International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Development testing requirements and evaluation non-destructive testing (NDT) methods for assessment age-related degradation of concrete containments in the nuclear power plantsElena Österberg, Swedish Nuclear Power Inspectorate, Stockholm, Sweden
Gregory Muravin, Margan Physical Diagnostics, Netanya, Israel
ABSTRACTA concrete containment in a nuclear power plant is a last barrier in the event of nuclear accident with radioactive release and therefore has significant role for public safety. In the past it has been a common belief that nuclear concrete containments will have life-time well beyond the life-time of nuclear reactors. That assumption was based on specific experimental and theoretical investigations and also the presence of many concrete structures several hundred years after construction. In view of this, it seemed to be no need in global inspection of concrete containments overall condition. The regulation testing and inspection demands almost limited to periodical leak integrity tests and visual examination of accessible concrete surfaces were believed to be sufficient.
However, the recent experiences of revealing degradation in concrete containments world wide lead to an increasing awareness of the fact that the nuclear concrete containments undergo age-related degradation and that the present testing and inspection practice can not reveal all degradations at an early stage. There is a risk that the undetected age-related degradation may lead to unexpected failure of nuclear concrete containments. Also, the nuclear power plants seeking to extend operation life-time must demonstrate that concrete containments continue fulfil designed functions. There is therefore reason to make a global assessment of concrete containments overall condition; to do this it is necessary to use non-destructive (NDT) methods.
Existing NDT methods for assessing reinforced and post-tensioned concrete structures include surface hardness methods, acoustic methods, radiography, electromagnetic methods, resistivity measurements. These NDT methods are widely used in the non-nuclear industries and some of them are outlined in well-known international standards and guidances. Successful application of NDT methods to nuclear conditions requires first of all understanding of acceptance criteria applied to nuclear concrete containments; the acceptance criteria must be decisive.
In practice, the established acceptance criteria for a particular concrete structure must be transferred into terms understandable for NDT measurements. Specific testing requirements must be developed with definition target flaws, detection sensitivity, characterisation and sizing ability, demands for accuracy and reliability. In general, the target for NDT testing of nuclear concrete containments will include concrete physical-mechanical properties, corrosion in embedded metal components, bond properties between concrete and reinforcement, level of post-tensioning force, cracks and porosity in concrete. Evaluation of NDT methods must be done against the specific testing requirements so that NDT results can be used to decide if the structure performs as expected; it is not acceptable when the decision on the structure's capability to fulfil designed functions is oriented at the capabilities of the used NDT methods.
This paper presents a systematic approach for evaluation NDT methods for a particular nuclear power plant containment; the acceptance criteria have been established and the testing requirements have been developed prior to evaluation NDT methods.