Postglacial Matrix Diffusion in a Boulder Sample

1995 ◽  
Vol 412 ◽  
Author(s):  
K. Rasilainen ◽  
J. Suksi ◽  
K.-H. Hellmuth ◽  
A. Lindberg ◽  
S. Kulmala
Keyword(s):  
Strong Mixing ◽  
Matrix Diffusion ◽  
Fracture Face ◽  
Rock Matrix ◽  
Land Uplift ◽  
Last Glaciation ◽  
Low Concentrations ◽  
Content Analyses ◽  
Activity Ratios ◽  
Possible Cause

AbstractA boulder sample was studied for its unusual U content. Analyses of U-series nuclides within the rock matrix perpendicular to an assumed fracture face show abrupt pulse-like concentration distributions with very low concentrations of U daughters. Both Th-230/U-234 and Pa-231/U-235 activity ratios are low, indicating recent U accumulation into the rock. Matrix diffusion is tested as a possible cause for the experimental observations. We assume that the diffusion process was triggered and controlled by rock expansion, strong mixing of different water types and rapid land uplift at the end phase of the last glaciation.

Diffusion Measurements on Crystalline Rock Matrix

1994 ◽  
Vol 353 ◽  
Author(s):  
Kari Hartikainen ◽  
A. HautojÄrvi ◽  
H. Pietarila ◽  
J. Timonen
Keyword(s):  
Power Law ◽  
Gas Flow ◽  
Crystalline Rock ◽  
New Technique ◽  
Matrix Diffusion ◽  
Rock Matrix ◽  
Model Calculations ◽  
Short Time ◽  
Drill Cores

AbstractA new gas flow technique is introduced such that experiments on very long samples are possible. This new technique together with increased accuracy of the measurements, allows the observation of power law tails in the break-through curves. Dispersion in these experiments can be controlled in great detail, and therefore the power law tails can be used to determine very accurately the parameters relevant in matrix diffusion. Results for rock and metal samples are shown, and they are fitted with model calculations which include both dispersion and matrix diffusion. The introduced technique, which is designed for ordinary drill cores, is suitable for scanning a large number of samples in a very short time.

Accumulation History of Uranium in Boulders - New Support for Postglacial Matrix Diffusion

1997 ◽  
Vol 506 ◽  
Author(s):  
Kari Rasilainen ◽  
Juhani Suksi ◽  
Antero Lindberg
Keyword(s):  
Time Scale ◽  
Early Stage ◽  
Matrix Diffusion ◽  
Natural Analogue ◽  
Land Uplift ◽  
Decay Stage ◽  
History Of ◽  
End Stage ◽  
Chain Decay ◽  
Accumulation History

Extended AbstractGeological formations are being considered as host media for nuclear waste disposal. The occurrence of natural U in rocks provides a possibility to test the radionuclide migration models used in safety studies of the disposal over comparable time periods. Here we study U accumulation into boulders as a process analogue for matrix diffusion; the boulders were found in glacial till in Hämeenlinna, southern Finland. Based on the glacial history of the site, matrix diffusion simulations, and independent U-series disequilibria (USD) dating, the U accumulation was interpreted to originate from the end stage of the latest glaciation, i.e. the system age is about 10 000 years1,2. The known time scale offers a rare opportunity for quantitative model testing; normally the time scale is difficult to determine for a single process in a natural analogue.The U accumulation was earlier1,2 interpreted to be due to matrix diffusion and sorption. The postulated accumulation history consists of short in-diffusion and out-diffusion stages, as well as a longer chain decay stage. The in-diffusion was caused by U-rich waters discharging on the boulders at the end stage of the glaciation. The subsequent partial out-diffusion represents the period the boulders were temporarily submerged in the Yoldia sea during the early stage of the Baltic Sea. The final isolated radioactive chain decay stage began when the boulders, and their surroundings, rose above the sea level due to land uplift.In this paper we report the first radiochemical results of a new larger boulder from the same area as the one studied earlier1; qualitatively, also the U distribution appears to be the same. Due to the larger dimensions, we can sample the inner zone of the boulder which matrix diffusion can not have reached within the postulated time, i.e. the state of the boulder before the U accumulation. The large amount of sample material containing almost only the recently accumulated U provides an opportunity to experimentally approach the kinetics of U fixation in situ. Understanding the long-term U fixation is essential in natural analogue studies, because the matrix diffusion model only has fast reversible adsorption (based on Kd) as the fixation process. Attempts to separate and quantify sorbed U in natural analogues have been reported elsewhere3.

