QUALIFICATION OF ULTRASONIC INSPECTIONS IN THE ITER VACUUM VESSEL MANUFACTURING PROJECT

Transcription

1 QUALIFICATION OF ULTRASONIC INSPECTIONS IN THE ITER VACUUM VESSEL MANUFACTURING PROJECT R. Martínez-Oña, A. García, M.C. Pérez, Tecnatom, Spain; G. Pirola, Ansaldo Nucleare, Italy ABSTRACT The vacuum vessel is one of the most important components of the ITER project in which the fusion will take place. It is a double wall torus made of nine sectors of 40 each one. The weld joints that are in the outer wall have one surface access only and their inspections have to be qualified. A consortium led by Ansaldo Nucleare with Mangiarotti and Walter Tosto is in charge of manufacturing seven (out of nine) sectors of the ITER vacuum vessel and Tecnatom is in charge of qualifying the inspection system for the examination of the weldments mentioned before. The objective of this paper is to describe the qualification methodology main features of the ultrasonic inspection system for the examination of vacuum vessel weldments with one surface access. The qualification approach required by the ITER European Domestic Agency, Fusion for Energy, in charge of the vacuum vessel manufacturing is the one specified by the CEN/TR and the RCC-MR 2007 Code. INTRODUCTION ITER project has as objective the construction of a fusion facility to demonstrate the scientific and technical viability of obtaining energy from fusion. It is a shared project between China, European Union, India, Japan, Russia, South Korea, and the USA. ITER reactor is being erected at Cadarache, in the South East of France. The fusion reactor is planned to start its operation at the end of this decade. One of the main components of the facility is the vacuum vessel in which the fusion will take place. The vacuum vessel (VV) is the first nuclear confinement barrier; therefore the integrity of the VV is essential. Furthermore, it is the structure on which are mounted the measurement equipment and other components within it, whereby it has to be manufactured with very high accuracy, overcoming the welds deformation and to satisfy the high dimensional requirements. The VV is made of austenitic stainless steel 316L (N) IG ITER Grade. It has a toroidal shape with double wall and the section of D shape. The torus dimensions are: Inner diameter: 6,456 mm; outer diameter: 19,400 mm; and height: 11,337 mm (see Figure 1). Figure 1. - General view and cut out of ITER Vacuum Vessel including ports and port extensions (ITER Organization courtesy) 36

2 Due to the manufacturing process and limited access, the VV outer wall plates, that are the most affected by the high pressure loads, are necessarily welded from one face only and this makes more complex the required volumetric inspection. Therefore, for this and other reasons associated to its constructive complexity, the European Domestic Agency named Fusion for Energy, responsible of managing the VV fabrication, requires the qualification of the ultrasonic inspections of weldments with only one surface access. Tecnatom is responsible of qualifying these inspections. In the sections that follow, first, are described the VV main characteristics regarding geometry, materials, and manufacturing process that could have an influence in the ultrasonic inspection techniques. After that, it is presented the qualification methodology and other applicable requirements. Finally, the inspection techniques initially devised and under development are described. DESCRIPTION The vacuum vessel is a torus of double wall with a section of D shape. Around the torus circumference, different upper, middle and lower ports for access, instrumentation and maintenance are distributed. The design includes, first, the fabrication of nine sectors of 40 each, then, the weldments between sectors and, finally, the port extensions welds; Fusion for Energy is in charge of manufacture 7 out of the 9 sectors. Each sector is constructed from four segments, named PS1, PS2, PS3 and PS4, that once manufactured are welded one to the other (see Figure 2). Each segment is made of pieces, flat and forged plates of different sizes and shapes that after welded produced the desired segment. The weld process, with pieces of 40 and 60 mm thickness in most of the cases, starts with the inner wall pieces and progresses to end with the outer wall welds. In all cases, the plates are of austenitic stainless steel 316L (N) IG ITER Grade. PS2 Port extension PS1 PS3 PS4 Port extension Figure 2. - ITER VV sector made of the assembly of four segments PS1, PS2, Ps3 and PS4 The VV design and manufacturing, and the associated inspections, are in accordance to the requirement of the RCC-MR Code 2007 Edition. The welds categories 1 and 2 are required to volumetric examination, either RT (Radiographic Testing) or UT (Ultrasound Testing). Inner wall weldments and their reinforcements are classified as category 1, and outer wall weldments and their reinforcements are classified as category 2. For UT inspection of austenitic steel full penetration welds, the RCC-MR Code requires examination from the two surfaces and two sides of the weld plus demonstration of the inspection 37

