Performance Assessment of Engineered Barriers Using the Vault Model
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PERFORMANCE ASSESSMENT OF ENGINEERED BARRIERS USING THE VAULT MODEL.
L.H. Johnson and D.M. LeNeveu, AECL Research, Pinawa, Manitoba, Canada ROE ILO
Whiteshell
Laboratories,
ABSTRACT The Vault Model for assessing engineered barrier performance has been developed as part of the preparation of an Environmental Impact Statement to be presented to a Federal Environmental Assessment Review Panel reviewing the Canadian nuclear fuel waste disposal concept. The model describes the behaviour of titanium containers, radionuclide release from used fuel, and migration of radionuclides through buffer and backfill materials and into the surrounding geosphere. Vault Model simulations have shown that the release of radionuclides from the engineered barrier system is dominated by the release from the fuel-sheath gap and grain boundaries in used fuel. Sensitivity and uncertainty analyses have illustrated how releases from the vault are affected by both the uncertainty in model parameters and the assumptions made in the development of the models. It is likely that the combined effects of a number of conservatisms in the model result in the releases from the engineered barrier system being overpredicted by several orders of magnitude. INTRODUCTION AECL Research is examining the concept of disposal of nuclear fuel waste 500 to 1000 m deep in plutonic rock of the Canadian Shield. The long-term performance of the engineered barriers (the used CANDU* fuel bundles, the containers and the vault sealing materials) will be described in documentation to be presented to the Federal Environmental Assessment Review Panel appointed by the Canadian government to review the acceptability of the disposal concept [1]. The consequences of disposal are estimated through the use of SYVAC (SYstems Variability Analysis Code) [21, which integrates the three major models representing the vault, geosphere, and biosphere. SYVAC repeatedly samples values of model input parameters from given probability distributions, calculates consequences, and produces a histogram of predicted consequences versus their frequency of occurrence. Here we outline the basic features of the models of engineered barrier performance (the Vault Model), and discuss some of the most important assumptions and uncertainties in the models. The disposal concept envisions the emplacement of used fuel in containers surrounded by sealing materials in a mined excavation at a depth of 500 to 1000 m in crystalline rock of the Canadian Shield. Although several engineered barrier options have been examined, the principal focus has been on the performance of a reference system involving borehole emplacement of used-fuel bundles in titanium containers. The containers would be surrounded by a 50:50 bentonite/sand buffer material, and rooms, drifts, and shafts would be filled with a 75:25 crushed granite/clay backfill, with additional seals present at various strategic locations. An example environmental assessment incorporating these engineered barriers into a hypothetical disposal vault has been p
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