Study of the Uranium Heterogeneous Diffusion through Crystalline Rocks and Effects of the “Clay-Mediated” Transport

2003 ◽  
Vol 807 ◽  
Author(s):  
U. Alonso ◽  
T. Missana ◽  
M. García-Gutiérrez ◽  
A. Patelli ◽  
J. Ravagnan ◽  
...  
Keyword(s):  
Surface Area ◽  
Bentonite Clay ◽  
Matrix Diffusion ◽  
Rock Matrix ◽  
Mineral Surface ◽  
Transport Models ◽  
Actual Surface ◽  
Disturbed Areas ◽  
High Level ◽  
Preferential Pathways

ABSTRACTRock matrix diffusion is one of the possible mechanisms for radionuclide retardation in a deep geological high-level radioactive waste repository, and it is usually considered that radionuclides diffuse as solutes through the rock. Nonetheless, the potential effects that clay, from the bentonite barrier, may induce on the radionuclides migration should be taken into account. Furthermore, transport models generally assume that the whole mineral surface is accessible to transport, whereas transport is highly conditioned by the heterogeneous mineral distribution, since different minerals may act as preferential pathways, while others may present higher sorption capability. It is therefore necessary to determine the actual surface area accessible to transport.The aim of the present work is the identification of the uranium preferential pathways to the granite, both in presence or absence of bentonite clay. Results showed that uranium as solute diffused in specific mineral areas, indicating that the actual surface area accessible to matrix diffusion, and/or sorption on the surface, is significantly lower than the whole mineral surface. By the other hand, the uranium in presence of the clay was randomly distributed on the surface, and penetrated into the granite mainly through “defects” (as fractures or grain boundaries); its migration being enhanced on specially fractured or disturbed areas.

Brownian Simulation of Matrix Diffusion

2000 ◽  
Vol 663 ◽  
Author(s):  
Sofie Andersson ◽  
Allan T. Emrén
Keyword(s):  
Effective Diffusion ◽  
Diffusion Constant ◽  
Boltzmann Distribution ◽  
Matrix Diffusion ◽  
Rock Matrix ◽  
Dissolved Species ◽  
Apparent Diffusion ◽  
Wide Variability ◽  
Far From Equilibrium ◽  
Solid Piece

ABSTRACTThe commonly used approach in dealing with matrix diffusion is to assign an effective diffusion constant for the radionuclide in the rock matrix. The idea behind this approach is that, on a scale much larger than the pore size, the irregularities tend to cancel out. Although it might look plausible at first sight, this approach has been questioned both for theoretical and experimental reasons.Here, Brownian simulation has been used to investigate the transport of dissolved material in a rock matrix modeled as a system of pores with a wide variability in size and shape. The Boltzmann distribution is used locally, although the system globally is far from equilibrium.The simulation consists of two main parts. First, the model rock is formed by precipitation of irregular mineral grains from a liquid phase. As the grains grow, they tend to form a mostly solid piece of rock.In the second part of the simulation, a dissolved species is introduced at one side of the rock and allowed to diffuse through its pore system. It is found that no apparent diffusion constant, D, can explain the properties of the system. Rather, D is found to be a function of both distance and time.

