Flow-Path Structure in Relation to Nuclide Migration in Sedimentary Rocks

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1 Journal of Nuclear Science and Technology ISSN: (Print) (Online) Journal homepage: Flow-Path Structure in Relation to Nuclide Migration in Sedimentary Rocks Hidekazu YOSHIDA, Mikazu YUI & Tomoki SHIBUTANI To cite this article: Hidekazu YOSHIDA, Mikazu YUI & Tomoki SHIBUTANI (1994) Flow-Path Structure in Relation to Nuclide Migration in Sedimentary Rocks, Journal of Nuclear Science and Technology, 31:8, , DOI: / To link to this article: Published online: 15 Mar Submit your article to this journal Article views: 132 Citing articles: 6 View citing articles Full Terms & Conditions of access and use can be found at

2 Journal of NUCLEAR SCIENCE and TECHNOLOGY, 31181, pp (August 1994). 803 Flow-Path Structure in Relation to Nuclide Migration in Sedimentary Rocks An Approach with Field Investigations and Experiments for Uranium Migration at Tono Uranium Deposit, Central Japan Hidekazu YOSHIDA, Tono Geoscience Center, Power Reactor and Nuclear Fuel Develofinaent Corfi.* Mikazu YUI and Tomoki SHIBUTANI Tokai Works, Power Reactor and Auclear Fuel Deuelopnzent Gorp.** (Received July 14, 1993), (Revised September 27, 1993) The migration of natural uranium in geosphere has been studied at Tono uranium deposit, located in the Gifu Prefecture, central Japan. Geological and geochemical investigations have been carried out, as well as laboratory experiments, in order to characterize those structural and geochemical factors governing nuclide migration and to develop a conceptual nuclide migration model in sedimentary rocks. The results can be summarized as: (1) Micro-fractures within quartz grains, cleavages of detrital biotite flakes, and pores between detrital grains play an important role in nuclide migration, both as pathways and retention sites. Natural feature of uranium concentration in minerals and hydrogeological estimations suggest that diffusion may also exert a significant role on nuclide migration. (2) Flow-path structures, e.g. connectivity of pores, influence the sorption capacity of the host formation. Connectivity of pores probably change the number of accessible sorption sites along the flow-path in the sedimentary rocks. KEYWORDS: radionuclide migration, uranium, grain boundaries, sedimentary rocks, geometrical factor, geochemical factor, impregnation experiment, hydraulic conductivities, flow-path structure, pores, sorption. diffusion I. INTRODUCTION Nuclide migration in the geosphere can be regarded as a heterogeneous process due to the complexity of the detailed flow-path structures(1)(2'. Flow-path structures may also influence geochemical interactions, such as sorp- tion and ion exchange, between nuclide and the rock at the water-rock interface"'. The objective of this study is to describe how such flow-path structures influence nuclide migration, in order to develop a nuclide migration model for assessment of contaminant transport disposal site) in the geo~phere'~"~). Direct field observations, together with supporting laboratory experiments, have been carried out to characterize the factors influencing nuclide migration in a sedimentary uranium deposit. Uranium deposit is potentially useful analogue of radioactive waste isolation in geological repository(6). Thus, the combination of observations of geological evidences and laboratory experiments is considered to be helpful for long-term estimation of nuclide migration in the geological formation. * Izumi-cho, Toki-shi (e.g. release of nuclide from radioactive waste ** Tokai-mura, Ibaraki-ken I. -47-

