, namely contact between the two in aqueous solution or coprecipitation of Am(III) during iron colloid generation. Dissolved organic carbon (DOC), especially humic substances, has a strong influence on the stability of inorganic colloids. In addition, humic substances interfere in the distribution and kinetics of exchange between groundwater and sediments. Four groundwaters from the Gorleben aquifer system are used with DOC concentrations varying between 0.9 and 81.6 mgC/L together with Pleistocene Aeolian quartz sand from this site. Batch and column experiments are conducted under near-natural conditions (Ar + 1% CO2). To study the influence of kinetics, contact times up to one month are studied. The dynamic light-scattering investigations show that the colloidal stability of the 2-line ferrihydrite increases with increasing DOC concentration. The low-crystalline iron colloids have a marginal influence on the Am(III) transport due to reversibility of americium sorption. Contrary to this, the crystalline hematite generated from coprecipitation of Am(III) leads to an increase of unretarded colloid-mediated Am(III) transport up to a factor of almost five. Chemical characterization of these hematite colloids shows that Am(III) is structurally entrapped in the hematite. The distribution of Am(III) and 2-line ferrihydrite between groundwater and sand sediment remained in disequilibrium even after one month. This shows that the kinetics of Am(III) distribution between the different phases (bulk solution/colloidal form/ sediment) is a key issue.

Functional group analysis of natural organic colloids and clay association kinetics using C(1s) spectromicroscopy

The quantification of natural humic colloid functional group content and chemical association of humic substances with clay minerals exerts a crucial role in the colloid-borne mobility of actinides due to the functional group dependent strong interaction with multivalent metal ions. Functional group quantification in isolated fulvic acids of the Gorleben groundwater (Lower Saxony, NW Germany) by comparison of high resolution C(ls) NEXAFS spectra deconvolution with 3 C-NMR measurements showed good correlation (r 2 > 0.9) and gives a potential quantification tool in complex natural groundwater systems. Time resolved soft X-ray spectromicroscopy on dissolved organic carbon stabilized SWy-2 smectite colloids revealed an enrichment of carboxyl groups on broken edges (silanol/aluminol groups) at short contact times (Ih). With longer contact times (7d, 6 months) the clay associated organic carbon increases and significantly higher aromatic content associated with basal surfaces were detected. The enhanced sorption of aromatic compounds can be related to an increase in mineral surface hydrophobicity and/or preferential sorption on charged siloxane surfaces.

Actinide Migration from Contaminated Soil to Surface Water at the Rocky Flats Environmental Technology Site

:Surficial soils of the Rocky Flats Environmental Technology Site (RFETS) contain elevated levels of 239,240Pu and 241Am due to wind dispersal of soil particles, contaminated in the 1960's by leaking drums stored on the 903 Pad. Over the past 4 years, actinide mobility in the surface environment at RFETS, Golden, Colorado, USA, was examined through field and laboratory experiments. From measurements of total 239,240Pu and 241Am concentrations in storm runoff and pond discharge samples, collected during spring and summer times, it was established that most of the actinide transport from contaminated soils to streams occurred in the particulate (0.45μm) and colloidal (3kDa – 0.45μm) phases. Controlled laboratory investigations of soil resuspension, indicated that remobilization of colloid-bound Pu during soil erosion events can be enhanced by humic acidsl).2-D Polyacrylamide Gel electrophoresis (PAGE) experiments of radiolabeled colloidal organic and inorganic matter, extracted from RFETS soils, suggested that colloidal Pu, which was focused at pHIEP p of 4.5, is mainly associated with organic (humic acids) colloids of 10-15 kDa molecular weight. Pu(IV) oxide and inorganic colloids such as iron and aluminum oxides have pHIEP of 8-10. While some clay minerals also have pHIEP of 3-5, no Al was found coincident with Pu. This finding has important ramifications for possible remediation, erosion controls, and land-management strategies.