Investigation of the Influence of Heterogeneous Porosity on Matrix Diffusion: a Novel Approach Using Adaptive Tree-Multigrid Technique and Real Porosity Data

1996 ◽  
Vol 465 ◽  
Author(s):  
P. Simbierowicz ◽  
M. Olin
Keyword(s):  
Matrix Diffusion ◽  
Two Dimensional ◽  
Rock Matrix ◽  
Empirical Parameter ◽  
Heterogeneous Model ◽  
Novel Approach ◽  
The Standard Model ◽  
Leaching Experiments ◽  
Heterogeneous Matrix ◽  
Infiltration Experiment

ABSTRACTLast year we developed a two-dimensional deterministic heterogeneous matrix diffusion model, which is capable of utilising porosity information originating from real drill-core samples. The results of numerical infiltration experiment we had performed with the model displayed substantial spatial variations in the penetration depth. Because it is practically impossible to verify experimentally those two-dimensional penetration profiles we had computed, this time we decided to try modelling of measured leaching curves. Unfortunately we have not succeeded in acquiring such curves for the exact same samples, which we have used in numerical leaching experiments. Nevertheless it can be seen, that the shape of leaching curves computed with the heterogeneous model is clearly closer to the shape of measured curves, than the shape of curves provided by the standard model. These differences can be utilised as a basis for an approximate numerical method of assessing the geometric factor, which has traditionally been a purely empirical parameter.The results of the new numerical experiments agree with our older results from last year: the heterogeneity of the rock matrix has highly significant impact on the diffusion. However, when interpreting the results, one must not neglect numerous limitations of the model, and hence, one should not attempt to overgeneralise the conclusions.

First-Passage-Time Modeling of Transport in Fractured Rock

1995 ◽  
Vol 412 ◽  
Author(s):  
M. W. Becker
Keyword(s):  
Time Domain ◽  
Breakthrough Curve ◽  
Fractured Rock ◽  
First Passage Time ◽  
Tracer Tests ◽  
Passage Time ◽  
Matrix Diffusion ◽  
Rock Matrix ◽  
Time Modeling ◽  
The Time Domain

AbstractThis paper presents theory intended for the design and interpretation of tracer tests in fractured rock. The objective is to provide an understanding of experiments in which particulate and solute tracers are injected simultaneously. In such experiments, it is expected that the solute tracer will diffuse into the rock matrix, while the particulates will be confined to the fractures. The theory allows information about matrix diffusion to be extracted from the breakthrough curve. Furthermore, it can accommodate tests in which a non-ideal source is used, and where some of the withdrawn fluid is re-injected. The solution is performed in Laplace space and transformed to the time domain using a commercial spreadsheet.

Radionuclide Retardation During Transport Through Fractured Granite

1981 ◽  
Vol 11 ◽  
Author(s):  
Ian G. McKinley ◽  
Julia M. West
Keyword(s):  
Matrix Diffusion ◽  
Parallel Plates ◽  
Rock Matrix ◽  
Porous Flow ◽  
Secondary Minerals ◽  
Dead End ◽  
Radio Nuclide ◽  
Radioactive Waste Repositories ◽  
Flow Through ◽  
Waste Repositories

In several countries low permeability crystalline rocks (e.g. granites) are under consideration as potential hosts for radioactive waste repositories. In such formations groundwater flow occurs predominantly in specific fractures rather than being a general porous flow through the entire rock matrix. By considering fractures to be simple parallel plates various authors have demonstrated the potential importance of diffusion into dead-end pores and the rock matrix itself (‘matrix diffusion’) as a mechanism for the retention of migrating radionuclides. Complementing these theoretical studies, several insitu migration experiments are planned in single fissures in crysalline rocks in Sweden, the U.K. and the U.S.A. The ‘parallel plate’ approximation to a single fissure is, however, acknowledged to be a gross simplification of any real case where “flowing” fractures are expected to be either filled or coated with secondary minerals, formed by hydrothermal alteration of fracture surfaces In the evaluation of net radio-nuclide retardation, therefore, the effect of sorption onto such secondary minerals must be carefully considered.

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