In SKB’s main alternative for final repository of radioactive material, KBS-3V, the backfilled deposition tunnels will be separated from the remaining tunnel system with concrete plugs. These concrete plugs will be designed for a life span of 100 years and their function shall maintain until the transport tunnels outside the plug are backfilled and the natural geohydrological conditions have been restored.
The purpose of this report is to document the results and the evaluation from this project and motivate the choice of the most appropriate design for closing the deposition tunnels in the spent fuel repository. The purpose has also been to investigate and present the loads acting on the plug system and determine the load capacity of the concrete plug. This report is the result of a project conducted between 2009-01-01 – 2010-12-31 and the project group has made its assessment based on the conditions and requirements that are present today.
The entire design of the plug system is part of this project, where the plug system consists of a filter, a bentonite seal and a cast-in-place concrete plug. Two different conceptual design alternatives for the concrete plug have been studied in this report, one long tapered plug and one dome shaped plug. The results in this report focus on the choice of the conceptual design for the concrete plug and its possibility to assist the entire plug system to satisfy its requirements.
It is a complicated task to dispose the radioactive waste and it sets high technical requirements on the design and the production of the backfill and the closing of the deposition tunnels. The aim of this project is to design and develop a plug system suitable for production. This is done by the means of numerical calculations and analyses. The primary function of the concrete plug is to act as a resistance to the external loads originated from the axial expansion of the backfill and the water pressure. However, the entire plug system has a requirement on being watertight, which also affects the design of the concrete plug. In the spent fuel repository, low-pH concrete should be used instead of conventional concrete. The reason for this is to the largest extent to reduce the negative effect that basic materials could have on the function of the bentonite clay. For this purpose, a new low-pH concrete recipe has been developed and this changes the conditions for using reinforcement, cooling and grouting compared to the use of conventional concrete.
The report shows the possibilities to use an unreinforced plug made of low-pH concrete as a resistance in the deposition tunnels. Today, some parameters are unknown and some data may be classified as uncertain, primarily regarding the long-term properties of the low-pH concrete material and the bentonite clay. It will take several years until all questions can be answered and a full-scale test is vital to validate the assumptions and the performed numerical simulations. The report should therefore be considered based on that data and conclusions will be studied further and be experimentally verified under realistic and controlled conditions.
Stockholm: Svensk Kärnbränslehantering AB , 2012. , 149 p.