Leaching of Spent Fuel in Cladding Segments and After Removal From Cladding

AbstractIn the early 1980s, tests of the leaching behavior of spent light water reactor fuel were conducted in Sweden by SKB and in the USA by the NNWSI Project. Both organizations used fuels with similar burnup, leaching solutions with similar chemical compositions, and conducted the tests at ambient hot cell temperature. Most of the test results were closely similar. The exception was in the recovery of actinide elements at the end of leaching cycles. In the NNWSI tests, the test vessels were stripped with nitric acid at the end of the leaching cycle. When the actinide inventories recovered in the original leaching cycle plus vessel rinse solutions were added to the amount recovered from the acid stripping, the relative abundances of uranium, plutonium, and other actinides were approximately the same as their inventories in the fuel samples. In the SKB tests, the materials recovered from stripping the leaching vessels were low in amount and interpreted to be fine fuel fragments. Only a few percent of the plutonium inventory that would correspond to the uranium recovered in solution was accounted for in the solution samples. To investigate the reasons for this difference in recovery of the actinides, a new test was undertaken using a fuel sample that had been leached for several years using the SKB methods. The fuel was removed from the cladding after a total of a bit more than two years of leaching inside the cladding. The bare fuel was then leached for several cycles using a geometry that simulated the NNWSI tests and a test procedure that was similar to that used in the NNWSI tests. The cladding from which the fuel was removed was also leached in a separate vessel in parallel with the bare fuel leaching. This paper presents the results of the test series and discusses the effects of specimen geometry on the mobility of actinides during leaching. During leaching of spent fuel inside the original cladding, a secondary phase containing Pu seems to form. This phase appears to be less easily dissolved than spent fuel itself.

Experimental investigation of cement, Topopah Spring tuff, and water interactions at 200°C

The stability of minerals that tend to form in cementitious materials at elevated temperatures has been investigated experimentally. This information is needed to predict the effect of these materials on the performance of geologic radioactive waste repositories. Reaction of large amounts of cement used to build the repository with the surrounding rock and groundwater may cause changes in the hydrologic properties of the repository, impact metal canister and waste form corrosion rates, and alter the mobility of actinides in the subsurface. We have determined that 11 Å-tobermorite, calcite and quartz is the stable (or at least metastable) mineral assemblage at 200°C of complex experiments containing mixtures of cement, Topopah Spring tuff, diesel fuel and 3 mM NaHCO 3 from analyses of the solids and solutions. Mesolite did not form in these experiments, despite predictions that it is the most stable phase. A solubility constant for 11 Å-tobermorite was calculated to be equal to 10 39.4(±1.3) at 200°C for the following reaction: Ca 5( SiO 2) 6( OH) 10∗0.5 H 2 O+10 H +=5 Ca 2++6 SiO 2( aq) +10.5 H 2 O. (1) AA− tobermorite} This work shows that reaction periods of at least 20 days are required to reach local equilibrium or steady-state.

Biotransformation of uranium and other actinides in radioactive wastes

Microorganisms affect the solubility, bioavailability, and mobility of actinides in radioactive wastes. Under appropriate conditions, actinides are solubilized or stabilized by the direct enzymatic or indirect nonenzymatic actions of microorganisms. Biotransformation of various forms of uranium (ionic, inorganic, and organic complexes) by aerobic and anaerobic microorganisms has been extensively studied, whereas limited information is available on other important actinides (Th, Np, Pu, and Am). Fundamental information on the mechanisms of biotransformation of actinides by microbes under various environmental conditions will be useful in predicting the long-term performance of waste repositories and in developing strategies for waste management and remediation of contaminated sites.

Incorporation mechanisms of actinide elements into the structures of U 6+ phases formed during the oxidation of spent nuclear fuel

