Analysis of Mass Transport in an Engineered Barriers System for the Disposal of Used Nuclear Fuel
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ANALYSIS OF MASS TRANSPORT IN AN ENGINEERED BARRIERS SYSTEM FOR THE DISPOSAL OF USED NUCLEAR FUEL
NAVA C. GARISTO AND DENNIS M. LENEVEU Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment Pinava, Manitoba, Canada ROE ILO ABSTRACT The Vault Model has been developed to assess the performance of engineered barriers in a conceptual geological disposal vault for used nuclear fuel. It represents container failure, release of radionuclides from used fuel and mass transport of released radionuclides through the clay-based sealing materials surrounding the waste containers. This paper focusses on mass-transport processes represented by the Vault Model, including diffusion, convection and retardation. In particular, we present results of several scoping calculations carried out with the Vault Model. We consider cases where the clay-based barriers are represented by either a one- or a two-layer system adjacent to an intact rock and a case where the two clay-based barriers are adjacent to a highly fractured rock. These calculations provide insight into the model and produce test cases for comparison with both relatively simple analytical The estimates and similar computer codes, as they become available. analytical estimates generally support the Vault Model results and thus enhance our confidence in the accuracy of the Vault Model calculations. 1.
INTRODUCTION
In the Canadian concept for the disposal of used fuel, the Vault Model (VM) represents container failure, release of radionuclides from used fuel and mass transport of released radionuclides through the sealing materials, e.g., clay-based buffer and backfill, surrounding the waste containers [131. It uses semi-analytical solutions to one-dimensional convectiondiffusion equations for radionuclides in a decay chain to simulate the migration of radionuclides through the buffer and backfill materials and into the host rock following container failure [4]. The relatively simple models in the VMare based on the results of extensive laboratory and field studies 151 and on detailed mechanistic models that simulate the hydrogeological, geochemical, thermal, mechanical and radiation environments of the vault. The VMhas been described in detail in Reference 3. In addition, scoping calculations analyzing the decay-chain approximations, container failure models and radionuclide chemistry were carried out with the VMand compared to other near-field models 16,71. In this paper we focus on the mass-transport aspects of the VM. In Section 2 we describe briefly the procedure used to calculate the mass transport of radionuclides through the engineered barriers system, and in Sections 3 and 4 we specify and discuss several deterministic mass transport calculations carried out with the VM. 2.
MASS TRANSPORT IN THE VAULT MODEL
The Canadian conceptual design for an underground nuclear fuel waste disposal vault consists of a series of parallel rooms excavated in the host rock. Boreholes would be drilled into the floor of the rooms, and waste containers would be empl
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