3 804 J. Nucl. Sci. Technol., Tono uranium deposit, located in the Gifu Prefecture, is one of the largest such deposits in Japan. Previous studies revealed that natural uranium has been transported by the groundwater and redeposited in the Tertiary sediments at ton^'^). Tono uranium deposit, therefore, provides an excellent area to study the structural and geochemical factors influencing nuclide migration in sedimentary rocks(8). The following studies have been carried out in order to characterize the most important factors governing nuclide migration : (1) Radiometric logging, petrological description, and geochemical analysis of the uranium ore. (2) Alpha autoradiography and electron probe microanalysis (EPMA), after impregnation experiment with red-colored resin, to characterize the detailed occurrence of uranium and the correlation between flow-paths and uranium migration. (3) Hydraulic conductivity measurements on the core specimens from the uranium mineralized zone to assess the hydrogeological influence on uranium migration. (4) Sorption experiments with 233U as tracer to estimate sorption capacity of the sedimentary rocks in the uranium mineralized zone. n. GEOLOGICAL AND EXPERIMENTAL STUDY 1. Tono Uranium Deposit Tono uranium deposit lies within Tertiary sedimentary rocks some 153 to 155m below ground, and the mineralization age was inferred to be approximately 10 million years(s). The uranium mineralization occurs in a fluvial or lacustrine, lignite-bearing formation which was deposited on a uranium-rich granitic basement as shown in Fig. 1. Tertiary sedimentary rocks, composed of conglomerate or sandstone and taffaceous rocks, are widely distributed in paleo-channels on the unconformable granite erosion surface'lo). Detrital materials of lignite and accessory minerals, such as pyrite and zeolite, are present in the lower part of Tertiary sedimentary rocks. The lignite and pyrite probably induce the reducing conditions necessary for uranium deposition'"). The uranium mineralization is found just above the unconformity over an area several hundred meters wide, 2 to 3km long and several meters thick'12). Fig. 1 Schematic view of Tono uraniuni deposit 2. Characterization of Mineralized Zone Bulk compositions and total uranium con- (1) Occurrence of Uranium tents have been measured by conventional Radiometric logging and petrological ob- wet chemical analysis, colorimetry and fluoroservation were carried out to characterize the metrical methods. The uranium distribution uranium occurrence in the mineralized zone. has been examined by alpha autoradiography, -448-

4 ~~~ ~ ~ ~ ~ Vol. 31, No. 8 (Aug. 1994) 805 using CR-39 plastic The rock thinsections were studied also with an optical microscope to identify the mineral phases corresponding to the alpha emissions and scanning electron microscopy (SEM) was used for observation of microstructure in relation to uranium occurrence. ( 2 ) Groundwater Groundwater was sampled from the mineralized zone via a double-packer system in a borehole in the highest mineralized area of the deposit. The water samples were being stored under N, atmosphere until analysis in the laboratory. Redox potential Eh, ph and electrical conductivity were measured in-situ by a remote monitoring system(16). The preliminary results are reported by Seo & YoshidaC8) and displayed in Table 1. Briefly, the groundwater is reduced (Ek-300 to -360 mv), alkaline (ph=8.7 to 9.5), and the chemical compositions indicate Na+ -HCO,- type groundwater. The uranium concentration is in the range of ppb(1b). Table 1 Physico-chemical characteristics of groundwater from mineralized zone (adapted from Seo & Yoshida, 1992) Sampling point : m (mbgl) Temperature : 18.5"C Eh : -355mV PH : 9.2 Dissolved oxygen : O.Oppb Electric conductivity : 1.68~ S/m Ion concentration Ion concentration (ppm) I (ppm) Si Al Fee+ Fe3+ Mne+ Mge+ Cae+ U 8.1 <o. 02 <0.02 <o. 02 <o (ppb) Na+ 45 K F- 4.1 CI Sop 1.26 HCOje- 79 CO32-8 ( 3 ) Hydraulic Conductivity Hydraulic conductivity and effective porosity were measured on 11 core specimens from the mineralized zone to characterize the detailed geometry of the sedimentary host rocks and to assess the hydrological influence on the uranium migration as shown in Fig. 2. mbgl : Meters below ground level Fig. 2 Radiometric profile and hydraulic conductivity of mineralized zone Two methods for measuring the hydraulic conductivity were applied ; the constant pressure method and the transient pulse method"'). In both the cases, a triaxial compressive system was set up to simulate the in-situ stress conditions(18). 3. Batch Sorption Experiments Sorption experiments were carried out to estimate the chemical retention capacity of the sedimentary rocks. Two rock samples were selected from cores of the HCZ and the LCZ within the mineralized zone shown in Fig. 3. The mineralogy of both samples were also established by X-ray diffraction. Batch sorption experiments were carried out in a glovebox under Ne atmosphere with low Oe (<I00 ppm) to simulate the in-situ conditions. Experimental conditions are detailed in Table 2. Briefly, the ratio of tuffaceous sandstone to the groundwater was 2 g/5 ml and the contact time intervals were from 1 to 60d. The isotope ss3u dissolved in dilute nitric acid was spiked to the groundwater as a tracer. Solutions were sequentially separated by membrane filter (0.45 pm) and followed by ultrafiltration (lo.0oo MWCO (Molecular -49-

