Discrete Dilatant Pathway Modeling of Gas Migration Through Compacted Bentonite Clay

AbstractIron canisters for high level nuclear waste embedded in compacted bentonite in deep geologic repositories will corrode forming hydrogen gas. The equilibrium pressure (when corrosion would stop) has been estimated to be between 500 and 1000 atm. under repository conditions. As this is much higher than the lithostatic pressure (weight of rock overburden) the gas must be allowed to escape before it disrupts the repository. Escape by diffusion alone is not sufficient but recent experiments have demonstrated that the larger pores in the bentonite are blown free of water and let the gas escape before excessive pressures build up.The potential effect of a capillary breaking layer (CBL) has been explored. A fine layer nearest the canister (e.q. quartz sand) would have much lower capillary suction pressures than the bentonite clay and would keep the water out as long as there is sufficient overpressure. As long as the CBL is void of liquid water no radionuclides can escape, even if the canister is penetrated.

Download Full-text

Diffusion of 22Na+, 85Sr2+, 134Cs+ and 57Co2+ in bentonite clay compacted to different densities: experiments and modeling

Radiochimica Acta ◽

10.1524/ract.2002.90.9-11_2002.753 ◽

2002 ◽

Vol 90 (9-11) ◽

Cited By ~ 63

Author(s):

M. Molera ◽

T. Eriksen

Keyword(s):

Porous Media ◽

Pore Water ◽

Water System ◽

Bentonite Clay ◽

Saturated Porous Media ◽

Diffusion Study ◽

Clay Surface ◽

Compacted Bentonite ◽

Water Saturated ◽

Sorption Mechanisms

SummaryThe diffusion of radionuclides in water-saturated porous media, such as compacted bentonite, is traditionally modeled assuming diffusion in the pore water and immobilization by adsorption on the clay surface. In reality there are several sorption mechanisms acting in the clay-water system. We have therefore carried out a careful diffusion study of the cations Na

Download Full-text

A Study on Gas Migration Behavior in Buffer Material using X-ray CT Method

MRS Proceedings ◽

10.1557/proc-932-114.1 ◽

2006 ◽

Vol 932 ◽

Author(s):

K. Tanai ◽

M. Yui

Keyword(s):

Water Saturation ◽

Gas Injection ◽

Swelling Pressure ◽

Digital Data ◽

Migration Behavior ◽

Gas Migration ◽

Migration Test ◽

X Ray ◽

Compacted Bentonite ◽

Buffer Material

ABSTRACTThis paper presents a study on gas migration behavior in a bentonite specimen with the aid of X-ray computer tomography (CT) scan data. The laboratory experiment was carried out to clarify gas migration behavior through saturated, compacted bentonite. X-ray CT was used to estimate the spatial distribution of gas and water saturation during gas migration test in the bentonite. For the gas migration test, the controlled flow rate of gas injection was adopted for pre-compacted samples of Kunigel V1 bentonite using helium gas, which is safer than hydrogen gas.A specimen was isotropically consolidated and saturated by synthetic seawater, simultaneously, by applying a backpressure. This was followed by injecting the gas using a syringe pump. Inlet and outlet gas fluxes were monitored. This test exhibited a significant threshold pressure for breakthrough, somewhat larger than the sum of the swelling pressure and the backpressure.The procedure of the X-ray CT measurement is as follows; i) measurement of the initial condition (saturated condition) of the compacted bentonite, ii) measurement of the gas injection condition as a function of time. The digital data obtained from the X-ray CT usually includes some noise. The stacking method can reduce the noise in CT values and enables to identify the gas migration area. The results indicate that gas is transported through preferential pathways in compacted bentonite, and is not homogenous.

Download Full-text

Transport and leaching of technetium and uranium from spent UO2 fuel in compacted bentonite clay

Journal of Nuclear Materials ◽

10.1016/s0022-3115(99)00152-x ◽

2000 ◽

Vol 277 (2-3) ◽

pp. 288-294 ◽

Cited By ~ 8

Author(s):

H Ramebäck ◽

Y Albinsson ◽

M Skålberg ◽

U.B Eklund ◽

L Kjellberg ◽

...

Keyword(s):

Bentonite Clay ◽

Compacted Bentonite

Download Full-text

Leaching/Migration of UO2-Fuel in Compacted Bentonite

MRS Proceedings ◽

10.1557/proc-176-559 ◽

1989 ◽

Vol 176 ◽

Cited By ~ 7

Author(s):

Y. Albinsson ◽

R. Forsyth ◽

G. Skarnemark ◽

M. Skålberg ◽

B. Torstenfelt ◽

...

