Laboratory migration experiments with radionuclides and natural colloids in a granite fracture

Natural colloids in groundwater could facilitate radionuclide transport, provided the colloids are mobile, are present in sufficient concentrations and can adsorb radionuclides. This paper describes the results of a laboratory migration study carried out with combinations of radionuclides and natural colloids within a fracture in a large granite block to experimentally determine the impact of colloids on radionuclide transport. The 85Sr used in this study is an example of a moderately sorbing radionuclide, while the 241Am is typical of a strongly sorbed radionuclide with very low solubility. The natural colloids used in this study were isolated from granite groundwater from Atomic Energy of Canada (AECL) Underground Research Laboratory (URL), and consisted of mostly 1-10 nm organic colloids, along with lesser amounts of 10-450 nm colloids (organics and aluminosilicates). The measured coefficients for radionuclide sorption onto these colloids were between 3 x 10(2) and 1 x 10(3) ml/g for 85Sr, and between 7 x 10(4) and 7 x 10(5) mg/l for 241Am. The 85Sr sorption on the natural colloids appeared to be reversible. Migration experiments in the granite block were carried out by establishing a flow field between two boreholes (out of a total of nine) intersecting a main horizontal fracture. These experiments showed that dissolved 85Sr behaved as a moderately sorbing tracer, while dissolved 241Am was completely adsorbed by the fracture surfaces and showed no evidence of transport. However, when natural colloids were injected together with dissolved 241Am, a small amount of 241Am transport was observed, demonstrating the ability of natural colloids to facilitate the transport of radionuclides with low solubility. Natural colloids had only a minor effect on the transport of 85Sr. In a separate experiment to test the effect of higher colloid concentrations on 85Sr migration, synthetic colloids were produced from Avonlea bentonite. The introduction of a relatively high concentration of bentonite colloids actually reduced 85Sr transport because, compared to natural colloids, the bentonite colloids were less mobile and they sorbed 85Sr more strongly.     

Impact of an exceptional Saharan dust outbreak in France: PM10 and artificial radionuclides concentrations in air and in dust deposit

The present study focuses on the exceptional Saharan dust event that affected most of France in February 2004. Activity levels of various artificial radionuclides (⁹⁰Sr, ¹³⁷Cs, uranium, thorium and plutonium isotopes, ²⁴¹Am) were examined. Activity or isotopic ratios are discussed in the context of atmospheric nuclear weapons tests, among them French tests performed in Sahara in the 1960s. The daily evolution of ¹³⁷Cs activity levels in the atmosphere was compared to daily PM₁₀ change. A link between airborne ¹³⁷Cs and PM₁₀, is given. It is estimated that this 2-day event deposited as much ¹³⁷Cs as would be deposited on average over a 10-month period. The amount of deposited ¹³⁷Cs and ^239+240Pu represents respectively about 0.1 and 1% of the activity already present in the soil. Such Saharan dust events correspond to an extreme type of “feeder” process of artificial radionuclides in the atmosphere. Therefore, they contribute to the long term background level of artificial radionuclides kept at trace levels in the atmosphere.     

Role of magnetite and humic acid in radionuclide migration in the environment

Sorption of ¹³⁷Cs, ⁹⁰Sr, ¹⁵⁴Eu and ¹⁴¹Ce by magnetite has been studied at varying pH (4 to 11) in the presence and absence of humic acid. The sorption studies have also been carried out at varying ionic strength (0.01 to 0.2 M NaClO₄) and humic acid concentration (2 to 20 mg/L). Percentage sorption of ¹³⁷Cs and ⁹⁰Sr was found to be pH dependent, with the sorption increasing with increasing pH of the suspension. At any pH, the percentage sorption of ⁹⁰Sr was higher than that of ¹³⁷Cs. The results have been explained in terms of the electrostatic interaction between the positively charged metal ions and the surface charge of the magnetite which becomes increasingly negative with increasing pH. On the other hand, ¹⁵⁴Eu and ¹⁴¹Ce were found to be strongly sorbed by the magnetite at all pH values, with the sorption being independent of pH. The strong sorption of trivalent and tetravalent metal ions suggests the role of complexation reactions during sorption, apart from the electrostatic interactions. However, in the case of ¹⁴¹Ce surface precipitation of Ce(III) formed by reduction of Ce(IV) in the presence of magnetite cannot be ruled out. Presence of humic acid (2 mg/L) was found to have negligible effect on sorption of all metal ions.     

