Gerard CATTIAUX – Thierry SOLLIERInstitut de Radioprotection et de Sûreté Nucléaire

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

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

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

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

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.

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

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,..

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)

Simulation is used to check the performance of focused immersed ultrasonic transducer, for RPV weld examination

Practical example – UT examination of reactor vessel weld with focused transducer 1/5

RESULTS : After computation, amplitude of each echo can be predicted

Such results are used to help IRSN expertise and have been used recently to ask improvements of procedures and qualifications

Side drilled holes for calibration, planar defects, flat bottom holes, and more realistic defects are then added inside the simulated component

Component, weld, surface shape, defects, can be described by CAD software

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.

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.

Practical example – RT examination on Dissimilar Metal Weld (DMW) 3/5

Weld with complex shape (crown) and defects can be described by CAD software (2D, 3D) or with CIVA GUI (holes, quality image indicator,..)

For all the DMW alloys, the chemical composition can be defined in CIVA to obtain and save the Alloy data file.

RESULTS : Assessment of detection performances, flaws appearing on the film, impact studies for numerous key parameters

This example (virtual case) illustrates complex simulation with a large variety of defects.. –Iridium 192 source, positioned along the nozzle centerline.

Practical example – RT on a weld with a thermal sleeve 4/5

RESULTS : Assessment of detection performances, flaws appearing on the film, impact studies for numerous key parameters

Lead object 4 mm thick

Nozzle with complex shape and defects described by CAD software (2D, 3D) or with CIVA GUI (holes, quality image indicator,..)

Practical example – RT of a small diameter, butt-welded pipe 5/5

RESULTS : Assessment of detection performances, flaws appearing on the film, impact studies for numerous key parameters like position of the source,..

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,..)

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

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

Notch

Driver

Receivers(differential)

ET signal

Eddy Current Testing (ET) modeling

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)

Eddy Current modeling: Qualification example COFREND Benchmark #2 : notch in a slab

Slab characteristicsThickness : 1.55 mmConductivity : 1 MS/mRelative permeability µr = 1

Probe characteristicsFrequency : 300 kHzInner diameter : 2 mmOuter diameter : 3.25 mmLift-off : 0.3Height : 2 mm

Notch characteristicsPosition : bottom of the slabLength : 10 mmWidth : 0.3 mmDepth : 40%

Reference notch filled with air

Same notch filled with an intermediate conductivity

(10% of the base material )

Probe

Eddy Current modeling: Qualification example

[2] conductivity value taken from an experimental academic work (Z. Chen et al. ENDE 2010)

-4 dB

The magnitude of the ET signal is reduced by -4dB considering a simplified model for a crack

(rectangular inclusion with intermediate conductivity)

ProbeScanning area

Modeling of a crack by a conductive inclusion having 10% of the

conductivity of the base material [2]

Reference case: notch filled with air

Eddy Current: modeling IGSCC

Example of a corrosion crack from a retired SG(cross section of the tube wall)

From NRC/NUREG CR-6924

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

Simplified model of a crack for the simulation of ET signals

10% of conductive bridges are introduced randomly in the

rectangular notch

Conductive bridges

Eddy Current modeling: Qualification example A more elaborate model for the cracks using CIVA 10 and

conductive bridges between the sides of the notch

Air

Conductive bridges(10% of the notch section)

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

The amplitude of the ET signal is not as homogeneous as in the previous example due to the random location of the conductive bridges

-4 dB

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

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