A Continuum One——Dimensional Model for Radionuclide Migration Testing in Unsaturated Loess

Unsaturated radionuclide migration experiments were conducted in a pit inside thetesting hall.Several types of radionuclides were used in the experiments.Tritium wasused as a tracer for water movement in unsaturated loess.Other kinds of radionuclideswere also used in order to obtain fundamental parameters for radionuclide migration sothat further environmental assessment of low—level radioactive waste disposal can be car-ried out.Mechanisms governing unsaturated flow in loess,that is,principles ofone—way lateral flow,are presented qualitatively in this paper.And a continuumone—dimensional model for radionuclide migration testing is developed based on the ex-periments conducted under the particular conditions at the test site.The data measuredfrom the tests were compared with solutions of this one—dimensional model.Resultsshow that this model is feasible for modeling radionuclide migration in unsaturatedloess.     

Effect of colloids on transfer of radionuclides by subsurface water

The fluid transfer of radionuclides in the geologic medium is considered under conditions when radionuclides are contained in fluids not only as solutes but also as colloids. The effect of colloidal transport of radionuclides on the rate of spreading of radioactive contamination in an underground medium is estimated, with assessment of this effect in mathematical models describing the transport of radionuclides by subsurface water. For this purpose, the exchange of radionuclides between subsurface water, colloid, and an immobile solid phase is considered, taking into account the precipitation of colloidal particles on both the immobile solid phase and other colloidal particles and their recurrent mobilization into the liquid phase. It is noted that, in real colloidal transfer, the heterogeneity of the geologic medium and colloidal particles in subsurface water is of great importance. The known models of colloidal transfer of radionuclides are evaluated on the basis of the analysis performed.     

核素迁移的示踪试验研究综述

按地下实验室或处置库的不同主岩,简述了以花岗岩,岩盐,粘土岩和凝灰岩等作处置库主岩的核素迁移示踪试验的研究现状,重点介绍了以花岗岩作高效废物处置库主岩的示踪试验方法和进展。     

Results of Marine Radioecological Study of Fjords of Western Spitsbergen

The patterns of the distribution of technogenic radionuclides ¹³⁷Cs and ⁹⁰Sr in the water mass and bottom deposits of three fjords of Western Spitsbergen (Isfjorden and its arms Grønfjorden and Billefjorden) were studied. The hydrological processes affecting migration of radionuclides in these water bodies were described. It was shown that the ice sheet of the archipelago that accumulates atmospheric precipitation is a source of radioactive contamination of the marine environment of the coast. Technogenic isotopes were accumulated during nuclear weapon testing in the second half of the twentieth century. The current climatic changes strengthen the role of glaciers in the contamination of arctic seas. It was noted that extrema of the ¹³⁷Cs and ⁹⁰Sr specific activity is mainly concentrated in the areas of glacier runoff discharge.

     

蒙脱石吸附模拟核素Sr、Cs的性能研究

基于蒙脱石具有良好的阳离子交换性能,采用静态吸附法,对阜新钙基蒙脱石吸附模拟核素Sr、Cs的吸附条件和吸附行为进行了研究。结果表明:蒙脱石样品对Sr2+、Cs+的饱和吸附量分别为0.271mmol/g和0.780mmol/g,在较大的pH值范围(4～11)内具有良好的吸附稳定性;蒙脱石对Sr2+、Cs+的吸附类型以阳离子交换吸附为主,符合Langmuir和Freundlich模型,但与Langmuir吻合更好,相关系数分别达到0.99924和0.99995。本实验研究对于处理低浓度的含Sr、Cs放射性废液具有重要的意义。     

Modelling seasonal variations of natural radioactivity in soils: A case study in southern Italy