The Grimsel (Switzerland) migration experiment: integrating field experiments, laboratory investigations and modelling

For several years tracer migration experiments are performed at Nagra's Grimsel Test Site in the Swiss Alps as a joint undertaking of Nagra, PNC and PSI. The aim is to develop methods for field experiments at possible sites for nuclear waste repositories and to test radionuclide transport models.A hydraulic dipole field is generated in a well-defined fracture zone in granite. The tracers used are non-sorbing (uranine, ³He, ⁴He, ⁸²Br⁻, ¹²³I⁻), mildly sorbing (²²Na⁺, ²⁴Na⁺), and more strongly sorbing (⁸⁵Sr²⁺, ⁸⁶Rb⁺, ¹³⁴Cs⁺, ¹³⁷Cs⁺). These experiments have been complemented by extensive laboratory investigations on petrography, on water-rock and nuclide-rock interaction as well as by migration experiments with bore cores.The main questions addressed are: What are the relevant geometric factors and mechanisms for transport, how well can breakthrough curves be extrapolated from one dipole arrangement to another, which parameters are scale dependent, is there a difference in sorption values between laboratory and field experiments or between static and dynamic experiments. Evaluating the experimental results for the non-sorbing uranine and the mildly sorbing tracers sorption, Strontium, we show that a consistent picture of tracer transport, and specifically of tracer sorption, is obtained when exploiting all available experimental information and using not too simplistic models.     

In situ tracer tests to determine retention properties of a block scale fracture network in granitic rock at the Aspö Hard Rock Laboratory, Sweden

Experiments were conducted at the Asp? Hard Rock Laboratory in order to improve the understanding of radionuclide retention properties of fractured crystalline bedrock in the 10-100 m scale (TRUE Block Scale Project, jointly funded by ANDRA, ENRESA, Nirex, JNC, Posiva and SKB). A series of tracer experiments were performed using sorbing tracers in three different flow paths. The different flow paths had Euclidian lengths of 14, 17 and 33 m, respectively, and one to three water conducting structures. Four tests were performed using different cocktails made up of radioactive sorbing tracers (22,24Na+, 42K+, 47Ca2+, 85Sr2+, 83,86Rb+, 131,133Ba2+ and 134,137Cs+). For each tracer injection, the breakthrough of sorbing tracers was compared to the breakthrough of a conservative tracer, 82Br-, 131I-, HTO and 186ReO4-, respectively. In the two longer flow paths, no breakthrough of 83Rb+ and 137Cs+ was observed after 8 months of pumping. Selected tracer tests were subject to basic modelling in which a one-dimensional (1D) advection-dispersion model, including surface sorption, and an unlimited matrix diffusion were used for the interpretation of the results. The results of the modelling indicated that there is a slightly higher mass transfer into a highly porous material in the block-scale experiment compared with in situ experiments performed over shorter distances and significantly higher than what would have been expected from laboratory data obtained from studies of the interactions in nonaltered intact rock.     

Mobility of radionuclides in soil/groundwater system: Comparing the influence of EDTA and four of its degradation products

The adsorption behavior of ²⁴¹Am, ⁶⁰Co, ¹³⁷Cs and ⁸⁵Sr in the presence and absence of chelating ligands (ethylenediaminetetraacetic acid, ethylenediaminediacetic acid, hydroxyethyliminodiacetic acid, iminodiaceiticacid and methyliminodiacetic acid) was investigated. Sorption affinity in the absence of chelating ligands followed: Am(III) > Co(II) > Cs(I) > Sr(II). The presence of chelating ligands generally had little effect on sorption of ⁸⁵Sr and ¹³⁷Cs with K_d values 110 and 690 mL g⁻¹, respectively. But at 0.02 M of ethylenediaminetetraacetic or hydroxyethyliminodiacetic, the K_d decreased to 5 or 63 mL g⁻¹, respectively, where thermochemical modeling indicated almost all ⁸⁵Sr is complexed with these ligands. The K_d values for ²⁴¹Am and ⁶⁰Co generally decreased with increasing chelating agent concentrations. In notable cases, the K_d values for Am increased at specific concentrations of 10⁻³ M for IDA, MIDA and 10⁻⁴ M for EDDA. This is proposed to be due to formation of a ternary surface complex.     

