NDT 2010 Conference Topics

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1 NDT 2010 Conference Topics Session 3B (2) Inspection Qualification Chairman M Mienczakowski Use of simulation in the NDE qualification process. Application to the EPR RPV Inspection Authors - Jack Fasham, Rebecca Toth, Laurent Truchetti, Yves Bouveret and Olivier Sornique The French Ministerial Order of 10 November 1999 requires the qualification of all NDE carried out on the Main Primary Circuit of a Nuclear Power Plant in France. This applies equally to the In-Service Inspection of the existing fleet as to the Pre-Service, and eventual In-Service, Inspection for the EPR New Build programme in France. The qualification process establishes the level of performance guaranteed by each inspection process (in terms of coverage of the area to be examined, sensitivity of the process, and defect sizing and location), and is supported by three elements: engineering reasoning, simulation and physical trials. This paper will explore the use of simulations used in the qualification process to be carried out on the EPR, building on the experience from the existing fleet. The focus will be on the influence of the identified variables on the process performances through the use of UT simulations (based on the CIVA software). Comparison will be made with the final physical trials to consolidate the findings. The process will be illustrated by real applications relating to the Reactor Pressure Vessel, in particular to the flange boltholes.

2 Use of simulation in the NDE qualification process in France. Application to the EPR RPV inspection Jack C Fasham Nuclear New Build Generation Company, part of EDF Energy Seconded to EDF Ceidre St Denis, France Jack.Fasham@edf.fr Rebecca Toth and Laurent Truchetti EDF Ceidre St Denis, France Yves Bouveret and Olivier Sornique AREVA Intercontrole Chalon-sur-Saône, France Abstract The French Ministerial Order of 10 November 1999 requires the qualification of all NDE carried out on the Main Primary Circuit of a Nuclear Power Plant in France. This applies equally to the In-Service Inspection of the existing fleet as to the Pre-Service, and eventual In-Service, Inspection (PSI and ISI) for the European Pressurised-water Reactor (EPR) New Build programme in France. The qualification procedure establishes the level of performance guaranteed by each inspection process (in terms of coverage of the zone to be inspected, sensitivity of the process, and defect sizing and positioning), and is supported by three elements: engineering reasoning, simulation and physical trials. This paper will explore the use of simulations in the qualification process to be carried out on the EPR in France, building on the experience from the existing fleet. The focus will be on the influence of the identified variables on the process performances through the use of ultrasound (US) simulations (based on CIVA software). Comparison will be made with the final physical trials to consolidate the findings. The process will be illustrated by real applications relating to the Reactor Pressure Vessel (RPV), in particular to the flange boltholes. 1. Introduction The qualification of NDE used on Nuclear Power Plants is commonplace throughout Europe, with the European Network for Inspection and Qualification (ENIQ) providing a framework for the process through its European Methodology for Qualification (1). In France, this requirement has been enshrined in law through the French Ministerial Order of 10 November 1999 (2). Increasingly, much of the Technical Justification which supports this qualification is conducted via computer simulation, due to the large

3 number of Influential Variables which can affect the performance of an NDE application. This article gives an overview of technical aspects of the French approach to Inspection Qualification. The three qualification categories are described briefly, before detailing the technical work required in compiling a Qualification Dossier. To illustrate this process, the use of simulation in supporting the qualification of the US 0º longitudinal wave inspection of the EPR RPV flange boltholes is explored here. The existence and position of a recessed junction between the flange and mating surfaces illustrates the treatment of variables in the sensitivity of this application. Positioning and sizing capability is explored via the geometry of the bolthole, its counterbore and threads. Limitations on the coverage of the examined volume brought about by the presence of a plug in the bolthole also serve to demonstrate the limitations of simulation, requiring as it did the use of practical trials on a test block. 2. Inspection Qualification in France NDE applications used on the Main Primary System and Main Secondary System of EDF Nuclear Power Plants in France are qualified to ensure that the demands of the Operational Engineering Unit (UNIE) within EDF are met by the technique developed, in accordance with the French Ministerial Order of 10 November 1999 (2). The required level of performances is demonstrated by the NDE subcontractor (AREVA Intercontrôle for the example used in this article), in coordination with the Expertise and Inspection Department for Manufacturing and Operation (Ceidre) within EDF. The qualification certificate is awarded by an internal Qualification Body whose independence is certified to ISO Type B Qualification Categories NDE on the French Nuclear fleet is conducted according to the RSE-M code (3) (ISI rules for the mechanical components of Pressurised Water Reactor (PWR) Nuclear Islands). This defines three categories of qualification, each having subtle differences in the qualification requirements Conventional Qualification RSE-M A4320 In the case of a Conventional Qualification, no defect has been postulated, neither from theory, nor resulting from operational experience. Given the crucial safety functions of the components concerned, inspections searching for an unspecified defect are conducted nonetheless. This helps to further justify the high reliability claims made for these components. The role of a Conventional Qualification is to demonstrate the sensitivity of a technique in a specified application relative to a conventional reference. In the application examined in this paper, a side-drilled hole of 2mm diameter is used to set the reference sensitivity. The levels of performance are expressed in terms of the diameter of flat-bottomed hole which can be detected at the detection threshold of 12dB, and characterised at a threshold of 6dB, relative to the reference reflector mentioned above. 2

