Diffusion of Technetium in Compacted Bentonites in the Reducing Condition with Corrosion Products of Iron

ABSTRACTIn the vicinity of a high-level waste repository, corrosion of carbon steel overpacks will create a reducing environment. Reducing conditions are expected to retard the migration of redox-sensitive radionuclides such as technetium.The apparent diffusion coefficients of technetium were measured in compacted bentonites (Kunigel VI® and Kunipia F®, JAPAN) in contact with carbon steel and its corrosion products under reducing conditions or without carbon steel under oxidizing conditions for comparison. The apparent diffusion coefficients measured were 10-12 to 10-13 m2/s under oxidizing conditions and 10-12 to 10-13 m2/s under reducing conditions. There were significant effects of redox condition, dry density (0.2 to 2.3 g / cm3) and montmorillonite content (50% for Kunigel VI or 100% for Kunipia F) on the apparent diffusion coefficients. Montmorillonite density could be a good index to explain density dependence of the diffusion coefficients under both reducing and oxidizing conditions.

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Diffusion of Cs and Sr in Compacted Bentonites Under Reducing Conditions and in the Presence of Corrosion Products of Iron

MRS Proceedings ◽

10.1557/proc-506-351 ◽

1997 ◽

Vol 506 ◽

Cited By ~ 6

Author(s):

K. Idemitsu ◽

Y Tachi ◽

H. Furuya ◽

Y. Inagaki ◽

T. Arima

Keyword(s):

Carbon Steel ◽

Corrosion Product ◽

Diffusion Coefficients ◽

Corrosion Products ◽

High Level Waste ◽

Dry Density ◽

Iron Corrosion ◽

Apparent Diffusion Coefficients ◽

Reducing Conditions ◽

Apparent Diffusion

ABSTRACTIn high-level waste repositories, a carbon steel overpack will be corroded by consuming oxygen trapped in the repository after closure. Iron corrosion products are expected to interfere with migration of radionuclides by filling the pore in bentonite and sorbing radionuclides. In this study the apparent diffusion coefficients of cesium and strontium were measured in compacted Na-bentonites (Kunigel VI® and Kunipia F®, JAPAN) contacted with carbon steel and its corrosion products under reducing conditions or without carbon steel under oxidizing conditions for comparison. The apparent diffusion coefficients of cesium with and without corrosion product were 2.2 to 13 × 10−12 m2/s. The apparent diffusion coefficients of strontium with and without corrosion product were 3.1 to 25 × 10−12 m2/s. There were significant effects of dry density (0.8 to 2.0 g/cm3) and montmorillonite contents (50% for Kunigel V1 or 100% for Kunipia F). The presence of corrosion product decreased the apparent diffusion coefficients of Cs in both bentonites and that of Sr in Kunigel V1, especially at low dry density. This may be due to corrosion product filling the pore in the bentonite, decreasing the free pore size and density for diffusion.

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Diffusion of Uranium in Compacted Bentonites in the Reducing Condition with Corrosion Products of Iron

MRS Proceedings ◽

10.1557/proc-412-683 ◽

1995 ◽

Vol 412 ◽

Cited By ~ 5

Author(s):

Kazuya Idemitsu ◽

Y. Tachi ◽

H. Furuya ◽

Y. Inagaki ◽

T. Arima

Keyword(s):

Carbon Steel ◽

Pore Water ◽

Diffusion Coefficients ◽

Corrosion Products ◽

Redox Conditions ◽

High Level Waste ◽

Dry Density ◽

Apparent Diffusion Coefficients ◽

Reducing Conditions ◽

Apparent Diffusion

AbstractIn high-level waste repository, a carbon steel overpack will be corroded after closure. This will create a reducing environment in the vicinity of the repository. Reducing conditions are expected to retard the migration of redox-sensitive radionuclides such as uranium.The apparent diffusion coefficients of uranium were measured in compacted bentonites (Kunigel VI® and Kunipia F®, JAPAN) in contact with carbon steel and its corrosion products under reducing conditions or without carbon steel under oxidizing conditions for comparison. The apparent diffusion coefficients measured were 10-12 to 10-14 m2/s under oxidizing conditions and 10-13 to 10-14 m2/s under reducing conditions. There were significant effects of redox conditions, dry density (0.8 to 2.0 g/cm3) and montmorillonite contents (60% for Kunigel VI or 100% for Kunipia F) on the apparent diffusion coefficients. The sorption of uranium on corrosion products of iron was smaller than that on montmorillonite. Montmorillonite density could be a good index to explain density dependence of the diffusion coefficients under the reducing conditions. Uranium would diffuse in free pore water with diffusion coefficients greater than 10-13 m2/s in low density bentonites under both redox conditions because the diffusion coefficients depended on sorption coefficients. Since diffusion coefficients were independent of sorption coefficients in high density bentonites where free pore water is scarce, surface diffusion might occur with coefficients were of about 10-13 m2/s.