Radioactive isotopes in atmospheric aerosols over Russia and the Sea of Japan following nuclear accident at Fukushima Nr. 1 Daiichi Nuclear Power Station in March 2011

Artificial radionuclides, such as iodine-131 (¹³¹I), cesium-134 (¹³⁴Cs), and cesium-137 (¹³⁷Cs), as well as natural isotopes of beryllium-7 (⁷Be) and potassium-40 (⁴⁰K) have been registered in atmospheric aerosols over Vladivostok selected from 11 March to 17 June 2011. Additionally, ¹³⁴Cs and ¹³⁷Cs were detected in atmospheric aerosols over Tomsk selected from 16 March to 17 June 2011. Artificial radionuclides were also discovered in atmospheric wet depositions sampled in Vladivostok from 3 to 17 May 2011. Moreover, these radionuclides have been registered in atmospheric aerosols over the sea surface of the Sea of Japan selected from 3 to 31 May 2011 during an expedition of the “Nadezhda” sailing ship. From 18 March to 15 April, an increase in concentrations of atmospheric aerosols over Vladivostok from 108.8 to 321.5 μg/m³ has been registered. It was accompanied by increased activity concentrations of ¹³⁴Cs, ¹³⁷Cs, and the ¹³¹I. During the period from 18 March to 15 April, activity concentrations of ¹³⁷Cs and ¹³⁴Cs in atmospheric aerosols increased 100 times compared with the minimum detectable concentration (MDC) level and peaked in the weekly sample gathered from 8 to 15 April (145.0 and 105.3 μBq/m³, respectively). Variability of concentrations of natural isotopes of ⁷Be and ⁴⁰K was not greater than 1 order of magnitude throughout the sampling period. Maximal values of ¹³⁷Cs and ¹³⁴Cs concentrations (1,281.5?±?141 and 384.4?±?42.3 μBq/m³, respectively) in Tomsk were reached in samples taken from 1 to 2 April. For the atmospheric aerosol samples from the Sea of Japan, the largest concentration of ¹³¹I (392.3?±?215.7 μBq/m³) was detected from 13 to 19 May, while all other samples had much lower concentration values. Synoptic analysis of back trajectories movement of air masses showed that the radioactive cloud came to Vladivostok from the regions of Siberia and northeastern part of China. Synoptic analysis for Tomsk showed that during the period of maximal activity concentrations (1–9 April), air masses were arriving from the European part of Russia and north of Kazakhstan.     

Solute transport in crystalline rocks at Äspö — II: Blind predictions, inverse modelling and lessons learnt from test STT1

Based on the results from detailed structural and petrological characterisation and on up-scaled laboratory values for sorption and diffusion, blind predictions were made for the STT1 dipole tracer test performed in the Swedish Äspö Hard Rock Laboratory. The tracers used were nonsorbing, such as uranine and tritiated water, weakly sorbing ²²Na⁺, ⁸⁵Sr²⁺, ⁴⁷Ca²⁺and more strongly sorbing ⁸⁶Rb⁺, ¹³³Ba²⁺, ¹³⁷Cs⁺.Our model consists of two parts: (1) a flow part based on a 2D-streamtube formalism accounting for the natural background flow field and with an underlying homogeneous and isotropic transmissivity field and (2) a transport part in terms of the dual porosity medium approach which is linked to the flow part by the flow porosity.The calibration of the model was done using the data from one single uranine breakthrough (PDT3). The study clearly showed that matrix diffusion into a highly porous material, fault gouge, had to be included in our model evidenced by the characteristic shape of the breakthrough curve and in line with geological observations.After the disclosure of the measurements, it turned out that, in spite of the simplicity of our model, the prediction for the nonsorbing and weakly sorbing tracers was fairly good. The blind prediction for the more strongly sorbing tracers was in general less accurate. The reason for the good predictions is deemed to be the result of the choice of a model structure strongly based on geological observation. The breakthrough curves were inversely modelled to determine in situ values for the transport parameters and to draw consequences on the model structure applied. For good fits, only one additional fracture family in contact with cataclasite had to be taken into account, but no new transport mechanisms had to be invoked. The in situ values for the effective diffusion coefficient for fault gouge are a factor of 2–15 larger than the laboratory data. For cataclasite, both data sets have values comparable to laboratory data. The extracted K_d values for the weakly sorbing tracers are larger than Swedish laboratory data by a factor of 25–60, but agree within a factor of 3–5 for the more strongly sorbing nuclides. The reason for the inconsistency concerning K_ds is the use of fresh granite in the laboratory studies, whereas tracers in the field experiments interact only with fracture fault gouge and to a lesser extent with cataclasite both being mineralogically very different (e.g. clay-bearing) from the intact wall rock.     