4 The sensitivity can be verified on a hypothetical reflector of more appropriate shape, if necessary. The coverage of the examined volume by the application is also assessed Generic Qualification RSE-M A4330 For a Generic Qualification, a defect has been postulated, either from theoretical considerations such as damage mechanisms, or calling on feedback from international experience. The characteristics of the target defect are thereby defined, with the application being qualified against its ability to detect, locate and size the postulated defect. Again, coverage of the examined volume is considered Specific Qualification RSE-M A4340 The distinction between a Generic Qualification and a Specific Qualification comes from the fact that, in the latter case, an actual defect has been seen on the same component and on a reactor of the same design as that for which the application is being qualified. The approach taken here is broadly similar to that for the Generic Qualification, albeit with greater demands placed on the representative nature of the defects used in the practical trials. 3. Qualification Dossier Treatment of the Influential Variables 3.1 Influential variables For each application, a comprehensive list of variables which could affect the performance of the NDE application is drawn up. Each of these Influential Variables is studied for its impact on the following three elements of the Technical Justification: Sensitivity of the technique; Positioning and sizing capability; Coverage of the examined volume. Those variables having an adverse affect on these levels of performance are treated as detailed in 3.3, for each of the three elements in turn. 3.2 Technical Justification The mainstay of the demonstration of the level of performance for a given NDE application is the Technical Justification. This document is based on engineering reasoning, simulation and practical trials. Whilst, superficially, demonstration by practical trials might seem to be the most sound approach, there is a large number of reasons why a sufficiently robust demonstration of performances by such an approach is neither achievable nor practicable: Large number (typically around 100) of Influential Variables which need to be accounted for; 3

5 Huge number of experimental repeats required for each configuration to give statistical significance to any positive result (whereas each non-detection carries a lot of weight); Lack of real defects, due to the inherently rare nature of the defects concerned; Difficulty and cost in manufacturing realistic defects where real ones do not exist; Cost of producing test blocks, bearing in mind that comparable levels of quality control are required as on the components themselves for these blocks to be sufficiently representative; Time required to run practical trials. Given these constraints, and the ever-increasing computing power available, the use of simulation as a major factor in the Technical Justifications is to be expected. A much wider range of variables and configurations can be considered via simulation. Experimental practical trials may be required for some variables for which simulation is unfeasible, and qualification test piece trials will also be conducted, as per Treatment of the Influential Variables The treatment within the Technical Justification of each Influential Variable identified in 3.1 is conducted via the following steps: Identify a reference value for each variable, serving as a base for the simulations; Identify the nominal value for EPR in permitted inspection conditions; Establish the effect of the difference between the reference value and the nominal value on the element being considered; Identify upper and lower bounds of permissible range of variation of variable; Establish the maximum impact that this range of variation has on the element; Obtain an overall level of performance for each element by the addition (3) of: the geometric sum of all impacts relating to random variables (2); and the algebraic sum for those variables relating to the component (1) itself: Comp = M i= 1 Comp, i...(1) Rand = N j= 1 2 Rand, j (2) = +..(3) Total Comp Rand Each of these Influential Variables is treated as an independent variable. Given the large number of Influential Variables, CIVA (4) simulation is used to determine most of these impacts. The levels of performance thereby established are then verified by trials on the qualification test piece or pieces. 3.4 Qualification test piece trials 4