5 806 J. Nucl. Sci. Technol., A - P : Ardysed point of mineralogy and uranium contents. Fig. 3 mbgl : Meters below ground level Geological section of HCZ (high concentration zone) and LCZ (low concentration zone) in mineralized zone Weight Cut Off)) to remove particulate and/or colloidal substances. Solution concentration of 233U was measured by a-spectrometry. Sorption coefficients (Rd) are given by the expression, Rd (ml/g)=cr/cs, where Cr=TJ concentration in the rock and CS=~~~U concentration in the Table 2 Tracer Initial concentration Solution Volume Specimen Grain size Mass of specimen Temperature Atmosphere condition Analytical method Sampling time (d) Conditions of batch sorption experiment 233U 5.0 x mol/l Groundwater 50 ml Tuffaceous sandstone (HCZ and LCZ) <710 pm 20.0 g 20 C Nz a-spectrometry 7, 14, 33, Impregnation Experiment An impregnation experiment with a redcolored resin was carried out in order to characterize the connectivity of pores and to find any correlation between flow-path structure and uranium migration. The resin was mixed with cyanoacrylate(eo) and injected into rock specimens, taken from both the high uranium concentration zone (referred to as HCZ) and the low uranium concentration zone (referred to as LCZ) under reduced pressure in a desiccator. After the air pressure had decreased sufficiently, the specimens were heated up to 100 to 120 C to cure the resin. Polished thin-sections were made using the center part of the specimen in order to exclude drilling artifacts that might occur near the surface of the specimens. m. RESULTS AND DISCUSSION 1. Structural Influence on Uranium Migration The result from the radiometric logging shows that the uranium distribution is heterogeneous within the mineralized zone shown in Fig. 2, in spite of uniform mineralogical compositions. Detailed analyses of the uranium contents, shown in Fig. 3, indicate that the uranium was concentrated in the coarse-grained tuffaceous sandstone and that uranium contents of the compacted, fine-grained tuffaceous sandstone is slightly above background contents. The hydraulic conductivity and the effective porosity of the mineralized zone vary between to m/s and 20 to 30%, respectively. The coarse-grained tuffaceous sandstone shows higher hydraulic conductivity than the compacted fine-grained tuffaceous sandstone shown in Fig. 3. The observation and the experimental data suggest a close correlation among the structure of the sedimentary rocks, the hydraulic conductivity, and the contents of uranium. The hydraulic conductivities of the HCZ and the LCZ were 5.4X and 4.8X m/s, respectively, and differ three orders of magnitude each other. The result suggests that uranium migrated through and concentrated in the relatively high permeable zone. The variation of the uranium contents in the mineralized zone is probably caused by the difference of hydraulic conductivities