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The Dependence of the Diffusion Coefficients of 3H and Cs on Grain Size in Compacted Montmorillonite

MRS Proceedings ◽

10.1557/proc-506-947 ◽

1997 ◽

Vol 506 ◽

Cited By ~ 2

Author(s):

Mamoru Nakajima ◽

Tamotsu Kozaki ◽

Hiroyasu Kato ◽

Seichi Sato ◽

Hiroshi Ohashi

Keyword(s):

Grain Size ◽

Diffusion Coefficients ◽

Dry Density ◽

Formation Factor ◽

Apparent Diffusion Coefficients ◽

Compacted Bentonite ◽

Buffer Material ◽

Apparent Diffusion ◽

Different Grain Size ◽

High Level

ABSTRACTCompacted bentonite is a candidate buffer material in geological disposal of high-level radioactive waste. The transport of radionuclides in compacted bentonite is dominated by diffusion, because of its very low permeability. In this study, we focused on the grain size of clay mineral, which is considered to be closely related to the formation factor in the pore water diffusion model[1,2]. The apparent diffusion coefficients (Da) of HTO and cesium ions in compacted clays were determined using montmorillonite samples with different grain size and dry density, and the effect of the grain size on diffusion behavior was discussed.

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Effect of pH on Plutonium Migration Behavior in Compacted Bentonite

MRS Advances ◽

10.1557/adv.2017.183 ◽

2016 ◽

Vol 1 (61) ◽

pp. 4011-4017

Author(s):

Ryo Hamada ◽

Noriyuki Maeda ◽

Kazuya Idemitsu ◽

Yaohiro Inagaki ◽

Tatsumi Arima ◽

...

Keyword(s):

Pore Water ◽

Diffusion Coefficients ◽

High Level Waste ◽

Migration Behavior ◽

Effect Of Ph ◽

Apparent Diffusion Coefficients ◽

Compacted Bentonite ◽

Ph Values ◽

Apparent Diffusion ◽

High Level

ABSTRACTIn disposing of high-level radioactive waste, the drop in pH in the repository as the iron overpack corrodes must be considered. Plutonium migration behavior may be affected by the pH of pore water in compacted bentonite barriers in high-level waste repositories. To examine the effect of pH on migration behavior, H-bentonite was prepared by treating Japanese Na-bentonite, Kunipia-F, with hydrochloric acid. Diffusion experiments were performed with mixtures of Na- and H-bentonites. The pH value in the pore water of the water-saturated bentonite mixtures decreased from 8 to 3 as the mixing ratio of H-bentonite increased. Diffusion experiments were carried out by using238Pu then apparent diffusion coefficients were determined from the plutonium distribution in the specimens. The apparent diffusion coefficients were on the order of 10-13to 10-12m2/s at pH values lower than 4, whereas they were less than 10-14m2/s at pH values higher than 6.5. These results indicate that plutonium diffused faster as Pu3+or PuO22+due to disproportionation at lower pH while plutonium could be retarded as Pu(OH)40by sorption on bentonite at higher pH.

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Corrosion of Iron and Migration of Corrosion Products in Compacted Bentonite

MRS Proceedings ◽

10.1557/proc-353-329 ◽

1994 ◽

Vol 353 ◽

Cited By ~ 8

Author(s):

T. Kozaki ◽

Y. Imamura ◽

J. Takada ◽

S. Sato ◽

H. Ohashi

Keyword(s):

Diffusion Coefficients ◽

Corrosion Products ◽

Iron Foil ◽

Iron Corrosion ◽

Apparent Diffusion Coefficients ◽

Corrosion Rates ◽

Compacted Bentonite ◽

Apparent Diffusion ◽

And Migration ◽

High Level

AbstractFor safety assessment of the geological disposal of the high level radioactive waste, it is necessary to study corrosion of the overpack materials and migration of the corrosion products in the compacted bentonite. In the present study, average corrosion rates of iron foil and apparent diffusion coefficients of the corrosion products were determined using a neutron- activated iron foil. The average corrosion rates were on the order of 10-6m/y, while apparent diffusion coefficients were in the range from 10-12 to 10-14m2/s. No tendency to decrease in the corrosion rates with increasing corrosion time was observed. This suggests that the iron foil corrodes under reducing condition. Asymmetric concentration profiles of iron corrosion products were obtained in some experiments. It can be considered that either cathodic or anodic reaction would dominantly occur on one side of the iron foil surfaces and that each reaction would change the pH in pore water of bentonite specimens adjacent to the iron foil surfaces.

