Radionuclide Transport Behavior in a Generic Geological Radioactive Waste Repository          下载免费PDF全文

Marco Bianchi  Hui‐Hai Liu  Jens T. Birkholzer 《Ground water》2015,53(3):440-451

We performed numerical simulations of groundwater flow and radionuclide transport to study the influence of several factors, including the ambient hydraulic gradient, groundwater pressure anomalies, and the properties of the excavation damaged zone (EDZ), on the prevailing transport mechanism (i.e., advection or molecular diffusion) in a generic nuclear waste repository within a clay‐rich geological formation. By comparing simulation results, we show that the EDZ plays a major role as a preferential flowpath for radionuclide transport. When the EDZ is not taken into account, transport is dominated by molecular diffusion in almost the totality of the simulated domain, and transport velocity is about 40% slower. Modeling results also show that a reduction in hydraulic gradient leads to a greater predominance of diffusive transport, slowing down radionuclide transport by about 30% with respect to a scenario assuming a unit gradient. In addition, inward flow caused by negative pressure anomalies in the clay‐rich formation further reduces transport velocity, enhancing the ability of the geological barrier to contain the radioactive waste. On the other hand, local high gradients associated with positive pressure anomalies can speed up radionuclide transport with respect to steady‐state flow systems having the same regional hydraulic gradients. Transport behavior was also found to be sensitive to both geometrical and hydrogeological parameters of the EDZ. Results from this work can provide useful knowledge toward correctly assessing the post‐closure safety of a geological disposal system.  相似文献   

A comparative simulation study of coupled THM processes and their effect on fractured rock permeability around nuclear waste repositories     

Jonny Rutqvist  Deborah Barr  Jens T. Birkholzer  Kiyoshi Fujisaki  Olaf Kolditz  Quan-Sheng Liu  Tomoo Fujita  Wenqing Wang  Cheng-Yuan Zhang 《Environmental Geology》2009,57(6):1347-1360

This paper presents an international, multiple-code, simulation study of coupled thermal, hydrological, and mechanical (THM) processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement drifts. Simulations were conducted considering two types of repository settings (1) open emplacement drifts in relatively shallow unsaturated volcanic rock, and (2) backfilled emplacement drifts in deeper saturated crystalline rock. The results showed that for the two assumed repository settings, the dominant mechanism of changes in rock permeability was thermal–mechanically induced closure (reduced aperture) of vertical fractures, caused by thermal stress resulting from repository-wide heating of the rock mass. The magnitude of thermal–mechanically induced changes in permeability was more substantial in the case of an emplacement drift located in a relatively shallow, low-stress environment where the rock is more compliant, allowing more substantial fracture closure during thermal stressing. However, in both of the assumed repository settings in this study, the thermal–mechanically induced changes in permeability caused relatively small changes in the flow field, with most changes occurring in the vicinity of the emplacement drifts.  相似文献   

Seasonality in the North Sea during the Allerød and Late Medieval Climate Optimum using bivalve sclerochronology     

Bernd R. Schöne  Jens Fiebig 《International Journal of Earth Sciences》2009,98(1):83-98

Seasonal temperature patterns may have changed through time in response to current global warming. However, the temporal resolution of available proxy records is not sufficient to quantify paleotemperature seasonality prior to anthropogenic forcing of the climate. In the present study, we reconstructed seasonal and inter-annual temperature patterns of the North Sea during the last 140 years, the Allerød Interglacial and the Late Medieval Climate Optimum using sclerochronological and δ¹⁸O_aragonite data from bivalve shells, Arctica islandica. On average, the climate during 1278–1353 AD was ca. 1.1°C colder and seasonality was ca. 60% less than today. During the Allerød, long-term temperatures remained about 3.2°C below present values, and absolute summer and winter anomalies were ca. ?4 and ?2.7°C, respectively. However, seasonality was statistically indistinguishable from today. Long-term average temperatures compare well with existing data for the Late Medieval and Allerød, but detailed information on seasonality during the studied time intervals has never been presented before. Our data also demonstrated that annual instrumental and δ¹⁸O_aragonite-derived temperatures did not always match. This difference is explained by (1) NAO-driven salinity changes, which influence the temperature estimation from δ¹⁸O_aragonite and (2) food-driven changes in growth rates; portions of the shell that formed more rapidly are overrepresented in carbonate samples. Our study indicated that individual bivalve shells can open discrete, near-century long, ultra-high-resolution windows into the climate past. Such information can be vital for testing and verifying numerical climate models.  相似文献   

Regional and global effects of southern ocean constraints in a global model     

Michael P. Schodlok  Manfred Wenzel  Jens G. Schröter  Hartmut H. Hellmer 《Ocean Dynamics》2008,58(3-4):155-168

