Karl-Heinz Hellmuth

Study of porosity and migration pathways in crystalline rock by impregnation with 14C-polymethylmethacrylate

Abstract A new method is described, which gives quantitative information on levels and spatial distributions of porosity in the matrix of rock in the sub-mm- to cm-scale. Migration pathways can be identified and visualized. The method employs impregnation of dried rock samples by 14 C-labelled methylmethacrylate (MMA), radiation polymerization, autoradiography and digital image processing. Infiltration and diffusion processes can be quenched by polymerization and the resulting profiles studied easily.

On the connected porosity of mineral aggregates in crystalline rocks

Abstract The 14C-PMMA impregnation of rock samples and mineral staining methods provide the connected porosity map and the spatial distribution of mineral aggregates, respectively. Combined image analysis of mineral and porosity maps allows quantification of the connected porosity distribution in rockforming mineral aggregates. After the two maps have been superimposed numerically, the 14C-PMMA method provides an indication of the extent of pore connectivity for each pixel in the analyzed area, which can be used to obtain the porosity distribution as a function of modal mineralogy. When applied to undeformed and deformed Kivetty granodiorite samples from Finland, the method allows for a detailed analysis of the rock porosity. Porosity distributions related to the undeformed rock are unimodal and approximately identical to each other. On the other hand, porosity distributions of the deformed granodiorite are multimodal and vary significantly. Constituent porosity sets of the deformed samples were albite crystals free of alkali feldspar patches and alkali feldspar phenocrysts (average porosity, Φ̅ = 0.21%), albite containing alkali feldspar patches (Φ̅ = 0.59%), rapakivi albitic mantles (Φ̅ = 1.15%), quartz (Φ̅ = 0.39%) and mafic minerals (Φ̅ = 5.8%). Moreover, the analysis indicates that the numerous micropores observed under SEM within alkali feldspar phenocrysts and albite crystals free of alkali feldspar patches are unconnected in 3D.

Rock Matrix Studies with Carbon-14-Polymethylmethacrylate (PMMA); Method Development and Applications

Abstract Application of the 14C-PMMA impregnation method has provided relevant information not available by other methods about the accessible pore space in crystalline rock. Such information is relevant for the assessment of the capability of the geosphere to retard the migration of harmful waste components. In this work further developments of the system (calibration, corrections, options), the impregnation method itself (problems related to infiltration and polymerization of the tracer) and an assessment of the method performance are presented, as well as additional examples of applications.

Effect of moisture content on emanation at different grain size fractions - a pilot study on granitic esker sand sample.

It is known that in soils and sediments moisture adsorbed on particle surfaces and in the pore system significantly affects the behaviour of recoiling radon ((222)Rn) atoms after decay of parent (226)Ra, leading to increased (222)Rn emanation. As a first step in an effort to characterize the (222)Rn source term in mineralised sediments in the present study, complementing previous studies in the area, granitic esker sand samples were collected in order to test how moisture content affects (222)Rn emanation at different grain size fractions. Emanation fractions measured for natural samples were compared with theoretical calculations. Six different grain size fractions were studied at 0%, 5% and 10% moisture contents relative to the mass of solids. In a further study necessary complementary information on the chemical and structural distribution of (226)Ra was gained by selective leaching experiments. The results showed that (226)Ra concentration increases from 50 Bq/kg at grain size 1-2 mm to 200 Bq/kg at grain size <0.063 mm. Respectively, the emanation factor increased from 0.12 to 0.30 at 5% moisture content. Both emanation factor and radium concentration increased significantly when grain size was below 0.125-0.250 mm. Above this fraction, the emanation fraction was approximately constant, 0.13 at 5% moisture content. In most of the grain size fractions, emanation reaches its maximum at 5% moisture content, being twice as high as in a dry sample. For the small particles (<0.063 mm) the (226)Ra distribution is rather complex and depends on the mineral composition compared to larger particles wherein emanation from the internal pore system and the adjacent matrix is dominating over the contribution from external surface.

Interpretation of out-diffusion experiments on crystalline rocks using random walk modeling

Matrix diffusion in saturated rocks with very low permeability is one of the major mechanisms of solute transport. Laboratory out-diffusion experiments on rock samples may provide an estimate of the bulk diffusion coefficient. However, numerous results have shown that this average parameter does not really depict the complex mechanism of diffusion as a function of the internal heterogeneity of crystalline rocks. Two-dimensional images of the porosity distribution in a granite sample were obtained by impregnation with a radioactive resin and autoradiography. Some examples based on these images and synthetic images were used to perform numerical simulations of out-diffusion using two different random walk methods. The simulated shapes of the out-diffusion curves depend on the spatial distribution of the porosity and on the pore connectivity with the border of the sample. Such relations might explain the multiple nested slopes or the convex shapes often observed on real experimental curves.

Multidisciplinary analysis of Finnish esker sediment in radon source identification.