6 Vol. 31, No. 8 (Aug. 1994) Effect of Microscopic Flow-path feldspar and lithic fragments (Photo. 1(A), (B)) Structures on Nuclide Migration and in the micro-fracture of detrital quartz The occurrence of uranium is closely re- grains (Photo. 1(C), (D)). The matrix and lated to the sandstone microstructure. Alpha autoradiographs of thin-sections from the HCZ show high uranium concentration both in the micro-fracture seem to be well connected by pores which may act as preferential pathways of uranium migration. matrix between detrital grains such as quartz, Photo. 1 (A) Photomicrograph showing- secondary pyrite (Pyt) between detrital quartz (Qtz) grains (reflected light) (B) Image of a-track (arrow) of same area in (A) showing concentration of uranium around the pyrite between detrital grains (C) Photomicrograph of micro-fracture within detrital quartz grain (Qtz) (transmitted light) (D) lmage of a-track (arrow) of same area in (C) showing concentration of uranium in micro-fracture (Scale bar=0.1 mm) Results from alpha autoradiographs and EPMA indicate intimate associations among uranium and pyrite, and biotite flakes as shown in Photo. 2(A) and (B), respectively. Framboidal and micro-subhedral pyrite occur in association with high concentration of uranium. The pyritization can provide favorable condition to concentrate uranium around the pyrite. Sulphides and uranium probably deposited contemporaneously, then crystallization of the pyrite might ejects the uranium. Characteristic X-ray photograph shows that the detrital biotite flakes also act as significant concentration sites (Photo. 2(B)). Wilson(21) reported that the distribution of uranium in the biotite flakes is caused by diffusion and (001)-cleavage planes appear to act as preferential diffusion paths and concentration sites probably due to its high sorption capability for uranium. Sato et a1.(22) performed the through diffusion experiment of the sedimentary rock (tuffaceous sandstone) sampled from mineralized zone and reported that the apparent diffusion coefficient is 4.0X

7 808 J. Nucl. Sci. Technol., m2/s. Conca ~t ul.(23) compiled the diffusion coefficients measured for a variety of repository and geological materials. The order of 10-l2 m2/s of diffusion coefficient is an acceptable value of the tuffaceous sandstone. With this diffusion coefficient and hydraulic gradient (<0.04) measured in the Tono calculated maximum peclet number (Pe<2) (derived from the equation; Pe=Ud*L/De, where Ud is Darcy velocity, L (<0.3m) is a concentration thickness within the mineralized zone (HCZ) and De is effective diffusivity) suggests that diffusion might be expected as a dominant process of groundwater movement in the mineralized zone(26'. If the same diffusional behaviour was expected on Tono deposit, uranium probably migrated by diffusion through connecting pores and fixed thereafter. Photo. 2 (A) (B ) SEM photograph (above) and characteristic X-ray photograph (below) showing uranium concentration around framboidal pyrite (Pyt) SEM photograph (above) and characteristic X-ray photograph (below) showing uranium concentration in a detrital biotite flake (Bit) between detrital quartz grain (Qtz) periments using YJ are shown in Fig. 4. Initial concentration of tracer was adjusted ^a". (5.0~ lo-' mol/l) lower than the solubility limit. of uranium under this experimental condi- 2 Y tions(26). Thus, the effect of the precipitation 'o of YJ was neglected. The effect of colloidal rc lo', substances was also excluded because no such substances were identified as a carrier of 10% Lw :f HCZ -52-

8 Vol. 31, No. 8 (Aug. 1994) (A) (B) (C) (D) Photomicrograph showing pores (P) impregnated by red-colorea resin between detrital quartz (Qtz) grains Photomicrographs of pores are present in detrital quartz grain (Qtz) Photomicrograph of a detrital biotite lake (Bit) which has planar type pore (P) split along (001)-cleavage Characteristic X-ray photograph of biotite flake (C) impregnated by red-colored resin (Scale bar=o.l mm) Photo. 3 Photomicrographs ((A)-( C ) : Transmitted light) of impregnated sandstones from HCZ

