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Numerical simulation of nondestructive testing, an advanced tool for safety analysis
Grard Cattiaux, Thierry Sollier
Institut de Radioprotection et de Sret Nuclaire (IRSN) Reactor Safety Division
BP 17 92262 Fontenay-aux-Roses cedex, France
Abstract: For safety analysis purposes, the IRSN needs to assess the performance and limitations of nondestructive test (NDT) methods being used in the nuclear industry. Numerical simulation is one of the tools that is suited to such assessments. This approach can improve understanding and facilitate analysis of the physical mechanisms involved in several types of NDT (ultrasonic, radiographic and eddy current techniques). For many years, the IRSN has participated in developing the numerical simulation functions required for its own safety analyses. These functions have been integrated into the CIVA NDT platform developed by the CEA and are now accessible to all other users, in particular nuclear facility operators. This paper presents various applications for numerical simulation in IRSN safety analyses and describes R&D currently being conducted on enhanced modelling of the relevant NDT methods.
1 QUALIFICATION OF NDT METHODS USED FOR IN-SERVICE INSPECTION OF NUCLEAR EQUIPMENT
NDT methods used for in-service monitoring of nuclear facilities or equipment (reactor pressure vessel, experimental and naval propulsion reactors, packaging for transport of radioactive materials, etc.) must be qualified or assessed to determine, with a high degree of confidence, their flaw detection and characterisation capabilities. For nuclear power plants, NDT practices are codified in the Rules for In-Service Inspection of Mechanical Components (RSE-M) and are qualified in accordance with article 8 of the French government ruling of November 10, 1999 on surveillance of the reactor coolant pressure and secondary system pressure boundaries. The process applicable to pressurised water reactors (PWRs) is described in . It is the IRSN's view that for other facilities or equipment for which practical requirements are either in the process of being formalised or have yet to be so, performance should be assessable under conditions similar to those considered for nuclear power plants.
NDT qualification is thus intended to demonstrate that a test method used for a given zone of equipment adequately detects and characterises any degradation likely to affect that equipment.
2 MEANS FOR ASSESSING THE PERFORMANCE OF NDT METHODS
NDT flaw detection and sizing performance must be sufficient to determine, with a high degree of confidence, any need for repair or special surveillance of the affected component. Such performance is often deduced from experiments performed on representative
mock-ups of tested components that contain artificial or realistic defects with characteristics1 resembling those of the flaws potentially induced by in-service damage mechanisms. Mock-ups, however, tend to be costly and are often specific to the component and zone (in particular welded zone) of interest. In most mock-ups, flaws cannot be reasonably replicated for more than a limited number of cases, which leads to inadequate assessment of NDT performance.
Numerical simulation offers a means for enhancing current knowledge of NDT performance, as ENIQ2 advocated at the end of its work on NDT qualification in the mid-1990s. Such simulation can be included in the qualification process to supplement mock-up experiments, for clearly identified flaws related to postulated or observed degradation mechanisms.
In some cases, however, there is no operational experience available for identifying particular degradation mechanisms and predicting related flaws. Where this is true, simulation can confirm the fitness of the NDT method for detecting hypothetical flaws. It likewise reinforces qualification of NDT methods under a defence-in-depth strategy considered vital by the IRSN for components subject to the break preclusion concept.
The following paragraphs describe the IRSN's contribution to the development of NDT simulation software and its application of this tool to safety analysis, with the help of a few examples.
3 USE OF NDT SIMULATION FOR IRSN ASSESSMENT PURPOSES
As a technical support entity for the French nuclear safety authorities3, the IRSN is regularly asked to assess NDT methods currently in use and, where necessary, to report on their real performance. After completion of work on this subject by ENIQ and NRWG4and the advent of regulations for qualification of NDT methods used in PWRs, the IRSN decided to avail itself of the best existing NDT simulation methods for its own assessments. It thus became closely involved in defining simulation requirements that would cover as broad as possible a range of nuclear facilities and equipment. This meant use of simulation tools that could verify the flaw detection and sizing performance of the most common nuclear industry NDT methods, independently of any utility, manufacturer or service provider. Initially developed to simulate ultrasonic testing, such software now permits simulation of gamma and X-ray inspections, as well as eddy current techniques. The many developments included in the CIVA platform are now available to a large community of users.
