Ultrasonic inspection systems for Atucha 222 May 2013
Difficult and restricted access conditions and complex geometrical constraints have created problems for developing automated inspection systems for the main reactor pressure vessel components of the Atucha 2 PHWR, including of the RPV welds itself, the closure head and main coolant pipe welds. By G. J. Campbell and L. J. Visconti
Nucleoeléctrica Argentina S.A. (NA-SA) is currently in the process of completing the construction of the Atucha 2 pressurised heavy water reactor (PHWR) nuclear power plant in Buenos Aires. The particular design of the Atucha unit is completely unique to Argentina. The fission reaction is heavy-water cooled, and heavy-water moderated, but unlike the Canadian heavy-water reactor design occurs not in a horizontal calandria vessel, but in a vertical reactor pressure vessel. Hot heavy water coolant travels to one of two steam generators. There are also four moderator cooling loops. The reactor is fuelled by 451 natural uranium dioxide fuel assemblies mounted in tubes that enter the RPV through vessel head penetrations, and which are replaced online, from above.
Doosan Power Systems (DPS) was awarded a contract by NA-SA to design and develop the automated ultrasonic inspection systems applicable for pre-service inspection (PSI), and subsequent in-service inspection (ISI), of the main reactor pressure vessel welds, closure head, nozzle belt and main coolant pipe welds factoring in geometry and access constraints.
In the design of the inspection equipment, techniques and systems, DPS also had to consider challenges in vessel dimensions, component design and specific operation of the plant. The design of the Atucha 2 reactor is completely unique to Argentina. At the time of its fabrication the RPV was one of the largest in the world. The diameter of the RPV is approximately 8m with a shell thickness of 290mm (plus 6mm cladding at the inner diameter). In terms of component design, there is a limited scan surface extent available for some inspection areas, complex and difficult geometries for some inspection areas, and physical access restrictions for some inspection areas. In terms of specific operation of the plant, all inspections have to be performed from the outside surface of the RPV. Normal plant operation does not require removal of studs and closure head; there is no access for inspections from the inside surface. These challenges also restricted the application of ASME XI inspection volumes.
DPS divided the inspection areas into two regions, in consideration of access: (1) the upper vessel, which includes inspection areas around the nozzle belt and closure head, where access is from the closure head region, and (2) the lower vessel: the inspection areas below the nozzle belt on the RPV where access is only through a small tunnel in the vicinity of the bottom dome. Figure 1 shows a schematic of these areas on the RPV.
This paper will focus in on a few important inspection areas that form only a part of the large number of areas that require inspection during the PSI.
Access to the lower vessel area is restricted to a small access tunnel near the lower dome of the RPV (-12m level), as indicated in Figure 1. The inspections required within the lower vessel area are the RPV shell-flange circumferential seam welds (1x), RPV shell-shell circumferential seam welds (2x), the bottom dome-shell circumferential seam weld (1x) and internal attachments: lower filler piece support pads (12x) and core barrel supports (12x).
To overcome the access problems, DPS have designed and manufactured a permanently-installed manipulator (LVM) that allows a probe mounted on a traversing carriage full access to all inspection areas from the nozzle belt down to the bottom dome. The manipulator (Figure 2) is installed within a 600mm space annulus between the RPV vessel and the insulation. Access to the manipulator is gained through the small access tunnel at the bottom dome.
The manipulator consists of a single J-shaped axial rail that follows the profile of the RPV and which is mounted on, and travels on, two circumferential support tracks. The manipulator has to be designed to withstand harsh environmental conditions (temperatures of approximately 245°C-270°C and high radiation levels up to approximately 0.5 Sv/h), and remain operational for the lifetime of the plant. The design is such that the main rails and tracks remain permanently installed within the space annulus, and non-permanent items are attached through the access tunnel. Non-permanent items include motor drives, ultrasonic probe carriages, cameras/lighting units and cabling. These items are installed through a series of pneumatic actuators that engage/disengage them on/from the rack, with assistance from a secondary manipulator. DPS has also developed control and installation software used during the installation procedure.