The activity of natural radionuclides in soil has become an environmental concern for local public and national authorities because of the harmful effects of radiation exposure on human health. In this context, modelling and mapping the activity of natural radionuclides in soil is an important research topic. The study was aimed to model, in a spatial sense, the soil radioactivity in an urban and peri-urban soils area in southern Italy to analyse the seasonal influence on soil radioactivity. Measures of gamma radiation naturally emitted through the decay of radioactive isotopes (potassium, uranium and thorium) were analysed using a geostatistical approach to map the spatial distribution of soil radioactivity. The activity of three radionuclides was measured at 181 locations using a high-resolution ?-ray spectrometry. To take into account the influence of season, the measurements were carried out in summer and in winter. Activity data were analysed by using a geostatistical approach and zones of relatively high or low radioactivity were delineated. Among the main processes which influence natural radioactivity such as geology, geochemical, pedological, and ecological processes, results of this study showed a prominent control of radio-emission measurements by seasonal changes. Low natural radioactivity levels were measured in December associated with winter weather and moist soil conditions (due to high rainfall and low temperature), and higher activity values in July, when the soil was dry and no precipitations occurred.     

Some geochemical and environmental aspects of the Chernobyl nuclear accident

The Chernobyl nuclear accident took place on 1986-04-26, and resulted in radionuclide pollution of vast regions of Byelorussia. This has raised several geochemical problems: (1) investigation of the distribution and behaviour of radionuclides in the natural and technogenic landscapes (which were examined as a result of a 5-a study); (2) modes of occurence of radioactive elements in the environment; and (3) preliminary estimation of radionuclide migration in the future. As background information, geochemical studies carried out within Byelorussia, as well as landscape investigations in the period 1953–1987 made under the guidance of K.I. Lukashev were very important for solving the above questions. According to the results of these investigations, the lithogeochemical and hydrogeochemical zoning of Byelorussia is understood and the radioactive background of the republic was studied in the 1960s, as well as the distribution and peculiarities of clay minerals, carbonates, FeMn oxides, organic matter, pH-Eh, and the contents of many elements in the surficial deposits of Byelorussia. All these factors are of great importance for understanding the behaviour of radionuclides.Radionuclide fallout on Byelorussia in the first days after the accident was mainly dependent on the mass movement of air and rain. In cities, fallout was confined to regions with intensive industrial dust emissions, as well as to river valleys, where degassing of deep-seated zones through faults occurred side by side with evaporation. Radionuclide washout from upland territories can be related to secondary processes.After 5 a, the radioactive emission near the surface of the Earth decreased due to the decay of short-lived isotopes and penetration of radionuclides deeper into the soil, although the major part still occurs at a depth of 1–5 cm. Bogs, peat-bog soils, aquifers with fluctuating groundwater levels, variable pH-Eh conditions and a igh biological activity are all factors contributing to radionuclide migration in the Byelorussian landscape. A part of the radionuclides is gradually removed from eluvial landscape and accumulated in subareal landscape (e.g. lakes, oxbow-lakes, water-storage basins).The Chernobyl debris are represented by the following substances: “hot” particles, pseudocolloids, aerosols and gaseous compounds. Study of the modes of occurrence of radionuclides by different methods (e.g. extracts, sorbents) made it possible to estimate the migration capacity of some radionuclides (Cs, Sr, Ce, Ru), and to distinguish two zones around the reactor—the nearest and remote ones—differing in the ratio of “hot” particles and condensate fallout, which causes a different migration capcity of radionuclides.A very important part is assigned to biological processes and organic matter, which cause the destruction of “hot” paricles, the formation of organomethallic complexes, and water migration of nuclides.In the future (within 300 and more years), the redistribution of radionuclides in the landscape involves processes of weathering, erosion and sedimentation which will strongly depend on climatic conditions, Side by side with a gradual decay of Cs and Sr, an appreciable accumulation of²⁴¹Am, which is very mobile in landscape, should be expected due to decaying²⁴¹Pu.A considerable development of scientific and applied exploration should also be expected in the future and this will help in solving problems of radioactive pollution of landscapes.     

Conceptual model for radionuclide release,Yucca Mountain,Nevada, USA

A new conceptual model for release rate of radionuclides from the proposed repository for high level nuclear waste located at Yucca Mountain, Nevada is developed. The model predicts that heat generated from radioactive decay combined with the unsaturated environment will lead to an inward flow system that, under many relevant conditions, will slow the release of and sometimes sequester radionuclides at locations of higher heat release and lower water percolation. The amount of protection will be greatest for more concentrated waste forms such as spent fuel and less for glass waste forms. The redistribution and concentration of the radionuclides is anticipated to significantly delay radionuclide release and create a tendency towards gradual release over time that is independent of localized penetrations of metallic barriers.     