3 procedure capability of detection. When the access is limited to only one surface weld, the ITER project requires the inspection qualification in accordance to the CEN/TR Methodology and the RCC-MR Code. Considering that the manufacturing of segments starts from the inner wall and progresses to the outer wall, inspection qualification is required in all closure welds of the VV outer wall; these welds have a V shape with a chamfer within a range of ±2 to ±6. In addition to this, due to the geometry complexity of the structure to fabricate and weld types (see Figure 3), it could also occur access limitation to fulfill the inspection as required by the code. Therefore, austenitic steel anisotropy, 60 mm thicknesses, difficult access to the examination area, and limited scanning surface make a challenge the inspection and qualification of these welds. (a) (b) Figure 3. a) Different weld types considered in the ITER VV project and b) Suggested (left) and required scanning according to RCC_MR Code (middle and right) INSPECTION QUALIFICATION METHODOLOGY The applicable Code for the inspection of ITER VV welds is the RCC-MR 2007 Edition and, for the qualification activities, the CEN/TR 14748:2004. The inspection system to be qualified is the prototype of the system to be used in the inspection of welds with one surface access during the manufacturing phase. In the qualification process, five phases can be distinguished (see Figure 4): Phase I. Prior to qualification: In this phase is necessary to collect all required input information, establish the qualification team and define the qualification objectives. Input information mainly refers to the essential variables of component and postulated defectology. The Qualification Team is the group of people that manage, plan, and carry out the qualification; it is made of representatives from ITER Organization, Fusion for Energy, Agreed Notified Body and Manufacturers. Phase II. Preparation for qualification: The main activities in this phase are the definition of the qualification programme, the production of the experimental evidence and the draft of the inspection procedure and the technical justification. The qualification programme explains the content, scope, the evaluation criteria, the means and how the qualification is carried out. Phase III. Executing the qualification: The main tasks are conduct the qualification programme, record the results during the qualification process, evaluate the qualification results and compile the 38

4 qualification dossier. The dossier includes the input information, the inspection procedure, its technical justification, the qualification programme, the test results, and the conclusions reached. Phase IV. Approval of qualification: Accredited documents are issued and explained their scope and their applicable conditions. Phase V. Application of qualification: The qualified inspection procedure is established and the qualification dossier is maintained with the experiences derived of its application. Phase I. Prior qualification To prepare Input information. To identify Qualification Team To define Qualification Objectives Phase II. Preparing for qualification To define Qualification Programme To prepare UT Procedure To prepare Technical Justification Phase III. Executing the qualification To apply Qualification Programme To evaluate Qualification Results To Compile Qualification Dossier Phase IV. Approving the qualification Phase V. Applying the qualification To establish Qualified Procedure To maintain Qualification Dossier Figure.- 4 Main phases of the qualification process (after CEN/TR 14748) INPUT INFORMATION Input information refers to the essential variables of the component weldments and postulated defectology that are necessary to know and specify to start the definition of the inspection technique. As mentioned above, there is a large number of weld types in the ITER VV design but we will concentrate on those welds, to be qualified, with one surface access. At present, these welds have V shape (see Figure 3a) but different chamfer parameters and welding parameters. The following possibilities can be distinguished: a) Chamfer angle (±2, ±5, ±6 ), b) Root preparation, and c) Weld procedure (First pass: GTAW manual, GTAW automatic; Filling: SMAW manual, GTAW manual, GTAW automatic). Currently the following component weldments essential variables are taken into account: a) Geometry, b) Surface conditions, c) Weld configuration, d) Material specification, e) Repairs, f) Access restrictions, and g) Environmental limitations. Based on the experience of manufacturers and discussion within the Qualification Team, three groups of defectology have been identified: a) Weld defects, b) Weld imperfections, and c) Perturbing factors. In table 1 are listed the cases considered in the three groups of defects. The essential variables of defects currently considered are: a) Size (length and through wall extension), b) Position (base metal, weld metal or weld interface), c) Location along the thickness (scanning surface breaking, embedded, or opposite to scanning surface breaking), d) Orientation (perpendicular or parallel to the weld), e) Skew, and f) Nature (planar or volumetric). 39

5 Table 1.- Postulated defectology for qualification of V welds Defect description Nature Weld defects Slag inclusion (for SMAW) Volumetric Tungsten inclusion (for GTAW) Volumetric Porosity and gas pore Volumetric Crack Planar Hot crack Planar Lack of fusion Planar Weld imperfections in the root area Hollow bead Excess penetration Undercut Shrinkage Disturbing factors Weld node -- Weld repair -- Misalignment -- WELDING MAP The VV manufacturers are preparing the so called Welding Atlas that compiles the relevant weld information. This includes, for each weld family, the weld geometry, the weld section, and the welding parameters. The Welding Atlas is part of the input information. Analyzing the Welding Atlas information, from the ultrasonic point of view, first the inspection volume is calculated, second, the inspection volume is assessed and, third, the scan plan is defined. These and other information related to the inspection technique form the named Welding Map that is made of a set of cards, each one describing a weld type; see an example of this above in Figure 5. Weld Section Minimum scanning area FORM D - Manual TIG for the penetration passes - SMAW for the filling passes ACCESIBLE SURFACE Inspection volume R Figure 5.- Example of Welding Map card for V shape weld SUGGESTED INSPECTION TECHNIQUES 40