Global ocean circulation models do not usually take high-latitude processes into account in an adequate form due to a limited model domain or insufficient resolution. Without the processes in key areas contributing to the lower part of the global thermohaline circulation, the characteristics and flow of deep and bottom waters often remain unrealistic in these models. In this study, various sections of the Bremerhaven Regional Ice Ocean Simulation model results are combined with a global inverse model by using temperature, salinity, and velocity constraints for the Hamburg Large Scale Geostrophic ocean general circulation model. The differences between the global model with and without additional constraints from the regional model demonstrate that the Weddell Sea circulation exerts a significant influence on the course of the Antarctic Circumpolar Current with consequences for Southern Ocean water mass characteristics and the spreading of deep and bottom waters in the South Atlantic. The influence of the Ross Sea is found to be less important in terms of global influences. However, regional changes in the Pacific sector of the Southern Ocean are found to be of Ross Sea origin. The additional constraints change the hydrographic conditions of the global model in the vicinity of the Antarctic Circumpolar Current in such a way that transport values, e.g., in Drake Passage no longer need to be prescribed to obtain observed transports. These changes not only improve the path and transport of the Antarctic Circumpolar Current but affect the meso- and large-scale circulation. With a higher (lower) mean Drake Passage transport, the mean Weddell Gyre transport is lower (higher). Furthermore, an increase (decrease) in the Antarctic Circumpolar Current leads to a decrease (increase) of the circum-Australian flow, i.e., a decrease (increase) of the Indonesian Throughflow.  相似文献   

Differences in the eutrophication dynamics of Ammersee and Starnberger See (Southern Germany), reflected by the diatom succession in varve-dated sediments     

Joachim Alefs  Jens Müller 《Journal of Paleolimnology》1999,21(4):395-407

Diatom assemblages in the annually laminated sediments of two neighboring, pre-alpine lakes (Ammersee and Starnberger See, Southern Germany) were analyzed and compared year by year. Within both varve records of the last decades, the same 4-phase-succession of planktonic diatom species is preserved, consisting of 1. a dominance of Cyclotella species 2. assemblages of Fragilaria crotonensis, Asterionella formosa and Stephanodiscus minutulus 3. a mass bloom of Aulacoseira islandica, and 4. a dominance of Stephanodiscus species. This diatom sequence is considered as a model of a 'basic species sequence of eutrophication'. Time lags and differences in the duration of the phases between both lakes show evidence of an earlier start for nutrient loading in Starnberger See than in Ammersee, and a faster development to a higher trophic state level in the latter. The different reactions of the lakes are attributed to hydrological differences such as the existence or lack of a major tributary, the size of the catchment area, and the water residence time. The reconstruction of the eutrophication dynamics appears to be reliably hindcast by the inference of total phosphorus (TP) concentrations using weighted averaging regression and calibration techniques. Remarkable differences in the occurrence of some diatoms in both stratigraphies, which are not related to the general changes of the 'basic species sequence of eutrophication' model, may be due to the epilimnetic silica content (e.g. Fragilaria crotonensis) or competitive weaknesses (e.g. Tabellaria flocculosa).  相似文献   

A multi-method approach for the identification and quantification of water balance components of two lentic small water bodies     

Lukas P. Loose  Jens Lange  Aaron Budde  Barbara Herbstritt  Nicola Fohrer  Uta Ulrich 《水文研究》2024,38(1):e15058

The protection of the globally widespread lentic small water bodies (LSWB) must be based on detailed knowledge about their hydrological connectivity and water balance. The study aimed to identify and quantify water balance components as well as surface-groundwater interaction of two LSWB in a characteristic lowland region with a combination of different methods. This includes the collection of hydrological data and the use of bromide and water stable isotopes (δ²H and δ¹⁸O) as tracers. With their help, mixing models were established, and daily water balances were assessed. The results show a strong bidirectional interaction of both LSWB systems with shallow groundwater. Bromide and stable isotope tracers allowed for the identification of the most relevant in- and outflow sources and pathways. Thereby, isotope data revealed isotopic enrichment typical for open-water bodies and only minor precipitation inputs mainly relevant at the end of the dry season. Water balance calculations suggested accentuated seasonal dynamics that were strongly influenced by shallow groundwater, which represented large inputs into both LSWB. By that, different phases could be identified, with high inflow rates in winter and spring and decreasing fluxes in summer. In one LSWB, a drainage system was found to have a major impact next to the shallow groundwater interaction. The findings of this research provide detailed insights into the influence and importance of shallow groundwater for LSWB in lowland regions. This impacts the diffuse input of agricultural pollutants into these ecologically important landscape features.  相似文献   

Role of effective permeability distribution in estimating overpressure using basin modelling     

Knut Bjørlykke  Jens Jahren  Per Aagaard  Quentin Fisher 《Marine and Petroleum Geology》2010

Overpressure generation is a function of the rates of sedimentation, compaction, fluid generation from kerogen and dehydration of minerals, and most importantly the lateral distribution of permeability within a basin as this controls lateral drainage. Sedimentary basins, however, are typically highly heterogeneous with respect to primary sedimentary facies, diagenesis and tectonic development. While fluid flow models based on idealised homogeneous basins may further our understanding of the processes that influence overpressure development, the results are very sensitive to the distribution of rock properties, particularly permeability. The absolute permeability of sedimentary rocks varies from more than 1 Darcy to less than 0.01 nanodarcy (nD) (10⁻¹¹ Darcy).  相似文献   

