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DEVELOPMENT OF PSI-ISI AUTOMATED ULTRASONIC INSPECTION SYSTEMS FOR

ATUCHA 2 PHWR

G.J Campbell, Doosan Power Systems, Porterfield Rd, Renfrew, United Kingdom.

L.J Visconti, Nucleoeléctrica S.A. (NA-SA) Buenos Aires, Argentina

INTRODUCTION

Nucleoeléctrica Argentina S.A. (NA-SA) are currently in the process of completing the construction of the

ATUCHA 2 Pressurised Heavy Water Reactor (PHWR) Nuclear Power Plant. This plant is located in the

Buenos Aires province of Argentina.

The particular design of the Atucha unit is completely unique to Argentina, and with the Reactor

Pressure Vessel (RPV) being one of the largest designs in the world, there are many difficult challenges

for application of the Automated Ultrasonic Pre-Service Inspection (PSI) and In-Service Inspection (ISI)

of the main RPV components. This is in part due to difficult and restricted access conditions and complex

geometrical constraints.

Doosan Power Systems (DPS) have been awarded a contract by NA-SA to design and develop the

automated ultrasonic inspection systems applicable for PSI, and subsequent ISI, of the main Reactor

Pressure Vessel Welds, Closure Head, Nozzle Belt and Main Coolant Pipe Welds taking into

consideration the geometry and access constraints.

This paper highlights some of the challenges encountered in the design of the equipment and

inspection techniques for the PSI/ISI inspections and identifies some of the solutions adopted to overcome

these challenges.

CHALLENGES FOR PERFORMANCE OF PSI / ISI

The particular design of the Atucha RPV unit is completely unique to Argentina, and with the Reactor

Pressure Vessel (RPV) being one of the largest in the world, there are many difficult challenges for

application of the Automated Ultrasonic Inspection of the main RPV components. This is in part due to

difficult and restricted access conditions and complex geometrical constraints.

DPS have had to consider these challenges in the design of the inspection equipment, techniques

and systems. These main challenges are detailed as follows:

1) Vessel Dimensions

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 ID);

2) Component Design

Limited scan surface extent available for some inspection areas;

Complex and difficult geometries for some inspection areas;

Physical access restrictions for some inspection areas;

3) 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;

No access for inspections from the inside surface;

The impact of the above can also provide some restrictions in the application of the ASME XI

inspection volumes.

RPV INSPECTION AREAS FOR PRE-SERVICE INSPECTION

The following describes the RPV Areas that DPS have been contracted to provide the PSI and ISI

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inspection system solutions. DPS have had to consider access to the relevant inspection areas and, as such,

they are split into two separate regions.

1) Upper Vessel: These are the inspection areas around the nozzle belt and Closure Head, where

access is from the Closure Head region.

2) Lower Vessel: These are the inspection areas below the nozzle belt on the RPV where access is

only from a small Access tunnel in the vicinity of the bottom dome.

Upper Vessel

• Closure Head Circumferential Weld (Area 1)

• Main Coolant Outlet Nozzles (Area 2A,2B,2C)

• Main Coolant Inlet Nozzles (Area 3A,3B,3C)

• Moderator Outlet Nozzles (Area 8A,8B,8C)

• Safety Injection Nozzles (Area 9A,9B,9C)

• Fuel Detection Nozzles (Area 10)

• Moderator Inlet Nozzle (Area 12)

• Closure Head Studs (Area 13)

• RPV Flange (threads) (Area 14)

• RPV Support Lugs (Area 16)

Lower Vessel

• Circumferential Seam Welds (Areas 4,5,6,7)

o Flange-to-shell weld

o Shell-to-shell welds

o Shell-to-bottom dome weld

• Internal Attachment Welds (Area 11 and 15)

Figure 1 shows a schematic of these inspection areas on the RPV.

1(a) 1(b)

Figure 1 ATUCHA 2 RPV Inspection Areas for PSI / ISI

Further to the above inspections on the RPV, DPS have also been contracted to provide an

inspection system solution for the Main Coolant Pipewelds for the two loops of the unit.

