Modelling the in Situ Performance of Bentonite-Sand Buffer
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MODELLING THE IN SITU PERFORMANCE OF BENTONITE-SAND BUFFER H.S. RADHAKRISHNA* AND K.-C LAU* * Ontario Hydro Research Division, 800 Kipling Ave., Toronto, Ontario M8Z 5S4 B.H. KJARTANSON** AND S.C.H. CHEUNG** ** Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba ROE ILO ABSTRACT In the Canadian nuclear fuel waste management concept, a number of engineered barriers, such as the bentonite-sand buffer which surrounds the waste container in the emplacement boreholes, are used to inhibit the transport of radionuclides. The buffer material is also required to effectively conduct heat from the fuel-waste containers to the surrounding rock. To a large extent, in situ buffer performance will depend on the degree of moisture within the buffer. The moisture content will in turn depend on temperature, temperature gradients, and buffer initial and moisture flux boundary conditions. Modelling of coupled heat and moisture transport in the buffer before resaturation is necessary to assess in situ buffer performance. This paper describes the results of a parametric study using the Philip and de Vries coupled heat and moisture transport model to assess the effects of variations in the moisture diffusivity parameters and the boundary conditions on buffer performance. The results show that the thermal performance of the buffer is affected by heat-induced moisture movement. In particular, the thermal vapour diffusivity, DTvap? has the most significant effect on thermal drying in a closed system. Work is currently underway to improve our capability to model coupled heat and moisture transport in buffer. Laboratory experiments are in progress to more accurately define the moisture diffusivity parameters and the model is being modified to include the effects of boundary moisture fluxes and pressure potentials so that the resaturation process may be modelled. A full scale buffer/container experiment is currently being planned for conduct in AECL's Underground Research Laboratory to assess further the effects of scale and in situ boundary conditions on buffer performance and to qualify the model. INTRODUCTION The Canadian concept for nuclear fuel waste disposal proposes that the waste be emplaced in a vault at a depth of 500 to 1000 m in stable plutonic rock of the Canadian Shield. In this concept, corrosion-resistant fuelwaste containers will be placed in boreholes drilled into the floors of emplacement rooms, and separated from the host rock by bentonite-sand buffer material. The buffer material is required to inhibit the transport of water and radionuclides, to protect and support the containers and to effectively conduct heat generated by radionuclide decay into the surrounding rock. A mixture of sodium bentonite and vell-graded silica sand in a 1:1 dry mass ratio has been selected as the reference buffer material [1,21. It will be compacted in situ to a minimum dry density of 1.65 Mg/mi3 at a moisture content of 17 to 19%. To a large extent, buffer performance will depend on the degree of moistu
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