Near-Field Transport of 129 I from a Point Source in an In-Room Disposal Vault
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NEAR-FIELD TRANSPORT OF 1291 FROM A POINT SOURCE IN AN IN-ROOM DISPOSAL VAULT MIROSLAV KOLAR, DENNIS M. LENEVEU AND LAWRENCE H. JOHNSON AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, Canada ROE 1LO ABSTRACT A very small number of disposal containers of heat generating nuclear waste may have initial manufacturing defects that would lead to pin-hole type failures at the time of or shortly after emplacement. For sufficiently long-lived containers, only the the initial defects need be considered in modelling of release rates from the disposal vault. Two approaches to modelling of near-field mass transport from a single point source within a disposal room have been compared: the finite-element code MOTIF (A Model Of Transport In Fractured/porous media) and a boundary integral method (BIM). These two approaches were found to give identical results for a simplified model of the disposal room without groundwater flow. MOTIF has been then used to study the effects of groundwater flow on the mass transport out of the emplacement room. INTRODUCTION AECL Research has recently completed an Environmental Impact Statement (EIS) [1] on the concept of disposal of nuclear fuel waste in plutonic rock of the Canadian Shield. The reference vault design that is quantitatively evaluated comprises used fuel bundles in Ti containers that are emplaced in boreholes in the floor of disposal rooms located at a depth of 500 m. The containers are surrounded by buffer material (a clay-sand mixture) and the rooms backfilled with a clay-crushed rock mixture. An additional study involving an engineering and safety assessment of in-room emplacement of long-lived Cu containers in a vault with different geosphere characteristics is in progress. This Alternative Postclosure Assessment will provide further information on alternative waste emplacement technologies and site characteristics to supplement the EIS. For our preliminary studies, a single emplacement room of an underground disposal vault is considered. The room contains rows of disposal containers placed in the buffer material (1:1 mixture of bentonite clay and silica sand) and is backfilled with a 1:3 mixture of clay and crushed rock. The nuclide considered in the modelling is 12'1 because it has the longest half-life (A = 1.4 x 10-15 s-1) among the readily leachable radionuclides released from a failed container and plays an important role in assessment studies [2]. Two methods, MOTIF and BIM, of simulating the transport of 1291 were compared. MOTIF can be applied relatively easily to almost any system geometry with arbitrary number of different components and any flow field. Its disadvantages are the need for auxiliary artificial boundary conditions in the rock at a finite distance from the disposal room because it can simulate only finite models, and the presence of oscillations in the calculated concentration (near the diffusion front, concentration may even assume negative values). Also, MOTIF at present handles the transport of only a single radionuclide. BIM gives higher preci
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