Three‐dimensional modelling of differential railway track settlement using a cycle domain constitutive model          下载免费PDF全文

Xin Li  Magnus Ekh  Jens C. O. Nielsen 《国际地质力学数值与分析法杂志》2016,40(12):1758-1770

A method for simulation of differential (spatially varying) track settlement in a ballasted railway track is presented. It employs a cycle domain constitutive model to determine accumulated plastic (permanent) deformation of the granular layers supporting the track. The constitutive model is adopted for both the ballast and the sub‐ballast but with different parameter sets. The proposed framework can be used to predict differential track settlement accounting for heterogeneous (space‐variant) track characteristics and loading conditions. Here, it is demonstrated for three‐dimensional continuum modelling of a railway crossing panel subjected to a large number of axle passages. Because of the design of the crossing panel and the transient character of the impact loads on the crossing, the load transferred into the track bed is not uniform along the track, and the resulting differential settlement leads to vertical irregularities in track geometry. The spatial variation of track settlement is calculated both along the sleepers and along the rails. The influences of the number of adjacent sleepers accounted for in the model and the stiffness of the subgrade on the predicted settlement at the crossing are studied. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Cosmic rays and the primordial gas     

Jens Jasche  Benedetta Ciardi  Torsten A. Enßlin 《Monthly notices of the Royal Astronomical Society》2007,380(2):417-429

One of the most-outstanding problems in the gravitational collapse scenario of early structure formation is the cooling of primordial gas to allow for small-mass objects to form. As the neutral primordial gas is a poor radiator at temperatures   T ≤ 10⁴ K  , molecular hydrogen is needed for further cooling down to temperatures   T ∼ 100 K  . The formation of molecular hydrogen is catalyzed by the presence of free electrons, which could be provided by the ionization due to an early population of cosmic rays (CRs). In order to investigate this possibility, we developed a code to study the effects of ionizing CRs on the thermal and chemical evolution of primordial gas. We found that CRs can provide enough free electrons needed for the formation of molecular hydrogen, and therefore can increase the cooling ability of such primordial gas under following conditions. A dissociating photon flux with   F < 10⁻¹⁸ erg cm⁻² Hz⁻¹ s⁻¹  , initial temperature of the gas  ∼10³ K  , total gas number densities   n ≥ 1 cm⁻³  , and cosmic-ray sources with     .  相似文献   

Constraining a SHRIMP U-Pb age: micro-scale characterization of zircons from Saxonian Rotliegend rhyolites   总被引：7，自引：0，他引：7  

Lutz Nasdala  Jens Götze  Robert T. Pidgeon  Ulf Kempe  Thomas Seifert 《Contributions to Mineralogy and Petrology》1998,132(3):300-306

We present results of a detailed investigation of zircons from two rhyolites from St. Egidien and Chemnitz, Saxony, using a combination of microprobe techniques (SHRIMP ion probe, Raman microprobe, SEM: SE, BSE, and CL imaging). These rhyolites belong to the so-called “lower volcanics”, which is the older of two series of Late Variscan volcanic rocks occurring in the Saxonian Sub-Erzgebirge basin (Germany). The purpose of the present contribution is to demonstrate that detailed characterization of zircons, as provided by the different micro-techniques, facilitates soundest interpretation of geochronological data. The zircons (at most 40 to 80 m in size) show oscillatory growth zoning, with reversely correlated CL and BSE signal intensities. These zircons are interpreted to have grown during crystallization of the rhyolite because, apart from some cracking, they do not appear to have experienced any alteration since the time of their growth: The shapes of the zircons and their internal structures revealed by CL and BSE imaging appear to be magmatic, and neither annealing of the accumulated alpha-decay damage nor disturbance of the U-Pb system is observed. The SHRIMP ion probe measurements on the zircons gave a Permian ²⁰⁶Pb/²³⁸U age of 278 ± 5 Ma (95% confidence). The concordance of this age is supported by the correlation between the low degrees of metamictization (estimated from Raman parameters) and the accumulated alpha fluxes (calculated from SHRIMP data). The 278 Ma zircon age is interpreted to represent the age of the “lower rhyolites” series and, with that, the age of postkinematic Late Variscan volcanism in the Sub-Erzgebirge basin, which has been related to anorogenic extension and uplift as a result of intracontinental rifting. Because of genetic association of rhyolites in the Sub-Erzgebirge basin and Li-F granites and lamprophyres in the neighbouring Erzgebirge, the rhyolite age also indirectly contributes to the understanding of the geological history of the Erzgebirge. The 278 Ma age is the first absolute dating result for rhyolites from the Saxonian Sub-Erzgebirge basin. Received: 16 December 1997 / Accepted: 4 May 1998  相似文献