Elegant NDE Techniques for Investigation of Concrete Containments
P. Shaw, J. Bergström, FORCE Institute, Helsingborg, Sweden
ABSTRACT
Safety related concrete structures in NPP are rarely exposed to extreme environments, which can significantly impact on their condition during their design life. There are a number of factors, however, which suggest that there is a need for regular in-depth inspection. These problems include corrosion of the steel liner, concrete creep and loss of pre-stressing forces, corrosion damage to pre-stressing systems and cracking of the containment walls. Because many damage processes are related to workmanship there is a need to confirm structural integrity, such as the homogeneity of the concrete or as-built reinforcing details. This need is greater if the structure is required to perform beyond the original design life or if it is to maintain its function after de-commissioning. Testing concrete containments using NDE is complicated by the fact that the concrete is often thick, heavily reinforced and difficult to access. Concrete is by nature a heterogeneous material and changes its properties with time, for example by the process of moisture movement and stress. The most promising NDE-techniques, which are available today for in-situ application, are high-energy radiography, high frequency radar and various seismic methods. These are in various stages of development, but can be used in their present form to a certain degree. Some relatively simple developments could improve their capability and reliability. At this stage it is the understanding of test method capability and sensitivity which should be given development priority, as this has not been systematically studied and documented for this type of special application. Until then, the methods will remain to a large extent qualitative, with the exception of radiography and to some extent radar. A typical concrete structure cannot be modelled in an ideal manner due to the number of variables that exist in construction, material and environment. It is therefore necessary to treat each structure individually and customise the inspection procedure according to the specific critical needs of that structure. The problems of applying NDE methods to concrete structures on NPP can be partly overcome by combining techniques optimally. This is demonstrated by examples from civil and nuclear structures using radiography, radar and seismic testing. Radar data is processed using advanced software and measurement procedures such as migration and time slicing to assist in identifying detail and interpretation of data. A promising new seismic method known as Spectral Analysis of Surface waves is used to obtain information about the structure of massive concrete elements.
Publication Source: First International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components , 20 - 22 October 1998, Amsterdam, Netherlands. Held by the Joint Research Centre of the European Commission. Publisher:Woodhead Publishing Limited
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