6 RESOLUTION Weld center line Inspection techniques are developed to satisfy the specified inspection requirements taking into account the input information. Inspection requirements are to detect and characterize all defects above the recording threshold; in addition to, any planar indication is unacceptable and any volumetric indication whose length exceeds the maximum permitted length is also unacceptable. When designing the inspection techniques is also worth to mention the reduced existing scanning space and the importance of minimizing the scanning duration, therefore the scanning lines for detection purposes are defined parallel to weld centre line (see Figure 6) and for characterization purposes perpendicular to it. At present, the following inspection techniques have been foreseen and are under development: a) Pulse-Echo (P-E) with phased array (PA) probes: For detection of weld defects, weld imperfections, and disturbing factors; an initial defect characterization assessment is also possible. b) P-E with creep probes: For detection of weld defects breaking to scanning surface. c) P-E with low frequency probes: For detection of weld defects perpendicular to the weld centre line breaking to opposite scanning surface and weld node defects. d) P-E with straight beam probe: For detection and characterization of weld imperfections. e) P-E with standard diffraction probes: For characterization of weld defects and weld imperfections. f) Tandem with standard probes: For characterization of weld defects located in the thickness middle part. g) Auto tandem with dual probes: For characterization of weld defects located in the thickness lower part. Salvo 2 Salvo 0 Salvo 1 Y+ Scan Y+ SCAN INDEX X+ Trajectory of the probe. One UT acquisition (three salvoes) is done each red bullet. Y- Y+ From the top of the Vacuum Vessel weld. X- Figure 6.- Scanning lines sketch for inspection detection phase PRESENT STATUS Different test blocks have been manufactured and others are under fabrication in order to have a set of representative defects (see Table 1) on which develop, demonstrate and qualify the different proposed inspection techniques. These test blocks reproduce the geometry and metallurgical configuration of the weld and parent material. The intended defects in the test blocks are mechanized reflectors of planar and volumetric shapes located in the areas of interest. Test trials and simulations have been carried out to select the essential variables of the candidate ultrasonic probes as well as to assess the capabilities of the inspection techniques. In that manner, the inspection techniques for weld defects detection and characterization have been developed. When weld imperfections and weld perturbing factors test blocks are ready the envisaged techniques will be implemented and tested. In order to perform the qualification, and taking into account the variety of weld configurations and elements of inspection system, the qualification groups are defined. Within a qualification group there is a set of welds with similar essential variables and an inspection system with similar essential variables; then, the objective is to qualify this inspection system for inspecting this set of welds. To define the qualification groups the criteria have been the following: 1 st ) select the weld V which has the most adverse welding procedure from the ultrasonic point of view, 2 nd ) consider the specificity of the inspection technique, and 3 rd ) consider the weld trajectory (straight, circumferential or mixed: part 41

7 curved / part straight) or the mechanical scanner necessary to carry out the ultrasonic inspection. For straight welds the Galaxy scanner will be used, for circumferential welds the Flexible Housing scanner will be applied, and for short length welds of either curve and/or straight trajectory the Wheel scanner which allows ultrasonic probe position recording will be utilized. With the present VV design status and these criteria nine qualification groups have been identified (see table 2 for details). However, it is anticipated that additional qualification groups could appear associated to any complex geometry not assessed yet. Inspection technique Table 2.- Summary of Qualification Groups Mechanical Scanner Galaxy Wheel Flexible Housing P-E with PA probe G1 G2 P-E with Creeping probe G3 P-E with Low frequency probe G4 G5 P-E with Straight beam probe G6 P-E with Standard diffraction probe -- G7 -- Tandem with standard probes -- G8 -- Auto tandem with dual probes -- G9 -- Inspection procedures are not developed yet. To do so, it is necessary to have the set of representative test blocks. Once the procedures are ready, the experimental evidence will be produced and the technical justifications could be prepared. The Qualification programme is being discussed and under preparation by the Qualification Team. It mainly includes: a) the assessment of the technical justification and the inspection procedure; b) conducting practical trials (type: open/blind, personnel requisites: EN 473 certification and specific training, NDT tests: on test pieces containing representative defects, following the procedure to the letter, scanner tests: acceptance tests on realistic mock-ups; c) evaluation of trials results: assessment of documentation, fulfillment of qualification objectives, and explanation and justification of inspection results using supporting information); d) details of qualification test pieces (purpose of trials, number of test pieces, number of defects per test piece, and test piece quality checks). CONCLUSIONS The ITER VV is a double wall torus made of many welded thick pieces of austenitic stainless steel. In order to perform a reliable inspection of those welds with one surface access the qualification of inspection techniques, according to CEN/TR and RCC-MR Edition 2007 Code, is required. The development of the inspection techniques covers a wide range to be able to detect and characterize the postulated weldments defectology. The qualification methodology managed by the Qualification Team, in which all the parts involved in the project are present, is developing as expected and guaranteeing the performance of the inspection qualification. REFERENCES 1) RCC-MR, Design and Construction Rules for Mechanical Components of Nuclear Installations, Section 3: Examination Methods and Section 4: Welding. Edition ) CEN/TR 14748:2004, Non-destructive testing Methodology for qualification of nondestructive tests. 42