Fusion Engineering and Design 88 (2013) 1973–1977

Authors' copy - Article presented at the 27th Symposium on Fusion Technology (SOFT-27), Sept. 2012, Liège, Belgium Contents lists available at ScienceDirect

Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes

Conceptual design finalisation of the ITER In-Vessel Viewing and Metrology System (IVVS) Gregory Dubus a,∗ , Adrian Puiu a , Carlo Damiani a , Marco Van Uffelen a , Alessandro Lo Bue a , Jesus Izquierdo a , Luigi Semeraro a , Jean-Pierre Martins b , Jim Palmer b a b

Fusion for Energy, c/ Josep Pla, n◦ 2 - Torres Diagonal Litoral - Edificio B3, 08019 Barcelona, Spain ITER Organization, Route de Vinon sur Verdon, 13115 Saint Paul Lez Durance, France

a r t i c l e

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Article history: Received 14 September 2012 Received in revised form 29 January 2013 Accepted 21 February 2013 Available online 28 March 2013 Keywords: ITER In-vessel viewing system IVVS Remote inspection Metrology Erosion monitoring

a b s t r a c t The In-Vessel Viewing and Metrology System (IVVS) is a fundamental tool for the ITER machine operations, aiming at performing inspections as well as providing information related to the erosion of in-vessel components. Periodically or on request, the IVVS probes will be deployed into the Vacuum Vessel from their storage positions (still within the ITER primary confinement) in order to perform both viewing and metrology on plasma facing components (blanket, divertor, heating/diagnostic plugs, test blanket modules) and, more generically, to provide information on the status of the in-vessel components. In 2011, the IO proposed to simplify and strengthen the six IVVS port extensions situated at the divertor level. Among other important consequences, such as the relocation of the Glow Discharge Cleaning (GDC) electrodes at other levels of the machine, this major design change implied the need for a substantial redesign of the IVVS plug, which took part to an on-going effort to bring the integrated IVVS concept – including the scanning probe and its deployment system – to the level of maturity suitable for the Conceptual Design Review. This paper gives an overview of the various design and R&D activities in progress: plug design integration, probe concept validation under environmental conditions, development of a metrology strategy, the whole supported by a nuclear analysis.

1. Introduction The In-Vessel Viewing and Metrology System (IVVS) is a fundamental tool for the ITER machine operations. It aims at performing visual inspections as well as providing information related to the erosion of the in-vessel components. Periodically – during planned shutdowns – or on request – in case of unforeseen events such as plasma disruptions with suspected damages for instance – the IVVS probes will be deployed by a mechanical system from their storage positions into the Vacuum Vessel (VV) in order to perform both viewing and metrology on the plasma facing components (blanket, divertor, heating/diagnostic plugs, test blanket modules) and, more generically, to provide information on the status of the in-vessel components. In terms of viewing capabilities, the IVVS will provide greyscale 2D images with a spatial resolution better than 1 mm at target distances of 0.5–4 m and better than 3 mm up to 10 m. With respect to metrology, the IVVS will be capable of providing range data with a standard deviation allowing the evaluation of the in-vessel erosion

– and thus of the amount of mobilised dust – with an indicative maximal error of ± 100 kg. The nominal operating scenario for the IVVS will be to carry out inspections while the tokamak is in Short Term Maintenance state (i.e. with toroidal field on). On a less frequent basis, the IVVS will be needed to carry out inspections during Long Term Maintenance periods. Whilst not in operation the IVVS units will be housed in a storage plug mounted in a long and narrow VV port extension, staying therefore within the ITER primary vacuum and close to the plasma. Accordingly, over the years, the IVVS probe has been developed and designed to maintain its mechanical performance and optical capability under the conditions listed in Table 1. The outline of this paper is as follows. The current conceptual design for the IVVS is detailed in Section 2. Then, Section 3 gives an overview of the various design and R&D activities in progress: plug design integration, probe concept validation under environmental conditions, development of a metrology strategy, the whole supported by a nuclear analysis. Finally, the outline of an alternative concept for the IVVS storage system is described in Section 4. 2. Current conceptual design

∗ Corresponding author. Tel.: +34 93 320 11 33. E-mail address: [email protected] (G. Dubus).

Authors' copy © 2013 Authors. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2013.02.125

In 2011, the IO proposed to simplify and strengthen the IVVS port extensions. Among other consequences, such as the relocation

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G. Dubus et al. / Fusion Engineering and Design 88 (2013) 1973–1977