Solute transport in crystalline rocks at Aspö--II: blind predictions,inverse modelling and lessons learnt from test STT1

Based on the results from detailed structural and petrological characterisation and on up-scaled laboratory values for sorption and diffusion, blind predictions were made for the STT1 dipole tracer test performed in the Swedish Asp? Hard Rock Laboratory. The tracers used were nonsorbing, such as uranine and tritiated water, weakly sorbing 22Na(+), 85Sr(2+), 47Ca(2+)and more strongly sorbing 86Rb(+), 133Ba(2+), 137Cs(+). Our model consists of two parts: (1) a flow part based on a 2D-streamtube formalism accounting for the natural background flow field and with an underlying homogeneous and isotropic transmissivity field and (2) a transport part in terms of the dual porosity medium approach which is linked to the flow part by the flow porosity. The calibration of the model was done using the data from one single uranine breakthrough (PDT3). The study clearly showed that matrix diffusion into a highly porous material, fault gouge, had to be included in our model evidenced by the characteristic shape of the breakthrough curve and in line with geological observations. After the disclosure of the measurements, it turned out that, in spite of the simplicity of our model, the prediction for the nonsorbing and weakly sorbing tracers was fairly good. The blind prediction for the more strongly sorbing tracers was in general less accurate. The reason for the good predictions is deemed to be the result of the choice of a model structure strongly based on geological observation. The breakthrough curves were inversely modelled to determine in situ values for the transport parameters and to draw consequences on the model structure applied. For good fits, only one additional fracture family in contact with cataclasite had to be taken into account, but no new transport mechanisms had to be invoked. The in situ values for the effective diffusion coefficient for fault gouge are a factor of 2-15 larger than the laboratory data. For cataclasite, both data sets have values comparable to laboratory data. The extracted K(d) values for the weakly sorbing tracers are larger than Swedish laboratory data by a factor of 25-60, but agree within a factor of 3-5 for the more strongly sorbing nuclides. The reason for the inconsistency concerning K(d)s is the use of fresh granite in the laboratory studies, whereas tracers in the field experiments interact only with fracture fault gouge and to a lesser extent with cataclasite both being mineralogically very different (e.g. clay-bearing) from the intact wall rock.     

Distribution coefficients (Kd) and desorption rates of Cs and Am in Black Sea sediments

The distribution coefficients (K_d) and desorption rates of ¹³⁷Cs and ²⁴¹Am radionuclides in bottom sediments at different locations in the Black Sea were studied under laboratory conditions. The K_d values were found to be 500 for ¹³⁷Cs and 3800 for ²⁴¹Am at the steady state and described exponential curves. Rapid uptake of the radionuclides occurred during the initial period and little accumulation happened after four days. The desorption rates for ¹³⁷Cs in different bottom sediments were best described by a three-component exponential model. The desorption half-times of ¹³⁷Cs ranged from 26 to 50 d at the slow components. However, the desorption rate of ²⁴¹Am described one component for all sediment samples and desorption half-time was found to be 75 d. In general, the results showed that the ²⁴¹Am radionuclide is more effectively transferred to bottom sediment and has longer turnover time than ¹³⁷Cs under Black Sea conditions.     

Fate of radiocesium in sewage treatment process released by the nuclear accident at Fukushima

The nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) which occurred after the Great East Japan Earthquake on March 11, 2011 resulted in releases of radionuclides such as ¹³⁴Cs (half-life:T_1/2 = 2.06 yr), ¹³⁷Cs (T_1/2 = 30.04 yr) and ¹³¹I (T_1/2 = 8.05 d) to the environment. For this paper, we observed the monthly variations of radiocesium (¹³⁴Cs and ¹³⁷Cs) and stable Cs concentrations in influent, effluent, sewage sludge, and sludge ash collected from a sewage treatment plant 280 km north of the FDNPP from July to December, 2011. Using the stable Cs results, we concluded the mass balance of Cs in the sewage treatment plant showed that about 10% of the Cs entering the sewage treatment plant would be transferred to the sewage sludge, and then Cs in the sewage sludge was totally recovered in the sludge ash. The behavior of Cs was similar to that of Rb, but it was not similar to that of K in the sewage treatment process.     