In every case, rigorous experimental validation is required for the three main types of NDT ultrasonic, eddy current and radiographic testing . For radiographic testing, joint studies recently begun by the IRSN and CEA/LIST have already enabled more realistic simulation of complex parts, materials and flaws. Such simulations in turn permit impact studies based on variation of numerous parameters. What now remains is to compare predictions of various flaws detected by simulation (including those at the edge of the visibility range) with results read from actual radiographic films. Future work will thus include experiments intended to validate models and to reinforce their flaw prediction capability, using parts of components illustrative of those present in nuclear facilities.
1 i.e. orientation, dimensions, type (volumetric or planar), opening, facies, etc. 2 ENIQ: European Network for Inspection Qualification, a working group made up of representatives from all the European utilities. 3 which include ASN (French Nuclear Safety Authority for civil facilities and activities) and ASND (French Nuclear Safety Authority for defense-related facilities and activities) 4 NRWG: Nuclear Regulatory Working Group, made up of representatives from the European safety authorities.
Areas of development for the three NDT methods are as follows:
- for ultrasonic testing: simulation of complex weld tests using flat, contact-type transducers, and reactor vessel tests using focused immersion transducers; performance of simulations on electrical discharge-machined planar flaws and flaws with complex shapes (e.g. cracks); making allowance for flaw misorientation effects; and simulation of tests most commonly applied to dissimilar metal welds (including validation against experimental data). A joint IRSN/NRC research project is currently focusing on coarse-grained heterogeneous materials.
- for eddy current testing: simulation of tests performed on straight sections of PWR steam generator tubes, in the vicinities of tube support plates, with various deposit build-ups, for flaws of both simple and complex shape; simulation of matrix-type transducers, etc.
- for radiographic testing: simulation of test cases most commonly encountered in nuclear facilities (nozzles, dissimilar metal welds, complex-shaped components and flaws, etc.).
In all these cases, CAD software is readily available to describe the most complex components and flaws.
Both the nuclear industry (in particular EDF) and the aerospace industry are contributing to the enhancement and extension of numerical simulation models, thereby supplementing studies undertaken by the IRSN. The radiographic test module implemented in CIVA simulates gamma and X-ray inspections by integrating the Moderato and Sindbad codes developed by EDF and CEA/LETI respectively. CIVA likewise proposes response models for film used in industrial radiography. The next paragraphs present several examples of simulations for nondestructive ultrasonic, radiographic and eddy current testing.
3.1 Simulation of ultrasonic testing with a focused immersion transducer
This simulation shows (Figure 1) ultrasonic tests conducted using a focused immersion transducer. The part tested is fictitious, for better illustration of the simulated functions, but nevertheless reflects a type of simulation appropriate for evaluating reactor vessel weld inspections.
This simulation serves to estimate the detectability of various crack-type flaws with simple or complex geometries, in the parts tested. It enables display of various acoustic phenomena generated by complex flaws (e.g. diffraction signals from crack tops or roots, which are used to size planar flaws). The simulated part likewise includes both flat and geometrically complex surfaces, thus enabling assessment of impact due to uneven surface state5 on flaw detectability and sizing. Ultrasonic signal amplitudes are expressed in decibels. In the example shown in Figure 1, amplitude values obtained for the different flaws are based on the value measured for a reference reflector6 .
5 This consists of a layer of partially grinded stainless steel cladding deposited on a ferritic steel base. 6 Reference amplitude measured for a cylindrical hole = 0 dB.