Numerical simulation of non-destructive testing, an advanced tool for safety analysis

Transcription

1 Gerard CATTIAUX Thierry SOLLIER Institut de Radioprotection et de Sûreté Nucléaire Numerical simulation of non-destructive testing, an advanced tool for safety analysis

2 Table of Contents Qualification of NDT methods used for ISI of nuclear equipment Means for assessing the performances of NDT methods Use of NDT simulation for IRSN assessment purposes Ultrasonic Testing (UT) Radiographic Testing (RT) Eddy Current Testing (ET) UT, RT and ET practical examples Conclusion

3 Qualification of NDT methods used for ISI of nuclear equipment NDT methods used for in-service inspection of nuclear facilities or equipment (RPV, packaging, experimental or naval equipment,..) must be qualified or assessed Purpose : assessing the flaw detection and sizing capabilities with a high degree of confidence As a technical support entity for the French nuclear safety authorities (ASN, ASND), the IRSN is regularly asked to assess NDT methods currently in use (UT, RT, ET, PT,..) French practice codified in RSE-M code and in accordance with French government ruling of November 10, 1999 for RPV For other facilities or equipment (practical requirement in process of being formalized or have yet to be so) it is IRSN s view that performances should be assessable under conditions similar to those considered for NPP

4 Means for assessing the performance of NDT methods 1/2 NDT flaw detection and sizing performance must be sufficient to determine with high degree of confidence any need of repair or special surveillance For clearly identified flaws, related to postulated or observed degradation mechanisms, such performances : Are often deduced from experiment on mock-ups of tested components Mock-ups contains artificial defects with characteristics similar to those of the flaws potentially induced by in-service damage mechanisms, However, mock-ups, tend to be costly, are often specific to components and zone (ie: welded zone), Flaws cannot be reasonably replicated for more than a limited number of cases can lead sometime, to inadequate or insufficient assessment of NDT performance, Numerical simulation offers a way to enhance current knowledge of NDT performance Simulation can be included in the qualification process to supplement mock-ups experiments

5 Means for assessing the performance of NDT methods 2/2 In some cases, there is no operational experience for identifying particular degradation mechanism and predicting related flaws Simulation is a way to confirm the fitness of the NDT method for detecting hypothetical flaws, Results of simulation can reinforce qualification of NDT methods under a defense in depth-strategy considered vital by the IRSN for components subject to break preclusion concept.

6 Use of NDT Simulation for IRSN Assessment purposes 1/3 After ENIQ and NRWG works (mid 90's), and the advent of regulations for qualification of NDT methods used in PWRs : IRSN decided to avail itself of the best existing simulation tools for its own assessment, IRSN became closely involved in defining simulation requirements that cover as broad as possible a range of nuclear facilities and equipment Works in collaboration with the CEA/LIST (CIVA Software) Works on UT, ET and RT simulation models, All development includes experimental validation on mock-ups, Benefits for IRSN Assessment of NDT methods independently of any utility, manufacturer or service provider

7 Use of NDT Simulation for IRSN Assessment purposes 2/3 Areas of development for the three NDT methods (UT, RT, ET) For UT Simulation of complex weld tests using flat, contact-type transducers, reactor vessels tests using focused immersion transducers, Works on electrical discharge-machined planar flaws (EDM), flaws with complex shapes (e.g. cracks), tilted and skewed flaws, works in progress : simulation of test most commonly applied to dissimilar metal welds (with experimental validation) and test on coarse-grained heterogeneous materials (cooperative research between IRSN-NRC and CEA-PNNL) For ET Works on free span section of PWR steam generator tubes, in the vicinity of tubes support plates, with various deposit build-ups, Simulation for flaws of both simple and complex shape, simulation on matrix-type transducers,..

8 Use of NDT Simulation for IRSN Assessment purposes 3/3 For RT Simulation on test cases (practical cases) most commonly encountered in all nuclear facilities (nozzles, dissimilar metal welds, complex-shaped components and flaws, etc.), works in progress experimental validation on realistic mock-up Nota : Radiographic test module implemented in CIVA For gamma-ray inspection : Moderato (EdF) For X-ray inspection : Sindbad (CEA)

9 Practical example UT examination of reactor vessel weld with focused transducer 1/5 Simulation is used to check the performance of focused immersed ultrasonic transducer, for RPV weld examination Component, weld, surface shape, defects, can be described by CAD software RESULTS : After computation, amplitude of each echo can be predicted Side drilled holes for calibration, planar defects, flat bottom holes, and more realistic defects are then added inside the simulated component Such results are used to help IRSN expertise and have been used recently to ask improvements of procedures and qualifications

10 Practical example Simulation of UT with contact type transducer (applied to transport packaging) 2/5 Simulation is used to check the performance of UT examination with 0 longitudinal wave contact transducer (2 MHz) for a component used in transport packaging for nuclear industry Side drilled holes for calibration, planar defects, flat bottom holes, or more realistic defects can be added inside the simulated component In the example, the effect of the flaw masking effect that prevents waves from reaching the backwall of the component can be predicted, for defects of different size RESULTS : Interpretations of the different simulated tests has been used to support recommendations made in IRSN assessment files.

11 Practical example RT examination on Dissimilar Metal Weld (DMW) 3/5 Simulation (virtual case) is used to check detection performance on DMW (DMW currently present on reactor pressure vessel, steam generator, pressurizer,..) Iridium 192 source, positioned along the nozzle centerline. Weld with complex shape (crown) and defects can be described by CAD software (2D, 3D) or with CIVA GUI (holes, quality image indicator,..) RESULTS : Assessment of detection performances, flaws appearing on the film, impact studies for numerous key parameters For all the DMW alloys, the chemical composition can be defined in CIVA to obtain and save the Alloy data file.

