Reflections on the use of monitoring technologies in geological nuclear waste repositories

The Finnish company Posiva Oy is constructing a repository for the final disposal of spent fuel in the bedrock of Olkiluoto, an island in the Gulf of Bothnia. The disposal is based on geological barriers and engineered barrier systems (EBS) that isolate the nuclear waste form the biosphere and maintain the protective function of the repository. Monitoring during implementation of the repository is used to provide information on the performance of the barriers and to strengthen confidence in long-term safety, which is the key objective of radioactive waste disposal. Posiva Oy has established a general monitoring program that ensures the applicability of the constructed facilities to the final disposal of nuclear spent fuel. A program defining the methods and systems that will be chosen to monitor and instrument the engineered barrier systems is currently under preparation. It gains substantial knowledge and monitoring experience from experiments and demonstrations in Posiva's underground research laboratories. The full-scale underground experiment POPLU is one of the demonstrations in a project of Euratom's Seventh Framework Programme called DOPAS ("Full-Scale Demonstration Of Plugs And Seals"), which is running from 2012-2016. The DOPAS project will compile the design basis of plugs and seals, develop new technology for plug and seal materials and for the assembly and construction of plug and seal systems, carry out full or partial design of the systems, and perform partly or wholly five full-scale plug and seal tests (http://www.posiva.fi/dopas). Within POPLU a deposition tunnel end plug for deep geological repositories is being constructed at Posiva's underground rock characterization facility ONKALO in the crystalline bedrock of Olkiluoto. It separates the demonstration tunnel from the vehicle connection and is instrumented with sensors continuously measuring displacements, strain, humidity, mechanical and pore water pressure and temperature under very demanding conditions. Furthermore, wired and wireless sensors allow for monitoring the pressure being applied by pumping water into a filter layer behind the plug and the observation and control of the main parameters needed during the pressurization of the structure. An artificial pressurization routine is conducted within the short timescale of the demonstration in order to simulate the 100 year expected lifetime of the concrete structure. The in-pumped water and any out-flowing leakage will also be determined by a monitoring system to analyse the performance of sealing mechanisms of the structure. The project monitoring and instrumentation plan builds upon the experiences gained in other ONKALO demonstrations and profits by the knowledge transfer from related disciplines with a strong background in Finland, like infrastructure monitoring, sensor development, wireless systems networks and ICT.