6 In certain situations, such as the bolthole plug discussed in 4.2.3, it will be necessary to resort to practical trials in order to obtain the data required for the Technical Justification. However, even in cases where all the Influential Variables can be treated satisfactorily through simulation, the inspection qualification process demanded by the French Qualification Body still requires these performances to be demonstrated in a physical environment as similar as possible to that found during the inspection on the station. This includes factors such as having the correct geometries of the components and ISI machine, along with the use of personnel qualified to the same level as that on site. The role of these qualification test piece trials is not to duplicate all the simulated data, rather to demonstrate that the results found through simulation can be replicated in trials which are representative of the operating conditions during an outage. A selection of configurations is chosen to confirm the claimed performances. In some instances, it will be instructive to use the nominal values, whilst in other situations, the worst-case scenario (in terms of the detection capability of the NDE application) will be more appropriate. The results of such trials are included in the Summary of Technical Evidence alongside the Technical Justification within the Qualification Dossier. 4. Use of CIVA in qualification of EPR RPV flange boltholes application 4.1 Description of the procedure This conventional qualification is being carried out on the flange boltholes for the PSI and subsequent ISI of the EPR generation of PWR in France. These 52 boltholes are found on the flange of the RPV, where they receive the studs, thereby enabling the RPV closure head to be held in place during operation of the plant. The good condition of the threads within the boltholes is therefore a key element in ensuring the integrity of the Reactor Coolant Pressure Boundary. In order to guarantee that the first ten working threads are examined, the inspection covers sufficient depth for the first 15 threads, thereby allowing for any repairs or degradation within the top five threads. Since the boltholes are not clad beyond the level of the flange surface cladding, the boltholes are plugged during the outages to prevent corrosion of the ferritic base metal. The inspection is qualified against its ability to detect defects within the 20 mm radially outwards from the back of the threads. The extent of the examined volume is therefore a ring of 20mm stretching around the bolthole from the first to the fifteenth thread. The details of this component are given in figure 1. Given the nature of the geometry of and loads on the component, the hypothetical reference reflector is a horizontal notch, with its dimensions and position relative to the threads shown in Figure. 2. Six such notches have been implanted on the qualification test piece, evenly distributed around the circumference of the bolthole, with two each at altitudes of 45 mm and 135 mm below the flange surface (the top and bottom of the examined volume respectively), and one each at the intermediate altitudes of 75 mm and 105 mm. Having two notches at either end of the volume enables one of each of these 5

7 to be placed within the zone of influence of the recessed junction between the mating and flange surfaces. The test piece has been manufactured with two junctions, to enable both of these configurations to be seen. Figure 1. Geometry of the component and examined volume Figure 2. Dimensions and positioning of the hypothetical reference reflector 4.2 Justification of performances Of all the various Influential Variables, those related to the component itself benefit the most from the use of simulation in the Technical Justification. This is due to the need that would otherwise exist for a different test block for each configuration, were such justifications to be conducted by practical trials. Of these various component variables, the position of the junction between the flange surface and the mating surface will be used to illustrate the impact on the sensitivity of the technique, whilst the effect of the geometry of the bolthole on the location and sizing capability will then be shown. Finally, the limitation of simulation will be apparent from the need to resort to physical 6

8 trials in the exploration of the bolthole plugs restrictions on the coverage of the examined volume Sensitivity of the technique For most of the circumference of the bolthole, the inspection area surrounding the hole is planar. However, towards the centre of the RPV, there is a step from the flange surface down to the mating surface. To investigate the disturbance caused by this on the US beam, CIVA was used to measure the attenuation of the beam at different distances from this junction, Figure 3. Figure 3. US field in the presence of the junction between the mating surface (to the left) and the flange surface (to the right), including the chamfer Since the Influential Variables are each to be modelled independently, the simplified geometry shown in Figure 4 contains only the junction between the two surfaces, and not the bolthole, counterbore and threads. Therefore when modelling the position of the beam relative to the bolthole, it is the distance from the junction that is used, with an origin placed 30 mm from the junction on the flange surface. For example, a radial distance from the bolthole of 20 mm, as shown in Figure 5, is in practice modelled as a beam at a distance of 10mm onto the flange surface from the junction. From Figure 5, it can be seen that the presence of the recessed junction leads to a beam attenuation of more than 4dB in the worst case of maximum depth and minimum distance form the junction. This is an example of how the existence of a variable can affect the level of performance in the nominal state for the EPR. This impact and any others need to be considered to establish the level of performances for this application in the nominal state of the EPR. 7

9 Figure 4. Model used for the junction between the mating surface (to the left) and the flange surface (to the right), including the chamfer Differences in amplitude in the FBH reference frame due to the flange / mating surface junction in the nominal state Attenuation (db) 0,0-1,0-2,0-3,0-4,0-5, ALT 45 ALT 75 ALT 105 ALT 135 Profil Radial distance from the back of the threads (mm) Figure 5. Beam attenuation resulting from the presence of the flange / mating surface The manufacturing tolerances of the RPV reveal an uncertainty in the distance from the junction between the flange and mating surfaces to the bolthole of ± 1.1mm. Further CIVA simulation is used to determine the uncertainty in the measurement of this variable. Again, the impact caused by this and all other variables having a range of variation around the nominal value need to be combined as described in 3.3 to obtain an overall performance quoted in the Performance Sheet in the Qualification Dossier Positioning and sizing capability In a similar vein, another aspect of geometry also affects performance of this application, being the layout of the bolthole itself. For the purposes of the element of the Technical Justification concerned with the location and sizing capability, however, the beam attenuation is no longer the aspect of interest. Rather, the beam deviation allows the performance of this application in this element to be determined. Figure 6 shows the deviation of the beam as a result of this geometry. 8