Keyword(s):

Spent Nuclear Fuel ◽

Fission Products ◽

High Mobility ◽

Bentonite Clay ◽

Compacted Bentonite ◽

Fuel Pellets ◽

Powdered Copper ◽

Trivalent Actinides ◽

And Migration ◽

Nuclear Fuel Pellets

ABSTRACTThe release and migration of the fission products cesium, europium and technetium, the actinides plutonium, americium and curium, and the activation product cobalt from spent nuclear fuel pellets in highly compacted bentonite clay has been measured after contact times of 101 and 386 days. Experiments at longer contact times are in progress. In some cases small amounts (0.5–1%) of powdered copper or iron metal, or vivianite (Fe3(PO4)2) have been mixed with the bentonite clay.The results indicate as expected a high mobility of cesium. The actinides have a very low mobility. After 386 days, plutonium has diffused less than 0.5 mm away from the fuel, while americium and curium appear to be somewhat more mobile. The behaviour of europium is similar to that of trivalent actinides. Very little technetium has been leached from all samples. Cobalt shows a strong retention in pure bentonite as well as in the presence of vivianite, while the mobility is much larger when iron or copper is added.

Download Full-text

Development of Numerical Simulation Method for Gas Migration Through Highly-Compacted Bentonite Using Model of Two-Phase Flow Through Deformable Porous Media

ASME 2010 13th International Conference on Environmental Remediation and Radioactive Waste Management, Volume 1 ◽

10.1115/icem2010-40012 ◽

2010 ◽

Cited By ~ 2

Author(s):

Yukihisa Tanaka

Keyword(s):

Porous Media ◽

Two Phase Flow ◽

Migration Behavior ◽

Phase Flow ◽

Gas Migration ◽

Two Phase ◽

Deformable Porous Media ◽

Compacted Bentonite ◽

Flow Through ◽

Engineered Barrier

In the current concept of repository for radioactive waste disposal, compacted bentonite will be used as an engineered barrier mainly for inhibiting migration of radioactive nuclides. Hydrogen gas can be generated inside of the engineered barrier by anaerobic corrosion of metals used for containers, etc. It is expected to be not easy for gas to entering into the bentonite as a discrete gaseous phase because the pore of compacted bentonite is so minute. Therefore it is necessary to investigate the effect of gas pressure generation and gas migration on the engineered barrier, peripheral facilities and ground. In this study, a method for simulating gas migration through the compacted bentonite is proposed. The proposed method can analyze coupled hydrological-mechanical processes using the model of two-phase flow through deformable porous media. Validity of the proposed analytical method is examined by comparing gas migration test results with the calculated results, which revealed that the proposed method can simulate gas migration behavior through compacted bentonite with accuracy.

Download Full-text

Gas transport in granular compacted bentonite: coupled hydro-mechanical interactions and microstructural features

E3S Web of Conferences ◽

10.1051/e3sconf/202019504008 ◽

2020 ◽

Vol 195 ◽

pp. 04008

Author(s):

Laura Gonzalez-Blanco ◽

Enrique Romero ◽

Paul Marschall

Keyword(s):

Gas Flow ◽

Gas Transport ◽

Initial Conditions ◽

Gas Injection ◽

Transport Processes ◽

Dry Density ◽

Gas Migration ◽

Micro Computed Tomography ◽

Saturation State ◽

Compacted Bentonite

The initial conditions (dry density and saturation state), the stress state and its history, and the deformation undergone during gas migration, affect the gas transport processes in granular compacted bentonite. Additionally, the sample microstructure set on compaction has a significant influence since gas tends to flow through preferential pathways. This experimental study intends to shed light on the gas transport and their coupled hydro-mechanical interactions with particular emphasis in the changes of the pore and pathway network. Controlled volume-rate gas injection followed by shut-off and dissipation stages have been performed under oedometer conditions. The microstructure of the samples has been characterised with three different techniques before and after the gas injection tests: Mercury Intrusion Porosimetry (MIP), Field-Emission Scanning Electron Microscopy (FESEM) and X-ray Micro-Computed Tomography (μ-CT). The results show a coupling of the deformational behaviour during the gas flow, revealing an expansion of the samples upon the development of gas pathways, which have been detected with the microstructural techniques. The opening of these pressure-dependent and connected pathways plays a major role in gas migration.

Download Full-text

Constitutive model describing the hydro-mechanical behaviors of compacted bentonite clay based on the crystal surface phenomena

E3S Web of Conferences ◽

10.1051/e3sconf/202020504012 ◽

2020 ◽

Vol 205 ◽

pp. 04012

Author(s):

Hiroyuki Kyokawa

Keyword(s):

Constitutive Model ◽

Crystal Surface ◽

Expansive Soil ◽

Bentonite Clay ◽

Degree Of Saturation ◽

Type Model ◽

Inelastic Behavior ◽

Surface Phenomena ◽

Compacted Bentonite ◽

Soil Skeleton

A constitutive model for unsaturated expansive soil based on the crystal surface phenomena is proposed. The behavior of the proposed model is described as a double structure, with the soil skeleton and the interlaminar behaviors. The soil skeleton behavior is modeled by the Cam clay type model based on the Bishop’s effective stress, which can consider the degree of saturation-induced hardening and inelastic behavior in the over-consolidated region and is capable of describing hydraulic collapse. On the other hand, the interlaminar behavior is give as a result of the interlaminar equilibrium of clay minerals. The diffusion double layer repulsive force in the interlaminar equilibrium varies with the degree of saturation, and it mainly causes the hydraulic swelling of expansive soil. The performance of the model is validated through the simulations of the suction-controlled oedometer tests on the heavily compacted bentonite.

Download Full-text