Containment nuclear plant structures evaluation by non destructive testing: strategy and results

Abstract

Containment nuclear plants structures are an ultimate barrier in the event of an accident. Mechanical resistance and tightness are the two functions that they are expected to provide. To evaluate their capacity to perform them, destructive testing cannot be used to characterize the material. Non-Destructive Tests then represent a relevant solution to test concrete and the struc- ture. The article positions NDT within the context of containment structures supervision and maintenance, and presents the objectives, means and results to date of the ENDE project. The 'Non Destructive Evaluation of containment nuclear plant structures' project is a French project based on 8 partners. Theirs aims are to control the containment structures by non-destructive testing. Solutions are expected today in order to evaluate the materials and structure over time during ten-yearly inspections as well as during a Loss Of Coolant Accident. The project first focusses on the characterization of the concrete thanks to durability indi- cators (elasticity modulus, porosity, water content) and the applied stress (figure 1). Then comes the nondestructive evaluation of the thermally damaged material with temperatures up to 180°C following a potential accident. Besides, the project proposes to evaluate through non-destructive techniques opening and the depth of the cracks partly responsible for the leak. The link between NDT and the leak has been eventually established. The techniques used are mostly ultrasonic (linear and non-linear acoustics). They are associated to the electromagnetic and electric fields as well as to the acoustic emission and image correlation to combine the measurement results. This article presents the methodology and experimental plans used to link the durability indica- tors to the parameters measured by NDT. A strategy of selection of these parameters is devel- oped. The values inferred by NDT are then selected on the basis of quality indicators then merged to reduce the measurement biases. The project results are synthetized (an example is provided in figure 1), the principle is then presented to transfer laboratory techniques to in situ measurements on two industrial structures: a wind tunnel (2016) and the EDF VeRCoRs mockup (2017). The article presents the ENDE project which integrates the latest research as- sociated to nondestructive techniques for an early evaluation of the material's evolutions. It shows the capabilities and the limitations of the non-destructive techniques in order to select them for industrial integration. The obstacles that will soon have to be removed are identified.