Buffer Production Line 2012: Design, Production, and Initial State of the Buffer

The buffer surrounds the canister and fills the void spaces between the canister and the rock wall in the deposition hole of a repository for spent nuclear fuel. The purpose of the buffer is to protect the canister from detrimental thermal, hydraulic, mechanical and chemical, including microbiological (THMC) processes that could compromise the safety function of complete containment, to maintain favourable conditions for the canister and to slow down the transport of radionuclides in case the canister starts leaking. The chemical composition of the buffer material shall also be compatible with the performance of the canister, host rock and backfill. The current buffer design consists of three main components: disk blocks under and above the canister, including the canister chasing disk block, ring shaped blocks around the canister and pellet filling in the outer gap between the blocks and deposition hole rock wall. A chamfer block that is used to fill part of the chamfer in the upper part of the deposition hole of the canisters of the Olkiluoto 3 power plant is also a component of the buffer. The chamfers facilitate the deposition of the longer canisters of Olkiluoto 1-3 power plants in the deposition tunnels. The reference buffer material is bentonite with montmorillonite content between 75-90 %. The whole production line for buffer production consists of material excavation and processing on site, delivery, acceptance of the material batch to production, transport, handling and storage of raw materials and components in different phases and manufacturing and installation of buffer components. Quality control is part of every phase in manufacturing and installation. The initial state of the buffer refers to the material properties of the components and the average properties of the buffer directly after emplacement according to its design when a direct control over the buffer ceases and only limited information can be made available on the subsequent development of conditions in the buffer or its near-field. It can be stated that the buffer design initially fulfils the performance targets with sufficient safety margins. The evolution of the buffer after emplacement is discussed separately in the Performance Assessment report. In the coming years further development will still be done concerning the buffer component manufacturing and the installation of buffer components and quality assurance.