Uranyl oxide hydrate and uranyl silicate phases will form due to the corrosion and alteration of spent nuclear fuel under oxidizing conditions in silica-bearing solution. The actinide elements in the spent fuel may be incorporated into the structures of these secondary U6+ phases during the long-term corrosion of the UO 2 in spent fuel. The incorporation of actinide elements into the crystal structures of the alteration products may decrease actinide mobility. The crystal chemistry of the various oxidation states of the actinide elements of environmental concern is examined to identify possible incorporation mechanisms. The substitutions Pu 6+U 6+ and (Pu 5+, Np 5+)U 6+ should readily occur in many U 6+ structures, although structural modification may be required to satisfy local bond-valence requirements. Crystal-chemical characteristics of the U 6+ phases indicate that An 4+ (An: actinide)U 6+ substitution is likely to occur in the sheets of uranyl polyhedra that occur in the structures of the minerals schoepite, [(UO 2) 8O 2(OH) 12](H 2O) 12, ianthinite, [U 2 4+ (UO 2) 4O 6(OH) 4(H 2O) 4](H 2O) 5, becquerelite, Ca[(UO 2) 3O 2(OH) 3] 2(H 2O) 8, compreignacite, K 2[(UO 2) 3O 2(OH) 3] 2(H 2O) 8, α-uranophane, Ca[(UO 2)(SiO 3OH)] 2(H 2O) 5, and boltwoodite, K(H 2O)[(UO 2)(SiO 4)], all of which are likely to form due to the oxidation and alteration of the UO 2 in spent fuel. The incorporation of An 3+ into the sheets of the structures of α-uranophane and boltwoodite, as well as interlayer sites of various uranyl phases, may occur.

238U [sbnd]234U and 232Th [sbnd]230Th in the Baltic Sea and in river water

The concentration ( C) of dissolved 238U, 234U, 232Th and 230Th in fresh and brackish waters from the Baltic Sea were determined using TIMS. The brackish waters range in salinity from that of sea water (SW) to 2.5‰. C 238U in oxygen-saturated, surface waters is well correlated with salinity and shows quasi-conservative behavior, as does Sr. Samples from the redox water interface show depletion in C 238U, demonstrating that dissolved U is being removed by FeMn oxyhydroxides. From a simple mixing relationship for the brackish water,δ 234U * = 1000‰ was calculated for the fresh water source in the northern Baltic. A study of the Kalixälven River over an annual cycle yields highδ 234U during spring and summer discharge and lower values during fall and winter, showing that different sources contribute to the U load in the river during different seasons. C 232Th and C 230Th in river water are governed by the discharge, reflecting the importance of the increased abundance of small particles ( < 0.45 μm) for the 232Th 230Th load at high discharge. 232Th/ 238U in river water is about 40 times less than in detrital material. In the brackish water, C 232 Th drops 2 orders of magnitude in the low salinity region ( < 5‰), reaching a value close to that of sea water at a salinity of 7.5‰. Almost all of the riverine 232Th must be deposited in the low-salinity regions of the estuary. The 230Th/ 232Th in river waters is about twice the equilibrium value for 232Th/ 238U (3.8). In the brackish waters, 230Th/ 232Th is greater by a factor of 10–100 than both river water and SW. The big increase in 230Th/ 232Th in the Baltic Sea waters over the riverine input indicates that the Th isotopes enter the estuary as a mixture of two carrier phases. We infer that about 96% of 232Th in river water is carried by detrital particles, whereas the other phase (solution, colloidal) has a much higher 232Th/ 232Th. Entering the estuary, the detrital particles sediment out rapidly, whereas the non-detrital phase is removed more slowly, causing a marked increase in 230Th/ 232Th in the brackish water. In SW, 230Th/ 232Th is closer to river input and detrital material than in brackish water. We conclude that in the deep sea, 232Th is almost exclusively dominated by windblown dust and can be used to monitor dust flux. The 230Th excess in Baltic rivers is produced in U-rich, 232Th-poor peatlands and trapped in authigenic particles and transported with the particles. Time scales for producing the 230Th excess are ∼ 2000–8000 yr. This is younger than, but comparable to, the time of the latest deglaciation, which ended some 9000 yr ago when the mires were forming. These results have implications for the possible mobility of actinides stored in repositories.