A generalised sorption model for the concentration dependent uptake of caesium by argillaceous rocks

A three-site cation exchange model is proposed to describe the concentration dependent uptake of Cs on natural argillaceous rock systems. Major premises in the model are that the sorption of Cs is dominated by the illite mineral component in the rock and that there is a fixed relationship between the site capacities of the three site types denoted as frayed edge, type II and planar sites. The definition of a “reference illite” with a cation exchange capacity of 0.2 equiv. kg^-1 allows the three site capacities to be fixed in the model calculations over the weight fraction of illite in the argillaceous rocks. Up to Cs equilibrium concentrations of 10^-3 M sorption occurs predominantly on the frayed edge and type II sites (higher affinity sites), with the planar site type playing only a minor role. Competition with Cs for sorption on the former two site types arises predominantly from monovalent cations such as K, Rb and NH₄ which have low hydration energies. H and Na (except at high concentrations) are considerably less competitive and bivalent cations such as Mg, Ca and Sr are effectively non-competitive. A consistent set of selectivity coefficients for Cs with respect to K, Rb, NH₄ and Na was derived from analyses and modelling of a wide range of Cs sorption data available in the open literature on pure illites from many different sources. The model was tested against four Cs sorption isotherm data sets determined on argillaceous rocks: Boom clay, Oxford clay, Palfris marl and Opalinus clay. The water chemistries and illite contents given in these experiments allowed the Cs sorption isotherms to be predicted. It is concluded that the Cs sorption model presented here, in which there are no free parameters, can be used to predict the uptake of Cs at equilibrium concentrations below 10^-3 M to within a factor of 2 to 3 in natural argillaceous rock systems.     

Artificial radioactivity in environmental media (air, rainwater, soil, vegetation) in Austria after the Fukushima nuclear accident

Several environmental media in Austria were monitored for artificial radionuclides released during the Fukushima nuclear accident. Air (up to 1.2 mBq/m³ particulate ¹³¹I) and rainwater (up to 5.2 Bq/L ¹³¹I) proved to be the media best suited for the environmental monitoring, allowing also a temporal resolution of the activity levels. Significant regional differences in the wet deposition of ¹³¹I with rain could be observed within the city of Vienna during the arrival of the contaminated air masses. Forward-trajectory analysis supported the hypothesis that the contaminated air masses coming from the northwest changed direction to northeast over Northern Austria, leading to a strong activity concentration gradient over Vienna. In the course of the environmental monitoring of the Fukushima releases, this phenomenon—significant differences of ¹³¹I activity concentrations in rainwater on a narrow local scale (8.1 km)—appears to be unique. Vegetation (grass) was contaminated with ¹³¹I and/or ¹³⁷Cs at a low level. Soil (up to 22 Bq/kg ¹³⁷Cs) was only affected by previous releases (nuclear weapon tests, Chernobyl). Here, also significant local differences can be observed due to different deposition rates during the Chernobyl accident. The effective ecological half-lives of ¹³⁷Cs in soil were calculated for four locations in Austria. They range from 7 to 30 years. No Austrian sample investigated herein exceeded the detection limit for ¹³⁴Cs; hence, the Fukushima nuclear accident did not contribute significantly to the total radiocesium inventory in Austrian environmental media. The levels of detected radioactivity were of no concern for public health.     

Possible change in Asian dust source suggested by atmospheric anthropogenic radionuclides during the 2000s

Decades-long monitoring of anthropogenic radionuclides in the atmospheric deposition in Tsukuba, Japan suggests not only the substantial impacts of the Asian dust (Kosa) on the deposition but also the possible change of the Kosa source region, especially during springs of the 2000s. In order to know more about such change, 4 single wet deposition events occurred in the spring of 2007 were scrutinized. The largest anthropogenic radionuclides wet deposition was supplied by the April 2–4 event. It brought several tens % of the monthly depositions (April 2007) of the dust (residue) mass (4.5 g m^?2) and anthropogenic radionulides (⁹⁰Sr: 16, ¹³⁷Cs: 97 and Pu: 3 mBq m^?2). None of the events observed fulfilled both criteria of the specific activities and ⁹⁰Sr/¹³⁷Cs activity ratio to the Tsukuba soil; they did not exhibit local soil dust signature. The Kosa events in fact have extensive impacts on the atmospheric environment over Japan in spring season. Considering the elevated specific activities as well as greater ¹³⁷Cs/⁹⁰Sr activity ratio in the deposited dust, it is hypothesized that the dust source areas in Asian continent would be shifting from the arid zone to the desert-steppe zone suffering from desertification during the 2000s. This type of the Kosa may be called as the ‘new-regime Kosa’. Chemical observation in the far downwind region of the Kosa dust could allow us to know possible shift in the source regions.     

