The Effect of Cracks on Diffusive Mass Transport through a Clay Barrier
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THE EFFECT OF CRACKS ON DIFFUSIVE MASS TRANSPORT THROUGH A CLAY BARRIER
NAVA C.
GARISTO AND FRANK GARISTO
Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment Pinawa, Manitoba, Canada ROE 1LO ABSRACT
Clay-based
buffers
are often
underground disposal vaults of
used
proposed nuclear
conceptual vault design, each used-fuel
as engineered fuel.
Thus,
barriers for
in the Canadian
container is emplaced in a vertical
borehole in rock and surrounded by a compacted buffer material, made up of a mixture of sodium bentonite and sand. There is some evidence, however, that the buffer may be susceptible to cracking due to, for example, cementation or moisture depletion. In this paper we estimate numerically the consequences of cracking on
the diffusion of uranium through a finite
buffer layer.
In particular, we
study how the effects of cracks depend on the rock conditions at the edge of the vault. For swept-away boundary conditions at the rock/buffer interface (e.g., representing a major fracture zone in granite), the total flux through the cracked buffer system is greater than through the corresponding uncracked buffer, particularly for buffers with a very small effective diffusion coefficient. On the other hand, for the case in which the rock at the rock/buffer interface is intact and, thus, the mass transport of material from the buffer into the rock is small, the effect of the cracks on the total release flux is negligible. INTRODUCTION In the disposal-vault design for the Canadian Nuclear Fuel Waste Management Program, cylindrical containers of used nuclear fuel would be
placed in vertical boreholes in rock and surrounded with a bentonite-based buffer material Ill. The buffer is expected to fulfil several functions: (i) to swell as groundwater seeps into the vault, thus providing mechanical support for the containers; (ii) to retard groundwater access to the containers and to ensure that the convective flow of groundwater in contact with the containers is negligible; and (iii) to absorb and retard radionuclides leaching from the fuel after failure of the containers. However, there is some evidence that the buffer may be susceptible to cracking due to, for example, excessive fluid pressure [2], cementation and mineral transformation [3-5], or moisture depletion 16]. The objective of the present paper is to estimate the effect of buffer cracks on the rate of dissolution of the used U02 fuel using solubilitylimited dissolution models [7]. In these models the used-fuel dissolution rate depends on the mass transport of uranium species through the buffer. In this paper we specifically investigate the effect of the cracks on the diffusion of uranium for various rock conditions at the rock/buffer interface, i.e., intact rock, highly fractured rock and intermediate cases.
DIFFUSION IN THE BUFFER/CRACK SYSTEM After the container has failed completely, dissolved uranium diffuses, in a radial direction, away from the whole length of the container. The radial mass transport of uranium in the un
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