9 810 J. Nucl. Sci. Technol., are different ; 3-5 and mllg, respectively. The relatively high Rd value of the LCZ is contradictory to the natural uranium content and the possible explanation would be the connectivity of pores. Impregnation experiment shows that good connectivity of pores was observed only in the specimen from the HCZ. These impregnated pores were identified among detrital grains (Photo. 3(A)) and also in the detrital grains such as quartz (Photo. 3(B)), biotite flake (Photo. 3(C), (D)) and feldspar. The planar-type pores along the (001)-cleavage planes in the biotite flakes were impregnated by colored resin (Photo. 3(C)) as was found for the micro-fractures in the detrital quartz grains which concentrate uranium. Characteristic X-ray photograph of same biotite flake (Photo. 3(D)) shows a good match of the impregnation site and the uranium concentration site. The impregnation experiments also demonstrate that groundwater in the pores between or within the detrital grains is readily accessible for fluid and serve the pathways of fluid movement. Microscopic observation after the impregnation experiment in the LCZ shows that no impregnated pores can be seen by optical microscopes. The effective porosity of the LCZ is around 20 %, even though the hydraulic conductivity is less than lo-'' m/s. The data show that the pores of the LCZ.are unevenly distributed and poorly inter-connected. The difference of results by impregnation experiment for the HCZ and the LCZ indicates that the connectivity of pores influences on nuclide migration. When uranium migrated into the sedimentary rocks, a portion of uranium moved through the pores, then, was presumably concentrated by chemical or physical retardants such as pore-filling materials(*'). However, sorption site of the LCZ were not functional for 233U due to the low connectivity of the pores. When uranium migrated in the sedimentary rocks, uranium could not access the sorption sites of the LCZ. The Rd of the LCZ derived from the batch experiment, however, may not correspond to the in-situ retention capability. The in-situ retardation process includes the effect of the long-term geochemical interactions along the flow-path during geological time period(28). Nohara et ~1."~) reported the result of uranium-series disequilibrium studies in the mineralized zone. The study revealed that uranium has not migrated through the rock matrix over a distance of less than few meters during the past one million years. The field data, therefore, suggest that the sedimentary rocks might be able to maintain the nuclide for long time in the geological environment if the nuclide has been fixed once along the flow-path, as seen in roll-front uranium deposits, such as Cigar Lake'"). IV. CONCLUSION Geological and geochemical field investigations as well as laboratory experiments were carried out in order to clarify the governing factors of nuclide migration in the sedimentary rocks of the Tono site and the following conclusions were reached. (1) Micro-fractures in quartz grains, cleavage in detrital biotite flakes and pores between the detrital grains play an im- portant role in nuclide retardation. The experimental evidence of uranium concentration in some minerals and hydrogeological estimations suggest that diffusion into the connected pores is expected to dominate nuclide migration. (2) Flow-path structures, e.g. connectivity of pores, influence the sorption capacity. Connectivity of pores probably change the volume of accessible retention sites along the flow-path in the sedimentary rocks. Qualitative data relating the diffusional behaviour to connectivity of pores in intact rock will have to be determined by diffusion experiment with uranium solution. With this experiment, microscopic investigation using alpha autoradiography must be carried out after thin-section prepared from the uranium diffused rock specimen in order to identify sorption sitec3". Diffusion profile of uranium is also gained in the same time and preferential pathways influencing nuclide migration

