Structural integrity of prestressed nuclear reactor containments
Abstract: A nuclear power plant is provided with an extensive defense intended to prevent radioactive discharge in case of a reactor accident. This defense is built up of several independent safety barriers, where the reactor containment constitutes the last barrier to the environment. The fundamental safety function of the containment is to withstand the internal pressure expected in the event of an accident. All Swedish containments are designed as shell structures, with an outer bearing prestressed concrete and an inner sealing of a tight-welded steel liner. The overall objective of this thesis is to increase the knowledge of the structural integrity for prestressed reactor containments. Two important structural components are highlighted; the prestressing system and the steel liner. The prestressing system is essential to keep the concrete structure in a linear elastic state up to the internal pressure load expected at an accident (design pressure). The steel liner is crucial for the leak tightness of the containment if the pressure load exceeds the design pressure, in case of a very severe accident. A Swedish containment is heavily prestressed with hundreds of tendons. The prestress will gradually be reduced due to the long-term behavior of concrete (shrinkage and creep) and due to relaxation of the tendons. The study concerning the prestressing system is mainly based on tendon force measurements made at Swedish plants. This study includes three parts. In the first part, techniques of measuring tendon force are investigated and one conclusion is that the friction between tendon and concrete structure could give misleading results. In the second part, results from 30 years of measured prestress in Swedish containments are presented. The main conclusions from these results are that the general loss of prestress is lower than predicted and that increased temperature during operation clearly influences the loss of prestress. Do the measured tendon forces meet the requirements? The third part intends to answer this question by using a reliability-based method. The steel liner is connected to the concrete structure by embedded studs or steel profiles. Different failure mechanisms for the liner are investigated in this thesis. It has been concluded from containment scale tests that concentration of strain occurs in the liner when the structure expands at high internal pressure loads. Strain concentration in the liner is in this thesis assumed to be caused by; concrete through-wall cracking and discontinuities like penetrations. From a study concerning through-wall cracking it is concluded that the level of plastic strain could be highly increased by the friction between liner and concrete, especially for thin liners. The effect of discontinuities is exemplified by a study of a penetration region in a containment scale test. From the analysis in this study it is shown that out-going folds in the liner tends to be straightened out when the containment expands and this behavior highly increases the strain in vicinity of the fold. The general conclusion from the study of the liner is that the non-linear plastic behavior of the liner is very sensitive to the detailed design and the interaction between liner and concrete.
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