Life extension of concrete structures in nuclear power plants

FULL VERSION
These years it is being reached the end of the initial period of operation of the Spanish Nuclear Power Plants, and they are working done to renew their license that allows them to continue operating safely and efficiently in the long-term. Large infrastructures such as nuclear power plants are associated with the need to verify the status of certain components and structures in order to ensure their integrity and useful life. Among the civil structures elements to be verified are concrete structures, which can be classified as follows:

  • Critical structures:
    • Containment building (biological shield).
    • Reactor Pressure Vessel support structures.
    • Foundations and underground structures.
    • Spent-fuel building.
  • Secondary structures:
    • Turbine building.
    • Auxiliary buildings.
    • Intake structures.
    • Cooling towers.

Today, specific regulations for the maintenance of concrete structures in nuclear power plants are being drawn up based on the operating experience of facilities with a longer operating life, but it is needed to develop inspection methods that could be capable of identify a concrete in perfect condition to certify its aptitude and extend its service life; or to detect possible discontinuities, control and evaluate them after repair.

In this way, the work is being done on the development of new Non-Destructive Testing methods (NDT). These are a branch of materials science that uses non-invasive techniques to determine the integrity of a multitude of components, such as concrete (either the individual component or the complete structure), in addition to allowing to measure certain characteristics of these.

The most appropriate method and technique to carry out an inspection is selected based on the objective of the inspection, typical defectology of the component, characteristics of the structure and conditions of the inspection itself. In the field of inspection of civil structures, in general, physical methods are used to study the cement and mortar matrix and to obtain information about the position and condition of the metallic reinforcement; while chemical, magnetic and electrical methods are used to study the state of degradation of metallic reinforcement (rebars). The objectives or scope of NDT techniques for the inspection of concrete structures in nuclear power plants can be classified in 4 groups:

  • Location of the reinforcement and checking the thickness of its cover.
  • Location of tendons ducts and checking the condition of the grout.
  • Determination of the state of the concrete matrix and the corrosion of the metallic structure (rebars).
  • Detection of cracks, voids, inclusions and delaminations in the matrix of the concrete structure.

In this way, and based on the operating experience of each plant and under the supervision of the Nuclear Safety Council, inspection plans are drawn up for certain structures to verify their integrity with the objective of the life extension of the Spanish Nuclear Power Plants.

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