The Effect of EDZ on the Migration of Radionuclides in a KBS-3 Type Repository
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7KH(IIHFWRI('=RQWKH0LJUDWLRQRI5DGLRQXFOLGHVLQD.%67\SH5HSRVLWRU\ Jorma Autio*, Thomas Hjerpe* and Marja Siitari-Kauppi** * Consulting Engineers Saanio&Riekkola Oy, Laulukuja 4, FIN-00420 Helsinki, Finland ** University of Helsinki, Department of Chemistry, Laboratory of Radiochemistry, P.O.Box 55, FIN-00014 University of Helsinki, Finland. $%675$&7 The role of excavation damaged zone (EDZ) in a KBS-3 type repository for spent nuclear fuel depends significantly on the design of the engineered barrier systems such as tunnel backfill since one obvious function of the backfill is to prevent the growth of EDZ. Main results of EDZ studies based on using 14C-PMMA method carried out at Äspö Hard Rock Laboratory in Sweden, and at Research Tunnel at Olkiluoto in Finland by Posiva and SKB in co-operation are presented. The significance of the results is evaluated quantitatively and qualitatively. The EDZ around the deposition hole on migration of radionuclides diffusing out of a waste canister was found unlikely to be a significant migration route in the absence of large hydraulic gradients. According to the study the EDZ caused by state-of-the-art drill and blast excavation adjacent to walls and roof of deposition tunnels was found negligible, however the EDZ adjacent to deposition tunnel floor is evidently more extensive and more significant. ,1752'8&7,21 Posiva Oy in Finland is planning to construct a KBS-3 type repository for spent nuclear fuel at the Olkiluoto site at the coast of the Baltic Sea. The main repository alternative is KBS-3V, the emplacement of fuel canisters in vertical deposition holes in the floor of a tunnel. An alternative KBS-3H, the emplacement of several canisters in long horizontal deposition holes is also studied. The backfilling can be based on use of homogeneous impermeable backfilling material such as crushed rock and bentonite or heterogeneous backfilling consisting of e.g. crushed rock and impermeable plugs [1]. The designs and objectives of the backfill and sealings in the Finnish KBS-3 type repository have been presented in [2]. The role of EDZ in KBS-3 type repository concept depends on the function of the barriers and the type of repository and backfilling. Therefore the role is different in KBS-3H and KBS-3V concepts, in different backfilling alternatives and in different parts of the repository. The abbreviation EDZ is used in this document for the zone of irreversible deformation of excavation damaged rock adjacent to the surface of underground openings such as the deposition tunnels and deposition holes. The excavation damage is caused by redistribution of stresses around an underground opening and the impact of excavation tool on rock (detonation of blasting agent in drill and blast excavation or impact of discs or hard metal buttons in mechanical excavation). In addition to the damaged zone there is a disturbed zone, which extends from the damaged zone to the undisturbed rock and which is characterized to be a zone having reversible deformations. The properties of exca
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