The inspection techniques have been designed to account for the 290mm wall thickness and 6mm stainless steel cladding. The inspection of the circumferential welds is required to provide coverage of the examination volume indicated by ASME XI IWB 2500-1. This volume extends to an axial distance of 145mm (half the wall thickness) on either side of the weld and the full wall thickness of the RPV (excluding cladding). DPS interpretation of ASME XI Appendix VIII Supplement 4 and 6 has lead to the assumption that detection of through-wall-oriented planar defects either parallel to the weld or perpendicular to the weld direction are required. It is also assumed that defects could be located near-surface and mid-wall.
To fulfil these requirements, DPS applied conventional ultrasonic (UT) techniques and 0°, 45°, 60° and 70° shear pulse-echo techniques in both axial and circumferential directions. In addition, the use of tandem techniques (sending with one probe and receiving with another) are provided in both directions to ensure coverage of through-wall-oriented defects over the midwall regions. The main benefits of applying tandem techniques are that significant defects are detected in numerous depth zones, and it improves detection capability and discrimination against false calls. The main disadvantage is that the tandem technique applied here requires a large array of probes to provide coverage of the wall thickness (Figure 3). DPS has also designed 'geometry code' algorithms to allow all ultrasonic inspection data to be plotted in a suitable weld coordinate system no matter where it is collected on the RPV surface.
The Atucha 2 closure head is not removed at any point during the lifecycle of the plant. It is held in place by a total of 76 studs equally spaced around the closure head flange (Figure 4). Each of the 76 identical studs is to be inspected from the 30mm stud bore using a volumetric inspection technique with the stud in its normal position in the closure head. The examination volume for the stud inspections is defined by the requirements of ASME XI Figure IWB-2500-12. The axial extent of the examination volume covers the whole of the upper and lower M220 x 8 threaded sections (outer diameter 210mm) and the entire length of the intervening smooth shank of the stud on the outside surface (OD 200mm). The limits of the examination volume in the radial direction encompass the entire through-thickness of the stud.
There are several problems associated with the inspection system design. First, studs are not removed for inspection. Second, access to a large portion of the studs is restricted due to the presence of obstructions such as the slanted control rod drive mechanism nozzles. Third, volumetric inspection is required from the 30mm diameter bore over a 1.8m stud length distance. Fourth, there is poor signal-to-noise capability within the threaded regions because of their geometry.
To overcome these problems DPS have designed and manufactured a bespoke stud manipulator that mounts on top of each inspection stud on the RPV (Figure 5). The main design features include:
- Flexible drive rod applied to ensure probe can raster axially (approx 2m travel) and circumferentially around the bore (Fig. 5a)
- Guide wheels to ensure flexible rod feeds into stud hole whilst positioned to avoid physical restrictions (Figure 5b)
- Probe-centring mechanism
- UT techniques applied with multi-element probe (Figure 5c) to ensure that all techniques are applied in one scan only
- Ease of application and transference to other studs to minimise any mechanical changes required.
The ultrasonic techniques are applied in contact-scanning by flooding each stud with water as couplant. The multi-element probe consists of individual conventional single crystal shear wave probes all contained within one housing. It has the following 4MHz beams applied for inspection: 45° shear wave in positive and negative axial directions, 60° shear wave in positive and negative axial directions, and 0° compression wave in positive radial direction. There are several benefits of this multi-element probe: the ability to provide coverage in two axial directions with one scan; higher probe frequency ensures adequate resolution, especially for defects located within the threaded regions at the outer surface; and the avoidance of mechanical changes required between scans or stud inspections.
Manipulator commissioning and technique trials have already been performed on a representative testpiece with artificial reflectors contained within it for the stud inspections. The testpiece itself represents the lower stud threaded section and smooth shank section. The results for defect detection are very favourable, with good resolution achieved in the outer diameter threaded region (where signal-to-noise can be low around the threads).
Coolant pipe welds
There are two primary-circuit main coolant loops in the Atucha 2 plant. Each loop transfers coolant from the reactor through the steam generator and back to the reactor via the primary pump (Figure 6). Each loop consists of three legs. A hot leg carries the hot primary coolant from the reactor to the inlet nozzle of the steam generator. A cross-over leg carries primary coolant from the steam generator outlet nozzle to the primary pump inlet nozzle and a cold leg carries primary coolant from the main coolant pump outlet nozzle back to the reactor.