In order to define the naturally-occurring radioactive materials that are the source of radon in natural environments, a comprehensive analytical (geochemical, physical and chemical) methodology was employed to study sand samples from the Hollola esker in the city of Hollola (Lahti area, Finland). Techniques such as gamma-spectrometry, emanation measurements, sequential chemical extraction, scanning electron microscopy (SEM), electron probe microanalyses (EPMA) and inductively-coupled plasma mass spectrometry (ICP-MS) were used to determine the potential source of radon. Monazite and xenotime, uranium- and thorium-bearing minerals and potential radon sources, occurred in significant amounts in the samples and were also the main reason for the distribution of uranium and thereby radium in separate grain-size fractions. Following deposition, the esker sand has been exposed to no significant weathering, and radium has not therefore been much separated from uranium. However, considering its non-compatibility with crystal lattices, it was recognized rather in easily leachable species (44% of the total (226)Ra) than uranium (21% of the total (238)U) in our analyses. The smallest grain-size fraction of the esker sand had a higher emanation power (0.24) than the other fractions (around 0.17). Due to the small relative proportion of this fraction, however, it contributed only slightly to the total emanation (4%). The emanation power of the leachable species was about three times higher (ca. 0.20) than that of the species tightly bound to the crystal lattice (ca. 0.07).

Calibration of digital autoradiograph technique for quantifying rock porosity using 14C-PMMA method

Digital autoradiography (DA) based on phosphor plate imaging was tested for porosity quantification using 14C-PMMA method, and was compared to the classical film autoradiography (FA) method. Validation was undertaken by characterizing porosity evolution of moraine deposits of Lamarck granodiorite (eastern Sierra Nevada, USA). The connected porosities obtained with FA and DA are comparable to density measurements. Using adapted exposure times, DA is more accurate than FA for analysing large variations of porosity within a given sample, and is faster than FA.

Development of a fracture network in crystalline rocks during weathering: Study of Bishop Creek chronosequence using X-ray computed tomography and 14C-PMMA impregnation method

The progressive development of porosity during subsoil weathering of granodiorite clasts was studied at the Bishop Creek moraine chronosequence in east-central California. Fractures and other pores were examined using two complementary imaging techniques, X-ray computed tomography (XRCT) and a 14C-PMMA (14C-polymethylmethacrylate) method. The well-known XRCT method allows the investigation of three-dimensional (3-D) pore space. 14C-PMMA is a less-known method based on the complete impregnation of pore space with 14C-doped PMMA, and subsequent autoradiograph of a rock section. These imaging methods allow us to decipher the evolution of pore space in the granodiorite during the 120 k.y. weathering period. The 14C-PMMA imaging technique was found to be more suitable for following the evolution of the whole sequence, from “intact” bedrock to saprock, in terms of crack density, porosity, and aperture. Working with hand specimens, this method was adapted to detect both the low-aperture fractures (microcracks) and macrocracks. Only a slight and progressive increase in total fracture density was observed during the whole weathering period. However, this trend does not hold if macrocracks and microcracks are separated: Microcrack density slightly decreases, whereas macrocrack density increases due to a progressive expansion of microcracks. The total porosity of the rock increases during weathering and is correlated to the progressive aperture increase of all types of cracks. This evolution is accompanied by a change of crack morphology and connectivity, and an overall increase in intragranular porosity of biotite and plagioclase aggregates.

Understanding uranium behaviour at the Askola uranium mineralization

Abstract Understanding the behaviour of uranium is essential when assessing the safety of a spent nuclear fuel repository. The geochemical behaviour of uranium, including its reactive transport chemistry, is also a matter of concern when assessing the environmental impact of uranium mining. Subsurface uranium mobility is believed to be primarily controlled by dissolution and (co)-precipitation of uranium mineral solids and adsorption to mineral surfaces. This paper describes a modelling exercise based on characterisation of samples taken from drilled cores at the uranium mineralization at Askola, Southern Finland. In the modelling exercise, current conditions are assumed to be oxidizing and saturated with groundwater. PHREEQC was used for modelling in conjunction with the Lawrence Livermore National Laboratory database, chosen for its extensive coverage of uranium species and mineral phases. It is postulated that weathering processes near the surface have led to uranium dissolution from the primary ore, leaching out from the matrix and migrating along water-conducting fractures with subsequent re-diffusion into the rock matrix. Electron microscopy studies show that precipitated uranium occupies intra-granular fractures in feldspars and quartz. In addition, secondary uranium was found to be distributed within goethite nodules as well as around the margins of iron-containing minerals in the form of silicate and phosphate precipitates. Equilibrium modelling calculations predict that uranium would be precipitated as uranyl silicates, most likely soddyite and uranophane, in the prevailing chemical conditions beneath Lakeakallio hill.

Natural Uranium Fluxes and their Use in Repository Safety Assessment

Studies of uranium series geochemistry have made relatively little impact on performance assessment where simplistic advection-dispersion models are still being used to derive dose and risk estimates. Supplementing the models with a scientifically sound representation of radionuclide migration would greatly enhance the rigour and transparency of the safety case. This paper outlines a methodology for employing natural geochemical fluxes as alternative indicators of repository safety by reference to case studies in Finland. The approach may be applied to any situation where a comparison between ambient exposure rates and increments received due to industrial activities is required.