STUDY OF THE PERFORMANCE OF SEALING SYSTEMS FOR ACCESS SHAFTS IN A HIGH-LEVEL WASTE REPOSITORY
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STUDY OF THE PERFORMANCE OF SEALING SYSTEMS FOR ACCESS SHAFTS IN A HIGH-LEVEL WASTE REPOSITORY
A.Saotome *, K.Hara° and J.Okamoto°" "*Power Reactor & Nuclear Fuel Development Corporation, Tokai-mura, Ibaraki, Japan "° Kajima Corporation, Akasaka, Minato-ku, Tokyo, Japan
ABSTRACT Shaft sealing in a high-level waste(HLW) disposal system functions to minimize the water flow passage, and retard the radionuclide transport from the repository to the accessible environment. It is important to estimate the radionuclide migration along the sealed shaft from the viewpoint of the design and the performance assessment of the sealing system. This study presents the results of sensitivity analyses on the radionuclide migration in the vicinity of the access shaft of a repository in order to evaluate the effects of the length of a plug, as well as the number of plugs, and curtain grouts. In this study, the upward hydraulic gradient of the groundwater flow along shafts was used, based on transient coupled thermo-hydraulic analyses around a repository. Hydraulic conductivities of the backfill material and the disturbed zones around the shaft tunnels were also assumed to be one order and two orders of magnitude higher than that of the host rock, respectively. The results show that the velocity of the groundwater within the shaft and the disturbed zone is reduced by a factor of one third by installing a few plugs into the shaft filled with backfill material. The curtain grouts have the effect of retarding the radionuclide migration from the repository to the ground surface at a factor of approximately five. A few plug installations have the same effect. The sealing system properly constituted with backfill, plugs, and grouts can provide the same performance as the original host rock. INTRODUCTION According to the current Japanese concept of HLWdisposal, HLWwill be disposed the underground at depths greater than several hundred meters after 30 to 50 years of interim storage for cooling. Several access shafts are necessary for the repository construction and the operation. At the time of closure, these shafts will be sealed with backfill material, plugs, and grouts to restrict the groundwater flow for the purpose of the retarding radionuclide transport. For the investigation of sealing, which includes backfill, plug and grout, Power Reactor and Nuclear Fuel Development Corporation(PNC) has been conducting the characterization of materials and the analytical study, and has been the accumulating information through the international cooperative works. This paper describes hydraulic analyses and radionuclide migration analyses of the sealing system for the access shaft in an HLW repository. An approach for design and performance assessing of the sealing system is discussed from the view point of the minimizing water flow and the retarding radionuclide transport. PRECONDITION Coupled thermo-hydraulic analyses were carried out to evaluate the effect of convective groundwater flow induced by heat from the repository[1]. The axisymmetric mod
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