Evaluation of retention properties of a semi-synthetic fractured block from modelling at performance assessment time scales (Äspö Hard Rock Laboratory, Sweden)

Modelling of radionuclide transport in fractured media is a primary task for safety evaluation of a deep nuclear waste repository. A performance assessment (PA) model has been derived from site characterization data with the aim of improving confidence for quantifying transport of sorbing radionuclides at a safety time scale of 10⁶ y. The study was conducted on a 200?×?200?×?200 m semi-synthetic fractured block, providing a realistic system derived from the Äspö Hard Rock Laboratory (Sweden) dataset. The block includes 5,632 fractures ranging from 0.5 to 100 m in length and a heterogeneous matrix structure (fracture coating, gouge, mylonite, altered and non-altered diorite). The PA model integrates steady-state flow conditions and transport of released radionuclides during the safety time scale. An original simulation method was developed involving Eulerian flow and transport within fracture planes with a mixed hybrid finite element scheme and a semi-analytical source term to account for heterogeneous matrix diffusion. Total mass flux of radionuclides (conservative to strongly sorbing) was computed. A method to simplify the system was demonstrated, leading to a major path of 12 fractures. This is mainly due to the low connectivity of the fracture network. Matrix diffusion and sorption proved to have major impact on block retention properties for PA conditions.     

Natural radionuclides as tracers of coastal biogeochemical processes

This paper presents an overview of the ways that natural radionuclides have been used as tracers of biogeochemical processes occurring in the coastal ocean. The radionuclides involved include those in the uranium and thorium decay series and those produced in the atmosphere by the interaction of cosmic rays with atmospheric gases (i.e., cosmogenic radionuclides). The property of radioactivity provides a chronometer with which to measure the rates of oceanic processes, and the fundamental biogeochemical behavior of the radionuclide determines which process(es) it may be used to trace. Examples from recent research will be presented in the talk.     

环境生态系统散落核素示踪研究新进展

环境生态系统中的放射性核素既具有生态毒性,对生物和人体健康产生重要影响,又具有稳定的衰变函数和输入通量,对环境生态系统具有重要示踪价值。人类可利用放射性核素来认识环境生态系统:①环境生态系统中放射性核素的基准:放射性强度与环境生态系统之间的平衡关系;②环境生态系统中放射性核素的行为:在环境生态系统不同宿体和组分间的运移关系。考察放射性核素在生态系统不同环节中比活度的变化有助于认识环境生态过程生物地球化学作用的影响。       

Model of colloidal transportation of radionuclides by groundwater

The transportation of colloidal radionuclides by groundwater was subject to theoretical analysis. The far field of radioactive contamination of the underground environment (liquid waste pumping sites or storage of solidified waste) is dominated by pseudocolloids, i.e., colloidal particles of natural origin contaminated with radionuclides upon contact of groundwater with radioactive materials. Properties of real pseudocolloids were analyzed at sites of radioactive contamination of the underground environment. Based on a probabilistic approach, we developed a mathematical model of pseudocolloid transportation by groundwater, taking into account the difference in size of colloidal particles and the occurrence of nonradioactive natural particles with a similar composition in the groundwater. It is proved that nonuniform dimensions of the particles considerably affect the water transportation rate.     

Influence of temperature decrease and soil drought on the geochemical fractionation of 60Co and 137Cs in fluvisol and cambisol soils

Consideration of the impact of substantial changes in soil temperature or moisture regime on the geochemical forms of radionuclides is important for more accurate assessment of the environmental risk posed by radionuclide migration and potential biological availability, especially in the first months after their release into the environment. This paper presents the results from a study of the influence of cooling, freezing and soil drought on the migration and potential bioavailability of ⁶⁰Co and ¹³⁷Cs in two soils (a fluvisol and a cambisol, according to the World Reference Base for Soil Resources/FAO) from Bulgaria. The changes in the geochemical fractionation of ⁶⁰Co, the exchangeable ¹³⁷Cs and water-soluble forms of both radionuclides were examined under different storage conditions up to 5 months after their introduction into the soils in solution form. Freezing or soil drought resulted in a significant increase of the water-soluble forms of ⁶⁰Co in the fluvisol soil, defining higher mobility and potential bioavailability. No influence of the storing conditions on the water-solubility of ⁶⁰Co in the cambisol soil was established. The cooling, freezing and soil drought caused an increase of the exchangeable ¹³⁷Cs in both soils.     