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Diffusion of Uranium in Compacted Bentonite in the Presence of Carbon Steel

MRS Proceedings ◽

10.1557/proc-333-939 ◽

1993 ◽

Vol 333 ◽

Cited By ~ 3

Author(s):

K. Idemitsu ◽

H. Furuya ◽

Y. Tachi ◽

Y. Inagaki

Keyword(s):

Carbon Steel ◽

Pore Water ◽

Silica Sand ◽

High Level Waste ◽

Dry Density ◽

Compacted Bentonite ◽

Reducing Conditions ◽

Order Of Magnitude ◽

The Difference ◽

High Level

ABSTRACTIn a high-level waste repository, a carbon steel overpack will be corroded by consuming oxygen trapped in the repository after closure. This will create a reducing environment in the vicinity of repository. Reducing conditions are expected to retard the migration of redox-sensitive radionuclides such as uranium.The apparent diffusivities of uranium were measured in compacted bentonite (Kunigel VI®, Japan) in contact with carbon steel under reducing conditions or without carbon steel under oxidizing conditions for comparison. The apparent diffusivities of uranium were 3.5 × 10-14 to 1.1 × 10-13 m2/s under reducing conditions and 9.0 × 10-13 to 1.4 × 10-12 m2/s under oxidizing conditions. There was no significant effect of dry density (1.6 to 1.8 g/cm3) and silica sand (0 or 40%) on the apparent diffusivities.Since the bentonite pore water would be buffered at a pH between 8 and 9, uranium in the bentonite pore water would probably exist as a neutral hydroxide complex under reducing conditions and as an anioníc carbonate or hydroxide complex under oxidizing conditions. The anion exclusion theory cannot explain the difference of diffusivities between the two conditions. The uranium concentrations in bentonite under oxidizing conditions were one order of magnitude higher than those under the reducing conditions. The uranium concentration in the bentonite pore water under the reducing condition is estimated to be two orders of magnitude lower than that under the oxidizing conditions under the assumption of diffusion in porous media.

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Hydraulic and Diffusive Properties of Clay-Based Backfill Material for a Low-and Intermediate-Level Waste Repository

MRS Proceedings ◽

10.1557/proc-353-299 ◽

1994 ◽

Vol 353 ◽

Author(s):

W.J. Cho ◽

J.O. Lee ◽

P.S. Hahn ◽

H.H. Park

Keyword(s):

Diffusion Coefficients ◽

Experimental Studies ◽

Clay Content ◽

Dry Density ◽

Compacted Clay ◽

Southeastern Part ◽

Apparent Diffusion Coefficients ◽

Crushed Granite ◽

Apparent Diffusion ◽

Water Saturated

AbstractThe results of experimental studies performed to determine the radionuclide diffusion coefficients in a compacted clay and the hydraulic conductivities of clay/crushed granite mixtures with various clay contents are presented. Clay used in the experiments is a natural clay from the southeastern part of Korea, and it contains mainly calcium bentonite. The hydraulic conductivities of clay/crushed granite mixtures decreased with increasing clay content. In case of clay content of 50 wt.%, they maintain the considerably lower values even at the dry density of 1.5 Mg/m3. The diffusion coefficients for 90Sr, 137Cs, 60Co and 125I in water saturated clay at a dry density of 1.4 Mg/m3 were measured at room temperature. The average apparent diffusion coefficients obtained are 4.5 × 10−12 m2/s, 9.0 ×10−13 m2/s, 3.4 × 10−13 m2/s and 6.7 × 10−11 m2/s for 90Sr, 137Cs, 60Co, and 125I, respectively.

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Phases Formation and Elements Partitioning in the CaO-Gd2O3(UO2)-MnO-TiO2 System: Application to Rare Earth - Actinide Waste Immobilization

MRS Proceedings ◽

10.1557/proc-757-ii6.5 ◽

2002 ◽

Vol 757 ◽

Author(s):

Olga I. Kirjanova ◽

Sergey V. Stefanovsky ◽

Sergey V. Yudintsev

Keyword(s):