There are 15 welds to be inspected in each loop, 30 welds in total. Each weld connects 880mm nominal diameter pipework. Many of the welds have some access restrictions, for example, where straight section pipe is joined to elbow pieces or nozzles. In addition the tapered inlet, or outlet, from the steam generator is joined to an elbow section. Therefore for most welds the inspection must be applied from one side that in some cases also has restricted scan surfaces. Further to this a large portion of the welds are located in difficult-to-access locations, for example, within the concrete biological shielding or between floor levels (Figure 6).
DPS have developed solutions for the inspection of the main coolant pipewelds by pairing phased array probes for inspection with NA-SA's existing simple magnetic crawler scanner (Figure 7). This has several advantages. First, it is a simple manipulator installation utilising current equipment owned by NA-SA (although additional manipulator equipment was also designed to remain low-profile so it can overcome all access problems, and minimise setup time). Second, this system requires minimal mechanical changes over all welds within the two loops. Third, the design of phased-array (PA) inspection techniques can reduce the number of scans required to ensure rapid deployment of the inspection.
Each weld has a similar geometry with a standard 20° U-shaped weld preparation. The inspection volume for each weld is similar and complies with the volume defined in ASME 2007 Section XI Figure IWB-2500-8(c). The nominal wall thickness on the straight leg adjacent to the weld is 57.5mm. The material is ferritic steel with ~5mm cladding on the inside surface.
Detection techniques have been developed utilising a 32-element linear array 1.5MHz PA probe attached to a rexolite wedge. The main advantage of application of a PA probe is that multiple inspection techniques can be applied with the one probe setup. DPS have utilised the probe to ensure detection capability is achieved over the required inspection volume. Applied techniques include:
- Self-tandem: in this case, a compression wave mode is generated by the probe. Any through-wall-orientated defect located at a specific depth zone would cause a mode conversion (compression to shear or shear to compression) at the inside surface, and be detected by the same probe.
- TLL: this also relies on wave mode conversion, but uses two different groups of probe elements, one as transmitter and one as receiver. This technique also relies on wave mode conversion, but the effective spacing between transmit and receive elements changes the depth zone for defect detection.
- TL skip: again relying on wave mode conversion, this technique is based on a low-angle shear wave being mode-converted to a high-angle compression wave at the inside surface. This can provide detection of defects tilted toward the probe.
- Corner trap: A defect close to the inside or outside surface that is nearly perpendicular to it (through-wall oriented) creates a corner that can reflect the ultrasonic beam back to the probe.
- L0: A compression-wave mode beam in a positive radial direction monitors the inside surface. In combination with other phased-array probes, it checks that coupling between probe and material is consistent.
Figure 7 shows the mechanical setup for a straight pipe section and also indicates the various techniques which are applied from the phased array probe. This arrangement of techniques provides a thorough inspection applied from one side of all the welds. Detection capability has been assessed on a full-scale representative testpiece that was developed in line with the ASME Appendix VIII requirements for defect population.
Sizing techniques have also been developed by DPS by the application of a 5MHz 32-element phased array probe. The 5MHz probe frequency was chosen to ensure adequate resolution, and to ensure focused beams could be applied within areas of the inspection volume. The results from the trials on the testpiece show excellent resolution for small defects at the clad/base metal interface existing within the examination volume. In addition, the techniques also show capability of sizing larger rough planar defects where the top edge is out of the examination volume (up to, and greater than 1/2 t through-wall). For example, Figure 8 shows the through-wall sizing for a large centreline crack (29mm through-wall).
The application of these inspection systems for pre-service inspection is currently ongoing.
G.J. Campbell, Doosan Power Systems, Porterfield Rd, Renfrew, United Kingdom.
L.J. Visconti, Nucleoeléctrica S.A. (NA-SA) Buenos Aires, Argentina
This paper was first presented at the 9th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, Seattle, Washington (22-24 May 2012).