12 Practical example RT on a weld with a thermal sleeve 4/5 This example (virtual case) illustrates complex simulation with a large variety of defects.. Iridium 192 source, positioned along the nozzle centerline. Nozzle with complex shape and defects described by CAD software (2D, 3D) or with CIVA GUI (holes, quality image indicator,..) Lead object 4 mm thick RESULTS : Assessment of detection performances, flaws appearing on the film, impact studies for numerous key parameters

13 Practical example RT of a small diameter, butt-welded pipe 5/5 This example illustrates complex simulation with a large variety of defects on a small diameter pipe Iridium 192 source, distance between source and film at least 8 x Ø Pipe with complex shape (OD and ID) and defects described by CAD software (2D, 3D) or with CIVA GUI (holes, quality image indicator, choice of the material,..) RESULTS : Assessment of detection performances, flaws appearing on the film, impact studies for numerous key parameters like position of the source,..

14 Eddy Current Testing (ET) modeling One of the major applications of Eddy Current testing in the nuclear industry is the inspection of Steam Generator (SG) tubes Tube integrity is an important safety issue since a tube rupture could release radio-elements in the environment EDF uses proprietary probes and procedures for periodic inspection and expertise Analysis of ET signal is difficult and comparison with foreign experience is limited (EDF uses specific probes and/or calibration) IRSN uses modeling tools for ET in order to: have a better understanding of the ET signals analyze topical subject and associated expertise conducted by EDF (SG Tube Support Plate clogging-up, corrosion under Tube Support Plate) recommend enhancements to the current practice

15 Eddy Current Testing (ET) modeling Notch Driver Receivers (differential) ET signal

16 Eddy Current modeling: Qualification example Practical trials for ET qualification are mostly done on artificial flaws such as notches Are those flaws representative for corrosion cracks? EDF proposes a conservative transfer coefficient between trials on artificial notches and signals from actual corrosion cracks [1] How conservative is this position? This approach is analyzed by IRSN using a simplified and independent methodology: Considering a benchmark proposed by the French federation for NDT (COFREND) Using academic data on corrosion cracks (conductivity) Using CIVA features to model a crack [1] The use of EC modeling for NDE qualification (F. Buvat et al. EPRI/SG workshop 2007)

17 Eddy Current modeling: Qualification example COFREND Benchmark #2 : notch in a slab Probe Reference notch filled with air Same notch filled with an intermediate conductivity (10% of the base material ) Slab characteristics Thickness : 1.55 mm Conductivity : 1 MS/m Relative permeability µ r = 1 Probe characteristics Frequency : 300 khz Inner diameter : 2 mm Outer diameter : 3.25 mm Lift-off : 0.3 Height : 2 mm Notch characteristics Position : bottom of the slab Length : 10 mm Width : 0.3 mm Depth : 40%

18 Eddy Current modeling: Qualification example Probe Reference case: notch filled with air Scanning area -4 db Modeling of a crack by a conductive inclusion having 10% of the conductivity of the base material [2] The magnitude of the ET signal is reduced by -4dB considering a simplified model for a crack (rectangular inclusion with intermediate conductivity) [2] conductivity value taken from an experimental academic work (Z. Chen et al. ENDE 2010)

19 Eddy Current: modeling IGSCC IGSCC : InterGranular Stress Corrosion Cracking in alloy 600 SG tubes is a complex degradation it follows the grain boundaries with a general orientation perpendicular to the main constraints secondary cracks or branches with slightly different orientations might be observed the cracks opening can be extremely tight with electrical contact Example of a corrosion crack from a retired SG (cross section of the tube wall) Simplified model of a crack for the simulation of ET signals 10% of conductive bridges are introduced randomly in the rectangular notch Conductive bridges From NRC/NUREG CR-6924

20 Eddy Current modeling: Qualification example A more elaborate model for the cracks using CIVA 10 and conductive bridges between the sides of the notch Conductive bridges (10% of the notch section) Air -4 db The amplitude of the ET signal is not as homogeneous as in the previous example due to the random location of the conductive bridges A simplified model of a crack gives us the necessary quantitative information and should normally be in agreement with EDF's position ET modeling gives insight into the physics of the technique and normally enhances the confidence in the performances of the inspection

21 Eddy Current modeling: conclusions Modeling is a useful tool for physical reasoning in ET, it is also a mean to learn and to get skill for the analysis of complex signals in the framework of expertise on topical subjects It has been used by IRSN for recent SGs safety issues Corrosion signals observed after a chemical cleaning Clogging-up of Tube Support Plates (TSP) Corrosion under TSPs (Bugey 3) Further R&D of IRSN in a joint program with CEA will address: Modeling the +Point probe getting a better understanding and benefit from foreign operational experience Modeling the X-Probe ET array probe used for the first time in France during last Bugey 3 outage Assessment of the pros and cons of this technology

22 Conclusions NUMERICAL SIMULATION IS NOW A MATURE TOOL The main benefits : Gives reliable means to assess for the most NDT examination, the detection and sizing performances on a numerous types of defects IRSN can justify improvement requirements of NDT procedures Improves the NDT assessment by IRSN expert independently of utilities Improves the expert knowledge's, particularly for the new technologies Gives to Safety regulators the benefit of IRSN knowledges