对重金属和辐射污染的土壤和地下水的微生物修复

由重金属和辐射产生的环境污染在世界范围内产生了一系列问题.利用特殊的微生物如金属还原和耐金属细菌对环境中的金属和辐射污染进行处理具有非常好的前景.现场的生物修复的成功应用将对清除污染环境中的重金属和辐射提供潜在方法.最近的研究还关注于了解在微生物群体内重金属和辐射对微生物的作用.生物毯和生物膜是在生物修复中具有代表性的两种微生物群落的机能.金属的种类和价态变化、转移过程以及微生物代谢作用是对金属和辐射生物修复的三种重要的组成部分.结合以上三方面,可以更好的了解自然中的微生物和生物修复过程之间的关系.     

Membrane luminescence determination of technogenic actinides and their speciation in environmental objects

Researchers of the Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences created a luminescence photometer of a new generation for the determination of trace amounts of uranium and transuranium elements (TUE). The limits of detection for actinides vary from 0.3 pg for uranium and neptunium to 2.0 pg for plutonium. For ²³⁷Np, the relative limit of detection is 0.008 Bq/L. The photometer was tested in the radioecological monitoring of a number of polluted zones in Russia. The dynamics of actinide migration in all of the studied zones enhanced in the series ²³⁹Pu < ²⁴¹Am < ²³⁷Np. In this series, concentrations of radionuclides in water-soluble and exchange forms that are most mobile and determine the migration mobility of chemical elements increased in all of the studied soil types. In the group of fulvic acids, concentrations of radionuclides decreased in the series ²³⁷Np > ²⁴¹Am > ²³⁹Pu irrespectively of the soil. In the group of humic acids, concentrations of radionuclides increased in the series ²³⁷Np < ²³⁹Pu < ²⁴¹Am. The sorption coefficients of radionuclides by bottom sediments of the Markha River (Kraton-3 underground nuclear explosion site) and Lake Kyzyltash (East Urals Radioactive Trace) were calculated. Bioaccumulation factors of radionuclides by different plants in the impact area of the Kraton underground nuclear explosion were determined depending on the plant type.     

Artificial radionuclides in the sediments of the Rhne river: a snapshot of the situation during winter 1994/95

Artificial radionuclides (⁶⁰Co, ¹⁰⁶Ru, ¹³⁴Cs and ¹³⁷Cs) produced in the nuclear power plants along the Rhne valley have been detected in most of the 35 sediment samples collected from the shores of the Rhne river in southern France. In one sample traces of ²⁴¹Am were found, suggesting the presence of transuranic elements. The wide use of river water for irrigation and the occasional big inundations of the Rhne have spread the artificial radionuclides over huge areas, quite far behind the river shores. Although artificial radionuclides are present in fields where food crops are collected, their activity is currently considerably lower than the activity of the natural radionuclides present in the river sediment.     

Mobilization and mechanisms of retardation in the Oklo natural reactor zone 2 (Gabon)—inferences from U, REE, Zr, Mo and Se isotopes