10 Figure 6. US field resulting from the geometry of the bolthole, counterbore and threads (image rotated through 90 compared to Figure 7) As might be expected, the further the beam is from the bolthole, the less it is affected by the geometry of the bolthole and threads, as shown in Figure 7. The black rectangles in these graphs delineate the examined volume. Again, the effects of all the other Influential Variables are combined to produce an overall level of performance in positioning and sizing capability. Figure 7. Graphic representation of the effect of the bolthole geometry on the deviation of the US beam, as a function of radial distance from the bolthole and depth Coverage of the examined volume Aside from the influence that the bolthole and thread geometry has on the ability to locate and size the defect, it is clear from Figure 6 that the US beam interacts with many surfaces relating to the threads and chamfers in this region. This results in a limitation to the coverage of the examined volume in the first 5 mm radially out from the threads. 9

11 However, feedback from experience of using this procedure on the existing French nuclear fleet has found that much more significant echoes are in fact caused by the bolthole plugs, extending far beyond this 5 mm limit. It has therefore been necessary to include the presence of the plugs as an influential variable in the case of the EPR. As is clear from Figure 8, the complex nature of the plug, with its numerous internal surfaces and materials, makes it unwieldy for computer modelling. Furthermore, a thin layer of air, water and oil will be found at the interface between the plug and the counterbore. Accounting for these materials in the correct distribution in CIVA would have been too uncertain to yield any reliable results. As such, the effect of the plug on the coverage of the examined volume had to be determined through physical tests, conducted on the qualification test piece at Intercontrôle s premises at Cadarache. Figure 8. EPR Bolthole plug, showing the numerous surfaces producing echoes The data acquisitions were analysed with Civacuve, being one of the software tools currently in use for ISI of PWR RPVs in France. Figure 9. Civacuve surface view of the plugged bolthole 10

12 In Figure 9, the echoes of the six notches positioned around the bolthole are clearly evident, emerging from the echoes of reflections coming from the plug. These latter echoes form a corona around the bolthole. Given that the defect of interest extends for 20 mm beyond the back of the threads, this corona only becomes a major problem should the margin between the maximum extent of this corona and 20 mm become too small. These trials lead to the creation of a mask extending for 15 mm beyond the back-of-thread radius, Figure 10. Figure 10. Determination of the mask resulting from the bolthole reflections Returning to the Civacuve analysis, the significant number of signals resulting from the plug echoes is clearly seen in Figure 11. The removal of all signals from the 240 mm corresponding to the mask s diameter leaves just the reflections for the mating surface reflections (which can be compensated for by procedures and a limiting of the coverage in that region) and the echoes relating to notches 1 and 5, along with their subsequent double reflections and mode transformations. This mask provides ample allowance for the maximum extent of the corona and the combined uncertainties relating both to the extent of the echoes and alignment of the ISI machine arm axis with the bolthole axis. The remaining margin of 5 mm to the defect size of 20 mm is also more than ample to cover the uncertainties relating to the sizing of 11

13 this defect. Therefore this mask of 15 mm, whilst limiting the coverage of the examined volume, still enables the detection of the target defect. Figure 11. Civacuve transverse view of the plugged bolthole, without (left) and with (right) application of mask 6. Conclusion In common with other European utility companies, EDF qualifies the NDE applications which are used for the ISI of the French nuclear reactor fleet. The Technical Justification constitutes the most important facet of the Qualification Dossier prepared for this purpose. Due to the large number of Influential Variables which can have an impact on the three elements whose levels of performance are to be demonstrated, simulation has become an invaluable tool in the production of the Technical Justification. Physical trials are nonetheless required, both to explore variables which cannot be satisfactorily simulated, and to consolidate the findings of the Technical Justification by demonstrating that this level of performance is seen in an environment representative of the inspection on site. References 1. European Network for Inspection and Qualification, European Methodology for Qualification of Non-Destructive Testing Third Issue, EC Joint Research Centre, 2007, EUR EN. 2. French Ministerial Order of 10 November 1999, NOR: ECOI A, 3. French Society for Design, Construction and In-Service Inspection Rules for Nuclear Island Components, In-Service Inspection Rules for the Mechanical Components of PWR Nuclear Islands RSE-M, AFCEN, 1997 Edition including 3 rd Modification October CIVA: Simulation software for Non-Destructive Testing, website 12