Rare Earth ◽

Rare Earths ◽

Corrosion Products ◽

High Level Waste ◽

Major Phase ◽

Reducing Conditions ◽

Transmission Electron ◽

Unit Cells ◽

Coupled Substitution ◽

High Level

ABSTRACTCeramics within the compositional series Ca4-xGdxMn2Ti7O20+x/2 (x = 0, 1, 2, 3, 4) and a sample with Ca2U2Ti7O20 formulation were studied as promising matrices for immobilization of rare earth (RE) and actinide (An) constituents of high level waste (HLW). The samples were prepared by cold pressing of oxide mixtures in pellets at 200 MPa followed by their sintering at 1400 °C or melting at 1500 °C and examined with X-ray diffraction, scanning and transmission electron microscopy. At x=0 and x=1 a perovskite – pyrophanite assemblage occurred. The sample with x=2 consisted of murataite and perovskite. Murataite was a major phase in the sample with x=3 (pyrochlore and perovskite were minor phases) and the only phase in the sample with x=4 prepared under oxidizing conditions (in air). The latter was composed of two murataite varieties with seven- and five-fold fluorite unit cells. The sample with the same formulation but synthesized under reducing conditions contained pyrochlore as an extra phase. Coupled substitution 2 Gd3+ = Ca2+ + U4+ resulted in formation of pyrochlore as the major phase. Mu-rataite and perovskite are considered as the host phases for rare earths and actinides mainly trivalent, including Pu(III), Am(III), and Cm(III), and corrosion products (Mn, Fe, Al) whereas pyrochlore is the host phase for rare earths and tetravalent actinides (U(IV), Np(IV), Pu(IV)). This makes the system of calcium, gadolinium, manganese, and titanium oxides prospective for immobilization of RE – An fraction of HLW containing minor corrosion products (iron group elements).

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Distribution Coefficients and Apparent Diffusion Coefficients of Cesium in Compacted Bentonites

MRS Proceedings ◽

10.1557/proc-556-1091 ◽

1999 ◽

Vol 556 ◽

Cited By ~ 4

Author(s):

Akiko Okamoto ◽

Kazuya Idemitsu ◽

Hirotaka Furuya ◽

Yaohiro Inagaki ◽

Tatsumi Arima

Keyword(s):

Diffusion Coefficients ◽

Distribution Coefficients ◽

Dry Density ◽

Apparent Diffusion Coefficients ◽

Compacted Bentonite ◽

Apparent Diffusion ◽

Tracer Solution ◽

Cesium Concentration ◽

Penetration Profile ◽

Very High

AbstractDistribution coefficients and apparent diffusion coefficients of cesium in some compacted bentonites were determined by the penetration profile method. Cylindrical compacted bentonites with the dry density of 0.8 to 1.6 Mg/m3 were contacted with tracer solutions containing 1000, 100 or 10 ppm of cesium. The apparent diffusion coefficients were obtained from the concentration profiles of cesium in compacted bentonites. The distribution coefficients were obtained concurrently by dividing the intercepts of the profiles by the concentration of the tracer solution. The apparent diffusion coefficients of cesium in compacted bentonite were obtained in the range of 0.42 to 9.6· 10−12 m2/s. The apparent diffusion coefficients in the compacted bentonite contacted with three different concentrations of cesium tended to decrease with increasing dry density of the specimen; but, they had no dependence on cesium concentration within a factor of 3 at the same dry density. The distribution coefficient of cesium for the specimens contacted with three different concentrations of cesium were obtained in the range of 0.3 to 90 L/kg and had little dependence on dry density. The distribution coefficients obtained in the compacted bentonites were dependent on pH of the solution rather than concentration of cesium. These distribution coefficients obtained in the compacted bentonites were 10 to 1000 times smaller than those obtained by batch experiments. The data suggest that not all sorption sites for cesium are available in highly compacted bentonite. It is necessary to consider surface diffusion as a significant migration mechanism of cesium through the compacted bentonites at very high pH condition such as 12.

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Analysis of Uranium Diffusion Coefficients in Compacted FEBEX Bentonite

MRS Proceedings ◽

10.1557/proc-807-603 ◽

2003 ◽

Vol 807 ◽

Cited By ~ 3

Author(s):

M. García-Gutiérrez ◽

J. L. Cormenzana ◽

T. Missana ◽

M. Mingarro ◽

U. Alonso

Keyword(s):

Radioactive Waste ◽

Diffusion Coefficients ◽

Solid Phase ◽

Dry Density ◽

Compacted Clay ◽

Spent Fuel ◽

Apparent Diffusion Coefficients ◽

Theoretical Approaches ◽

Through Diffusion ◽

High Level

ABSTRACTIn the Spanish concept of a high level radioactive waste (HLRW) repository, the canister with the spent fuel would be surrounded by a layer of FEBEX bentonite (Ca-Mg smectite clay) compacted to a dry density of 1.65 g/cm3. Since the permeability of the compacted bentonite is very low, the main mechanism for radionuclide transport in the clay barrier is expected to be diffusion, retarded by the sorption on the solid. Since the mayor component of the radioactive waste is uranium it is very important to study its diffusion behaviour in the compacted clay.Generally, effective (De) and apparent (Da) diffusion coefficients are obtained from "through-diffusion" and "in-diffusion" experiments, respectively. The obtention of both parameters in the same sample is a very interesting issue because the former includes the information on the porous medium structure (connectivity, constrictivity, tortuosity) and the latter takes into account the sorption on the solid phase. In this work, through-diffusion experiments were carried out for studying uranium diffusion and both effective and apparent diffusion coefficients were estimated, from the same experiment, by using different theoretical approaches.

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