Although there are a large number of inspection areas that require inspection during the PSI the

remainder of this paper will focus only on the access problems and the inspection solutions for three of

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these inspection areas:

• The Lower Vessel Inspection Areas;

• The Upper Vessel Inspection of the RPV Studs;

• The RPV Main Coolant Pipewelds;

LOWER VESSEL MANIPULATOR (LVM) INSPECTION SYSTEM Access to the Lower Vessel area is restricted to a small Access Tunnel near the lower dome of the RPV (-

12m level), this is indicated in Figure 1. The inspections required within the Lower Vessel area are the

RPV Shell-Flange Circumferential Seam welds (1 off), RPV Shell-Shell Circumferential Seam welds (2

off), the Bottom Dome-Shell Circumferential Seam Weld (1 off) and the Internal Attachments: Lower

filler piece support pads (12 off); Core barrel supports (12 off).

To overcome the access problems DPS have designed and manufactured a permanently installed

manipulator (Lower Vessel Manipulator LVM)) which allows full access to all inspection areas from the

nozzle belt down to the bottom dome. The manipulator 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. Figure 2 shows an illustration and a photograph of the manipulator.

Figure 2 ATUCHA 2 RPV Lower Vessel Manipulator for PSI / ISI

The manipulator consists of a ‘J-frame’ axial rail which follows the profile of the RPV and travels

on 2 circumferential support tracks. The manipulator has to be designed to withstand the harsh

environmental conditions, and remain operational for the lifetime of the plant. This includes high

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temperatures (approximately 245° – 270°) and high radiation levels (up to approximately 0.5Sv/h). The

design is such that the main rails and tracks remain permanently installed within the space annulus, and

the ‘non-permanent’ items are attached through the access tunnel. The non-permanent items include the

Motor drives, Ultrasonic Probe Carriages, Cameras/Lighting units and cabling.

Further to the manipulator design, DPS have also developed the control and installation software.

This software is used during the installation of the ‘Non-permanent’ items onto the J-frame. These items

are installed through a series of pneumatic actuators to engage and disengage onto the rack assisted by a

secondary manipulator.

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 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 Supplement 6 has lead to the

assumptions that throughwall oriented planar defects either parallel to the weld or perpendicular to the

weld direction are required to be detected. It is also assumed that defects could be located near surface

and mid-wall.

To account for the defect requirements, DPS have applied conventional UT techniques L0° and 45°,

60° and 70° shear Pulse Echo techniques in both the axial and circumferential directions. Further to this,

the application of ‘Tandem’ Techniques are provided in both directions to ensure coverage of throughwall

oriented defects over the midwall regions. The main benefits in applying the Tandem technique are that:

• Significant defects are detected in numerous depth zones;

• 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 shows the inspection techniques applied to the RPV

Seam Welds.

Figure 3 ATUCHA 2 RPV Seam Weld Inspection Techniques

DPS have also designed ‘geometry code’ algorithms to allow all ultrasonic inspection data to be

plotted in a suitable weld co-ordinate system no matter where it is collected on the RPV surface.

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RPV STUD MANIPULATOR INSPECTION SYSTEM

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 seventy six studs, hence inspection of these studs is required while the stud are in

position. Each of the 76 identical studs is to be inspected from the stud bore using a volumetric inspection

technique with the stud in its normal position in the closure head. The studs are equi-spaced around the

closure head flange.

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 and the entire length of the intervening smooth shank of the stud on the outside

surface (∅200). The limits of the examination volume in the radial direction encompass the entire through

thickness of the stud. The studs are show in Figure 4 below.

Figure 4 ATUCHA 2 RPV Studs

The main problems associated with the inspection system design are:

• Studs are not removed for inspection;

• Access to a large portion of studs is restricted due to the presence of obstructions such as the

slanted CRDM nozzles;

• Require volumetric inspection from the 30mm diameter bore over a distance of 1.8m stud length;

• Poor S:N capability within the threaded regions;

To overcome these problems DPS have designed and manufactured a bespoke ATUCHA 2 Stud

manipulator which allows full access to all inspection studs on the RPV. The main design features include:

• Flexible drive rod applied to ensure probe can raster axially (approx 2m travel) and

circumferentially around the bore;

• Guide wheels to ensure flexible rod feeds into stud hole whilst positioned to avoid physical

restrictions;

• Probe centralising mechanism;

• UT Techniques applied with multi element probe (∅ 30mm) which ensures that all techniques are

applied in one scan only;

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• Ease of application and transference to other studs which minimises any mechanical changes

required;

Figure 5 shows the mechanical setup for the Stud Manipulator and the multi-element probe housing.