Mineralogical and isotopic studies were carried out on the natural nuclear reaction zone 2 from the Oklo deposit to evaluate the mobility of several nuclear reaction products in response to the alteration of the reaction zone and to identify the mechanisms which could retard the transport of released radionuclides. To address these issues, in situ isotopic analyses by SHRIMP and a selective extraction procedure were performed to constrain the structural location of nuclear reaction products (exchangeable and non exchangeable) and their association with mineral phases. The distribution patterns of U, REE, Zr and Mo isotopes reveal that substantial amounts were released from the core and migrated through the hydrothermal alteration halo over metric distances, owing to uraninite dissolution and advective transport by hydrothermal solutions during and soon after criticality. The results emphasize the mobility of Zr at Oklo, this element being often considered as “immobile” during water–rock interactions. The main output is the demonstration of the net effects of sorption and coprecipitation processes. Chlorite and to a lesser extent illite were found to have adsorbed significant amounts of U, REE, Zr (and probably Th) and less sorbing elements such as Mo. Coprecipitation of secondary UO₂ and P-rich coffinite within the alteration halo is also an important means of retardation. The concentration of radionuclides released from the reactor were probably high and they display solubility limited transport behaviour. No retention effect was found for Se in the immediate vicinity of the reactor and this element may have moved farther from its source of production. These results have interesting implications for the evaluation of long-term containment of radionuclides. They provide a simple illustration of the performance of a clay barrier in the uptake of radionuclides by sorption onto clays and reincorporation in secondary U-minerals. This study also demonstrates the robustness of these retention processes over extremely long periods of time.     

Long-core profiles of137Cs,134Cs,60Co and210Pb in sediment near the Rhône River (Northwestern Mediterranean Sea)

Various anthropogenic radionuclides and²¹⁰Pb were analyzed in a 4.3-m-long core, sampled near the Rhône River mouth in March 1991, to evaluate the extent of industrial releases that accumulate in this area. The whole core was significantly marked by radionuclide inputs from the nuclear facilities located along the river (¹³⁷Cs,¹³⁴Cs,⁶⁰Co). Irregular profiles in natural and artificial radionuclides should be related to variations in their respective inputs from the Rhône River to the Mediterranean Sea. Minimum concentrations were found during high flow periods. Using both the¹³⁷Cs/¹³⁴Cs profile in the core and the range of this ratio in Rhône waters, mean apparent accumulation rates were estimated to range between 37 cm yr^?1 and 48 cm yr^?1. This core would then represent a sedimentary record over a 7–10 year period. However, the presence of a signal from the Chernobyl accident, which occurred on April 26, 1986, was not clearly observed in the core. Inventories of both artificial and natural radionuclides were greater than expected from atmospheric inputs. The increased sedimentation occurring in close vicinity to the mouth of the Rhône River is thus responsible for trapping of elements transported by the river to the Mediterranean Sea. In this area, inventories of artificial radionuclides are well in excess of aerial deposition from Chernobyl and atmospheric weapons tests and are linked primarily to industrial releases.     

A model of radionuclide transfer by groundwater on the territory of the Russian Research Centre Kurchatov Institute

The convective transfer of ¹³⁷Cs and ⁹⁰Sr by groundwater on the territory of the Russian Research Centre Kurchatov Institute (RRCKI) was modeled. Geological data on the RRCKI site and possible sources of radionuclides show that the uppermost aquifer, composed of Quaternary sediments, is the most probable region of spreading of radioactive contamination. Since the lateral migration of radionuclides is predominant, a 2D horizontal model was used for the forecasting of spreading of radioactive contaminants in the subsurface medium. Transient or long-term repositories of radioactive materials at the RRCKI site (concentrated sources) and aquifer rocks contaminated in the course of removal of radionuclides from these repositories (distributed source) are responsible for groundwater pollution. The initial ¹³⁷Cs and ⁹⁰Sr distributions used in the forecasting of radionuclide migration were determined from their contents in core samples taken from wells drilled in contaminated areas of the RRCKI site. The zone of radionuclide migration is limited by poorly permeable moraine loam from below and by the water table from above. To determine the upper and lower boundaries, these surfaces were mapped in plan view. The data of meteorological observations over a long period were used to map the intensity of precipitation in the studied territory. The density of rocks in the uppermost aquifer and partition coefficients of radionuclides between rocks and groundwater were estimated from the data of laboratory examination of the core samples. The available data on the permeability of rock samples and the results of test pumping out of some wells were involved in the consideration. The results obtained and the data on the water table allowed us to define a gauge problem for determining the distribution of the filtration coefficient in the uppermost aquifer. This problem was solved taking the intensity of precipitation into account. The properties of the uppermost aquifer and the initial radionuclide distribution therein were used as initial data for modeling ¹³⁷Cs and ⁹⁰Sr migration on the territory of the RRCKI over 50 years.