10 Vol. 31, No. 8 (Aug. 1994) 811 can be described. The correlation between the result of diffusion experiment and the observation of natural evidence is considered to be helpful for further discussion of the nuclide migration in sedimentary rock. The diffusion experiment is now conducting in PNC Tono Geoscience Center and the result of experiment will be reported in the near future. ACKNOWLEDGMENT The authors acknowledge with T. Ashida of Power Reactor and Nuclear Fuel Development Corp. (PNC) Tokai Works for valuable advises on experiments and K. Kodama of PNC Tono Geoscience Center for the preparation of characteristic X-ray photographs. Dr. Y. Yusa and PNC scientists at Tono Geoscience Center are gratefully thanked for constructive discussions. Dr. I. Mckinley and Dr. R. Alexander of NAGRA (Nationale Genossenschaft fur die Lagerung radioaktiver Abfalle) are also thanked for helpful comments and benefitting of manuscript. -REFERENCES- PITTMAN, E. D. : Porosity, diagenesis and productive capability of sandstone reservoirs, SEPM Spec. Publ., 26, (1979). DOYEN, P. M. : Permeability, conductivity, and pore geometry of sandstone, J. Geophys. Res., 93, (1988). BROOKINS, D. G. : Sandstone uranium deposits, an analogue for SURF disposal in some sedimentary rocks, EUR11037EN (1987). NERETNIEKS, I. : Diffusion in the rock matrix; an important factor in radionuclide retardation?, J. Geophys. RQS., (1980). HADERMANN, J. : The influence of speciation on the geospheric migration of radionuclides, Nucl. Technol., (1982). CHAPMAN, N.A., MCKINLEY, I.G., SMELLIE, J.A.T. : The potential of natural analogues in assessing systems for deep disposal of highlevel radioactive waste, NAGRA NTB, (1984). DOI, K., HIRONO, S., SAKAMAKI, S. : Uranium mineralization by groundwater in sedimentary rocks, Jpn. Econ. Geol., 70, (1975). SEO, T., YOSHIDA, H. : Natural analogue studies of the Tono uranium deposit in Japan, CEC Rep. pre-print, (Proc. 5th NAWG), (1992). c\-.--..,hiai, Y., YAMAKAWA, M., TAKEDA, S., HARASHIMA, F.: A natural analogue study on Tono uranium deposit in Japan, CEC Rep. (Proc. 3rd NAWG), (1989). NOHARA, T., OCHIAI, Y., SEO, T., YOSHIDA, H. : Uranium-series disequilibrium studies in the Tono uranium deposit, Japan, Radiochim. Acta, 58/59, (1992). SAKAMAKI, Y. : Geological environments of Ningyo-Toge and Tono uranium deposit, Japan, Geological environments of sandstone-type uranium deposits, IAEA-TECDOC. 328, (1985). YAMAKAWA, M. : Geochemical behaviour of natural uranium nuclides in geological formation, Proc. 3rd Int. Symp. Nucl. Energy Res. -Global Environment and Nuclear Energy, (1991). BASHAM, I. R. : Some applications of autoradiographs in textual analysis of uranium-bearing samples, Econ. Geol., 76, (1981). ISHIGURE, N., MATSUOKA, 0.: Property of TS-16N solid state nuclear track detectors as an imaging medium for macroautoradiography of alpha-emitters in biological specimens, Radioisotopes, 37, (1988). SEO, T., OCHIAI, Y., TAKEDA, S., NAKASHIMA, N.: A natural analogue study on Tono sandstone-type uranium deposit in Japan, Proc. Joint Int. Waste Management Conf., Vol. 2, (1989). KANAI, Y.. SAKAMAKI, Y., SEO, T.: Geochemical behaviour of uranium series nuclides (zs*u, 234U, 226Ra, 222Rn) around the Tono uranium mine, Gifu Prefecture, Central Japan, Geochem., (in Japanese), 24, (1990). BRACE, W.F., WALSH, J.B., FRANGOS, W.T.: Permeability of granite under high pressure, J. Geophys. Res., 73, (1968). NAKANO, K., SAITO, A., NISHIGAKI, M.: Laboratory measurement technique for low permeability of rock sample, J. Jpn. SOC. Soil Mech. Found. Eng., (in Japanese), 31, (1991). ALEXANDER, W. R.. MCKINLEY, I. G. : Natural analogues in assessment ; Improving models of radionuclide transport in groundwaters by studying the natural environment, EUR13014EN, (1990). NISHIYAMA, T., KUSUDA, H., SAITO, T.: A few remarks on examination of cavities or microcracks produced in rocks using a fluorescent substance, J. Jpn. SOC. Eng. Geol., (in Japanese), 33, (1992). WILSON, C. J.L. : Combined diffusion-infiltration of uranium in micaceous schists, Contrib. Mineral. Petrol., 65, (1977). SATO, H., ASHIDA, K., KOHARA, Y., YUI, M., UMEKI, H., ISHIGURO, K. : Diffusion coefficient of radionuclide in bentonite and rock, PNC -55-

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