Analysis of the migration of instantaneously injected cesium in artificial fractures of Lac du Bonnet granite, Manitoba, Canada

Breakthrough curves of ¹³⁷Cs and tritiated water injected instantaneously into artificial fractures in Lac du Bonnet granite were analyzed using the analytical solution for a single rock-fracture system and assuming the linear sorption isotherm of the solute. Parameters of nuclide diffusion and sorption in rock matrices, obtained by fitting, varied depending on the flow velocity in the fractures. According to theoretical calculations, different fracture flow velocities lead to different diffusion distances of nuclides in matrices at the same injection volume. As microscopic inhomogeneity is considered to exist in the rock matrix, the average diffusion-sorption characteristics of the matrix within the diffusion distance may have varied with the fracture flow velocity. Surface sorption was marked in fractures that had relatively high matrix sorption-diffusion capacities. The phenomenon was interpreted using the theoretical relationships developed between the surface sorption, matrix sorption and pore diffusion coefficient, and the porosity of matrices.The effect of the nonlinear sorption of solute was examined by numerically solving model equations that incorporate the nonlinear isotherm. This incorporation may contribute to the reduction of deviations between theoretical and experimental BTC's.     

Sources and pathways of artificial radionuclides to soils at a High Arctic site

Activity concentrations, inventories and activity ratios of ¹³⁷Cs, ²³⁸Pu, ^239?+?240Pu and ²⁴¹Am in soil profiles were surveyed in the dry tundra and the adjoining proglacial zones of glaciers at a High Arctic site on Svalbard. Vertical profiles of radionuclide activities were determined in up to 14-cm-thick soil sequences. Additionally, soil properties (pH, organic matter, texture, mineral composition and sorption capacity) were analyzed. Results obtained in this study revealed a large range of activity concentrations and inventories of the fallout radionuclides from the undetectable to the uncommonly high levels (inventories of 30,900?±?940, 47?±?6, 886?±?80 and 296?±?19 Bq/m² for ¹³⁷Cs, ²³⁸Pu, ^239?+?240Pu and ²⁴¹Am, respectively) found in two profiles from the proglacial zone. Concentration of these initially airborne radionuclides in the proglacial zone soils is related to their accumulation in cryoconites that have a large ability to concentrate trace metals. The cryoconites develop on the surface of glaciers, and the material they accumulate is deposited on land surface after the glaciers retreat. The radionuclide inventories in the tundra soils, which effectively retain radionuclides due to high organic matter contents, were comparable to the global fallout deposition for this region of the world. The ²³⁸Pu/^239?+?240Pu activity ratios for tundra soils suggested global fallout as the dominant source of Pu. The ²³⁸Pu/^239?+?240Pu and ^239?+?240Pu/¹³⁷Cs activity ratios in the proglacial soils pointed to possible contributions of these radionuclides from other, unidentified sources.     

Comparative kinetic desorption of 60Co, 85Sr and 134Cs from a contaminated natural silica sand column: influence of varying physicochemical conditions and dissolved organic matter

In order to determine the mechanisms of the retention of 60Co, 85Sr and 134Cs in natural silica sand columns, desorption experiments were performed by changes of pH and ionic strength and by injection of natural organic matter (NOM). Injection of KCl (0.1 M) resulted in a high release of 60Co (60-100%) and 85Sr (72-100%) but a smaller release of 134Cs (31-66%). Only limited release of 60Co (66%) and 85Sr (71%) and no release of 134Cs were observed by injection of NOM. The different percentages of desorption were related to the chemical characteristics of the organic colloids previously retained in columns before the desorption step. The results evidenced different sorption processes on energetically heterogeneous surface sites. According to the initial conditions, the binding of the radionuclides to the solid phase resulted from weak and easily reversible sorption processes to strong association probably by inner sphere complexes. The rather weak release of 134Cs by KCl was attributed to the strong retention of 134Cs by clay coatings on the natural silica sand surfaces.     

