Using C3D Simulation Software to Study the Eddy Current Non Destructive Inspection of In-Core Instrumentation Thimbles in Nuclear Power Plants

Zorni, Chiara; EDF Centre d'Expertise et d'Inspection dans les Domaines de la Realisation et de l'Exploitation; France

Cordeiro, S.; EDF-CEIDRE; France
Dabouis, F.; EDF-CEIDRE; France
Zorni, C.; EDF CEIDRE; France

ID: ECNDT-0216-2018
Session: Eddy Current-Application 1
Room: H2
Date: 2018-06-15
Time: 09:00 - 09:20

Eddy Current Non Destructive Testing (ECNDT) is a standard technique in industry for the detection of surface breaking flaws in conductive materials. The in-core instrumentation thimble is a small tube made of austenitic stainless steel which guides nuclear measurement detector into the fuel assembly and therefore is a part of the primary system pressure boundary. External wear scars have been observed at contact points of the thimble with its guide structure. These wears are caused by vibrations and repeated contacts between the two interfaces. EDF employs an eddy current testing technique to verify the integrity of the in-core instrumentation thimbles of its 900 MWe, 1300 MWe and 1450 MWe pressurized water reactors. The probe is an axial EC probe, composed by an absolute bobbin coil with a ferrite core. The technique employed is able to characterize the depth and the volume of the detected external wears thanks to charts, which define the relation between EC signals and the defects dimensions. These charts have been defined using experimental measurements on mock-ups containing external wears with different profiles and dimensions. These measurements were then validated by comparison with real defects dimensions verified after the extraction of inspected tubes and the destructive metallurgical exam. During the inspections on nuclear power plants, some EC signals are placed outside the area described by the chart. In these cases, we cannot easily estimate the dimensions of the detected wear and our EC experts have to study these atypical cases and give their diagnostic. The study presented in this paper is focused on these atypical signals.
In order to study the response of the EC probe in presence of external wear characterized by different profiles, a 3D simulation software named C3D and developed by EDF R&D, has been used. C3D software is based on a finite elements model and allows the simulation of complex 3D geometries which have to been defined as close as possible to the real inspection’s conditions.
Comparisons between modeling results and real EC signals acquired during inspections carried on nuclear power plants will be presented. These studies contributed to the establishment of new geometries for external wears which could affect the in-core instrumentation thimbles. New mock-ups had been created and allowed us to validate our simulation results. This study will allow us to extend the domain of validity of our charts.