The aqueous geochemistry of the rare-earth elements: Critical stability constants for complexes with simple car☐ylic acids at 25°C and 1 bar and their application to nuclear waste management

The mobility of the rare-earth elements (REE) is relevant to nuclear waste management in that REE are produced during radioactive decay in high-level waste and they are good chemical analogues for some of the trivalent actinides (e.g., Am 3+ and Cm 3+). Car☐ylic acids are present naturally in many geologic fluids and also have been disposed of along with radionuclides; such complexing agents may increase the mobility of the REE (actinides) and need to be considered in geochemical models of the transport of these metals into the far-field environment. In this study, available stability constants for the formation of REE complexes with the organic ligands acetate, adipate, benzoate, citrate, ETDA, formate, fumarate, glutarate, glycolate, α-hydroxyisobutyrate, isobutyrate, lactate, maleate, malonate, α-mercaptoacetate, oxalate, m-phthalate, o-phthalate, propionate, salicylate and succinate have been critically evaluated. Those stability constants considered most reliable were, where necessary, corrected to 25°C using published enthalpy data and, where possible, corrected to infinite dilution using a model based on the Pitzer equations. These data provide the basis for future geochemical modelling of the behavior of the REE in nuclear waste disposal sites. Some simple geochemical calculations presented in this paper suggest that monodentate ligands such as acetate are unlikely to contribute significantly to REE (actinide) mobility in most nuclear waste disposal environments. However, ligands capable of chelating in a multi-dentate manner, e.g., oxalate, malonate, succinate, EDTA, etc., may increase the mobility of trivalent lanthanides and actinides significantly, even though their concentrations are lower than monodentate ligands in most natural environments. However, the effectiveness of any of these organic ligands depends on the concentrations of competing ions, such as Ca 2+, Mg 2+, etc., being low relative to the total ligand concentration.

Colloid Formation During Waste Form Reaction: Implications for Nuclear Waste Disposal

Insoluble plutonium- and americium-bearing colloidal particles formed during simulated weathering of a high-level nuclear waste glass. Nearly 100 percent of the total plutonium and americium in test ground water was concentrated in these submicrometer particles. These results indicate that models of actinide mobility and repository integrity, which assume complete solubility of actinides in ground water, underestimate the potential for radionuclide release into the environment. A colloid-trapping mechanism may be necessary for a waste repository to meet long-term performance specifications.

Investigations Into the Mobilization of Actinides From ILW Conditioned in Cement and Bitumen and Studies of their Sorptive and Migration Behaviour in Both the Near- and Far-Fields for a Repository Contained in a Deep Salt Environment

AbstractExperiments have been carried out into the leaching of ILW conditioned in cement and bitumen in concentrated salt solutions. Although difficult to observe with real waste leachates, investigations into the leaching of a simulated waste in cement have indicated leached transuranic levels of ca. 10−9 M ; the amount of activity leached by a quinary salt solution being 102 - 103 times higher . This has been interpreted in terms of a pH effect. For the real waste, spectroscopy has indicated a significantly larger release of Cs from cement than from bitumen. For all waste samples a notable absence of colloidal material was observed ; an observation which can be explained in terms of the high solution ionic strengths and the corresponding influence upon radionuclide solvation.Transuranic mobility studies through salt and sand from a salt dome in Northern Germany have shown the presence of at least two types of species of wildly differing mobility ; one migrating with approximately the same velocity as that of the solvent front and the other strongly retarded. Actinide recoveries (i.e. that passing through the columns) could be strongly influenced by either changing the system pH or by the addition of a competitor such as Ce ; the latter effect pointing to a competitive sorption.

Radionuclide adsorption by manganese oxides and implications for radioactive waste disposal

COBALT-60 and certain transuranics such as 244Cm, 241Am and 238Pu are migrating in trace amounts from original intermediate-level waste disposal pits and trenches in the radioactive waste burial grounds of Oak Ridge National Laboratory (ORNL) in Tennessee1. These radionuclides are transported in trace amounts from the trench 7 area and are to some extent adsorbed by soil and shale along the route of migration. We show here that the adsorbed radionuclides, which are very strongly bound in the soil, are being retained primarily by manganese oxides in the soils. Our data illustrate the importance of the manganese oxide component of soils and sediments in general in controlling 60Co and actinide mobility. Certain implications of these results for the field of radioactive waste management will also be discussed.