Figure 5 ATUCHA 2 RPV Stud Manipulator

The Ultrasonic techniques are applied in contact scanning with each stud flooded with water which

is used as couplant. The multi-element probe consists of individual conventional single crystal shear wave

probes all contained within one housing with the following beams applied for inspection:

• 45°, 4 MHz shear wave +ve axial direction;

• 45°, 4 MHz shear wave -ve axial direction;

• 60°, 4 MHz shear wave +ve axial direction;

• 60°, 4 MHz shear wave -ve axial direction;

• 0° 4 MHz compression wave +R - B/W monitor

The obvious benefits of this multi-element probe are:

• the ability to provide coverage in two axial directions with one scan;

• the higher probe frequency ensures adequate resolution, especially for defects located within the

threaded regions at the outer surface;

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• no 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 M220 x 8 threaded sections and intervening smooth shank of the stud. The results for

defect detection are very favourable, with good resolution achieved in the outer diameter threaded region

(where S:N can be low around the geometric signals from the threads).

MAIN COOLANT PIPEWELDS INSPECTION SYSTEM

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. Each loop

consists of three legs. A hot leg where the hot primary coolant travels from the reactor to the inlet nozzle

of the steam generator. A cross-over leg where the main coolant flows from the steam generator outlet

nozzle to the primary pump inlet nozzle and a cold leg where the primary coolant travels from the main

coolant pump outlet nozzle back to the reactor.

There are fifteen welds to be inspected in each loop, thirty 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 which in some cases also have restricted scan surfaces. Further to this a large portion of the

welds are located in difficult access locations, for example, within the concrete biological shielding or

between floor levels. Figure 6 below shows some examples of the Main Coolant Pipewelds.

Figure 6 ATUCHA 2 Main Coolant Pipewelds

DPS have developed solutions for the inspection of the Main Coolant Pipewelds by utilising NA-

SA’s existing simple magnetic crawler scanner and introducing Phased Array Probes for the inspection

techniques. The advantages of this are as follows:

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• Simple manipulator installation utilising current equipment owned by NA-SA;

• Design of additional manipulator equipment to remain low profile and overcome all access

problems and minimise setup time;

• Minimal mechanical changes required over all welds within the 2 loops;

• Design of Phased Array (PA) inspection techniques to reduce number of scans required and 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 and have designed Self Tandem, TLL, TL skip

and corner trap shear wave techniques all from the one probe. The main detection techniques applied to

the welds are as follows:

- L0 Coupling monitoring on backwall

- L55 Self Tandem utilising front 16 elements

- L55 Self Tandem utilising back 16 elements

- L55 TLL utilising 2 groups of 16 elements as Tx and Rx

- L55 TL skip technique

- T45 PE corner echo utilising the front 12 elements

- T45 PE corner echo utilising the back 12 elements

- T45 Tandem utilising 2 groups of 16 elements as Tx and Rx

- T45 PE corner echo all elements with DDF

Figure 7 ATUCHA 2 Main Coolant Pipewelds Equipment

Figure 7 above 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 which has been

developed in line with the ASME Appendix VIII requirements for defect population.

Sizing techniques have also been developed by DPS with 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 to this the techniques also show capability of sizing larger rough planar

defects where the top edge is out with the examination volume (up to, and greater than 1/2 t throughwall).

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For example, Figure 8 below shows the throughwall sizing for a large rough planar defect (28mm

throughwall).

Figure 8 ATUCHA 2 Main Coolant Pipewelds Large Rough Planar Defect

SUMMARY

Nucleoelectrica S.A (NA-SA) are currently in the process of completing the construction of the ATUCHA

2 Pressurised Heavy Water Reactor (PHWR) Nuclear Power Plant. The design and operating conditions

of this plant provide some challenges for PSI/ISI inspection design;

DPS have developed :

• A lower vessel inspection system with major structures designed to remain in place for the reactor

lifetime while the inspection head is deployed through a small access tunnel;

• A stud inspection system which meets the challenges posed by the difficult access conditions;

• A primary piping system which utilises Phased Array technology to overcome access issues,

restricted scanning surfaces and to minimise inspection time;

The development work for some inspection areas is still ongoing. DPS are currently working

closely with NA-SA to complete all preparatory inspection development activities in readiness for Pre-

Service Inspection in the near future.