Atmospheric emissions from the Windscale accident of October 1957

Although it occurred nearly 50 years ago, the nuclear reactor fire of October 1957 at Windscale Works, Sellafield, England, continues to attract interest. Several attempts have been made to quantify the releases of radionuclides and their radiological consequences, but additional information and a re-analysis of meteorological data encourage a further examination of emissions. The limited instrumentation of the reactor provided little relevant information and, as in previous estimates, the discharges are deduced from environmental evidence, but here the recent meteorological analysis is used. The interpretation of the meteorological and environmental evidence requires both timing and quantity of the emitted radionuclides to be considered together.Significant fission product emission continued from about 15:00 or 16:00 on 10 October 1957 until noon the following day. There were two main peaks in discharge rate, during the evening and early hours and from roughly 06:00 until 10:30, and the amounts emitted during each of these periods were probably comparable.Iodine-131 (¹³¹I), caesium-137 (¹³⁷Cs) and polonium-210 (²¹⁰Po) activities dominated the radioactive emissions and there is sufficient environmental evidence for releases of these radionuclides to be estimated within a factor of about two. (Some additional ¹³¹I may have escaped in a chemical form that was not included in the estimate, but it appears likely that the fraction was small.) There is evidence that the plume extended further east than accepted in previous assessments and the estimates of quantities emitted have been increased to allow for this. For other radionuclides the environmental measurements were fewer and the uncertainties are greater.     

Migration of Cs through quartz sand: experimental results and modeling approaches

We present results from experiments on the migration of ¹³⁷Cs through columns containing quartz sand. Times for ¹³⁷Cs movement through these columns and the quantity of ¹³⁷Cs adsorbed by the sand decreased as the ionic strength of the pore water increased from 0.002 to 0.1 m. The breakthrough curves were characterized by a slow approach towards steady-state concentrations as well as by long tails, indicating that ¹³⁷Cs adsorption to the sand grains was, at least in part, controlled by rate-limited reactions. Various formulations for solute mass transfer were tested for their ability to fit the experimental breakthrough curves. Based on a statistical analysis, a nonlinear, two-site model was identified as the most appropriate for describing the suite of experimental data. Variation in the model parameter that describes the rate of ¹³⁷Cs adsorption to the sand showed no consistent pattern with changes in ionic strength. In contrast, model parameters describing the sorption capacity of the sand grains and the fraction of kinetic sorption sites on the sand decreased with increasing ionic strength. The parameter describing the rate of ¹³⁷Cs desorption varied directly with changes in ionic strength.     

Reactive barriers for Cs retention

¹³⁷Cs was dispersed globally by cold war activities and, more recently, by the Chernobyl accident. Engineered extraction of ¹³⁷Cs from soils and groundwaters is exceedingly difficult. Because the half-life of ¹³⁷Cs is only 30.2 years, remediation might be more effective (and less costly) if ¹³⁷Cs bioavailability could be demonstrably limited for even a few decades by use of a reactive barrier. Essentially permanent isolation must be demonstrated in those few settings where high nuclear level wastes contaminated the environment with ¹³⁵Cs (half-life 2.3×10⁶ years) in addition to ¹³⁷Cs. Clays are potentially a low-cost barrier to Cs movement, though their long-term effectiveness remains untested. To identify optimal clays for Cs retention, Cs desorption was measured for five common clays: Wyoming Montmorillonite (SWy-1), Georgia Kaolinites (KGa-1 and KGa-2), Fithian Illite (F-Ill), and K-Metabentonite (K-Mbt). Exchange sites were pre-saturated with 0.16 M CsCl for 14 days and readily exchangeable Cs was removed by a series of LiNO₃ and LiCl washes. Washed clays were then placed into dialysis bags and the Cs release to the deionized water outside the bags measured. Release rates from 75 to 139 days for SWy-1, K-Mbt and F-Ill were similar; 0.017% to 0.021% sorbed Cs released per day. Both kaolinites released Cs more rapidly (0.12% to 0.05% of the sorbed Cs per day). In a second set of experiments, clays were Cs-doped for 110 days and subjected to an extreme and prolonged rinsing process. All the clays exhibited some capacity for irreversible Cs uptake. However, the residual loading was greatest on K-Mbt (0.33 wt.% Cs). Thus, this clay would be the optimal material for constructing artifical reactive barriers.