Table 1 Environmental conditions for the IVVS probe & plug. Pressure During operation: ≤10−5 Pa During shutdown: 1 atm (dry air or nitrogen) Temperature Operating temperature: ≤120 ◦ C Bake-out temperature: ≤200 ◦ C Radiation Gamma dose rate: ≤2 kGy/h (deployed probe) Gamma integrated dose: ≤5 MGy (probe) Neutron fluence: ≤2 × 1017 n/cm2 Neutron flux spectrum: from 10−7 to 2 MeV Magnetic field In deployed position: ≤8 T In storage: 0.3 T Available space Probe maximal cross section: 150 × 150 mm2

of the Glow Discharge Cleaning (GDC) electrodes, this major design change implied the need for a substantial redesign of the plug, which took part to an on-going effort to bring the integrated IVVS concept to a level of maturity suitable for the Conceptual Design Review (CDR), currently scheduled for Q1 2013. This section describes the current IVVS concept design. 2.1. The IVVS port and port extension Six IVVS units will be installed at lower port positions: 03, 05, 09, 11, 15 and 17. Each unit comprises a viewing/metrology probe mounted on a deployment arm inserted into the ITER plasma chamber between the divertor outer target and the lower blanket modules. Among other benefits, the relocation of the GDC allowed the redesign of the IVVS port extension into a simpler and more robust system, able to accommodate relative displacements between the VV and the Cryostat. 2.2. The IVVS plug The main components of the IVVS are contained in an integrated plug composed of two main parts (Fig. 1). A rigid guide tube, permanently installed inside the port, represents the main frame of the plug. It includes radial rails guiding the IVVS probe from its storage position to its various working positions. It is supported at one end by a rolling and sliding spherical collar and on the other end by a bending spider support, both designed such that the expected movements of the VV relative to the cryostat do not induce excessive stresses. An extractable cartridge, embedding the scanning probe, its deployment system and a cable management system, can be fully tested off-site and remotely installed/removed as one piece using a specific Cask and Plug Remote Handling System. This strategy reduces installation and commissioning times. It also decreases the risks associated with on-site assembly or repair. 2.3. The IVVS probe deployment system The deployment system has been designed to allow the radial transport of the IVVS probe, from its storage position at the rear of the plug to its maximal deployed position, up to 6 m away towards the vessel centre. During this translation, the IVVS carrier is guided by channels machined inside the guide tube. The IVVS probe deployment also includes a tilt motion up to 60◦ . An indicative length for the tilt body is 1–1.5 m. The selected actuation method is a single stepper motor located at the rear of the cartridge, where the toroidal field is sufficiently

low to allow the use of magnetic motors. Through an in-line planetary gearbox, this motor extends a rigid chain from the back of the port. The chain front interface is connected to the carrier such that, when the chain is pulled out of its magazine, the carrier is pushed forward, and vice versa. Comparable rigid chains already proved their reliability in the nuclear industry. Similar stepper motors and gearboxes were successfully tested under ionizing radiation levels up to 10 MGy. All ball bearings use MoS2 or WS2 solid lubrication, which allows the system to operate in both vacuum and dry air. Special attention shall be paid to store these components in a humidity controlled environment prior to installation. This system also actuates the probe tilt motion. In that case, the translation of the push rod equipping the carrier is converted into a rotation thanks to a lever mechanism at the hinge, assisted by two counterbalance springs. The choice between the two motions relies on the selective deployment of latches. 2.4. Latching systems A traverse latch secures the carrier trolley: two lobed arms extend through the carrier sides and engage with pockets machined in the guide tube. They are moved by a non-backdrivable leadscrew mechanism actuated by piezoelectric technology, which is immune to the surrounding magnetic field. A tilt latch operates in a similar manner: arms mounted within the carrier body clamp inwards onto a toothed rack, locking the tilt body. In normal operation, both latches cannot be released simultaneously. If the traverse latch is released, the push rod acts on the carrier and linear motion is achieved. If the tilt latch is released, the push rod acts on the tilt mechanism to raise the probe. Once in position, both latches are locked. As they operate at discrete positions, the probe is deployed in a very repeatable way which, nonetheless, is not mandatory in order to perform a scan. 2.5. Cable management Since the various services connected to the probe and carrier must accommodate the 5–6 m radial translation, the umbilical management relies on a system able to control both tension and bending applied on the cables. This cable reeling system is made of the arrangement of four pulleys: two of them are fixed to the rear of the cartridge and the other two are mounted on a sliding carriage. The motion of this carriage is controlled by a ball screw mechanism attached to the chain drive, so that the pulleys move synchronously with the rigid chain. 2.6. The scanning probe The probe is the active scanning tool. It now benefits from 15 years of R&D and has already been well documented [1]. It is a hybrid viewing and ranging system relying on the amplitude modulation of a coherent single mode laser. Both intensity and phase of the light backscattered from the target are detected concurrently and compared to the emitted signal. Greyscale 2D images are obtained from the intensity, while range data is generated from the phase shifting. The probe design relies on an intrinsic radiation resistant concept: all electronics is external to the VV. Therefore its layout only contains an optical box and a rotating head, held together by a mechanical structure. The optical box hosts an assembly of mirrors, lenses, and optical fibres connectors, segregated into one transmitting and one receiving stage. This arrangement is equipped with a step focus, realised by means of nano/micro-positioning linear piezo motors. The rotating head comprises a fused silica prism, actuated in pan and tilt by piezo rotary stages equipped with optical encoders.

G. Dubus et al. / Fusion Engineering and Design 88 (2013) 1973–1977

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Fig. 1. Section view of the IVVS port extension including the IVVS plug.

3. On-going design and R&D activities This section describes the activities launched by F4E in 2012, and which will continue until early 2013, to provide the CDR panel with a complete design concept.

3.1. Plug design integration The IVVS plug is still at “pre-concept” level and a significant amount of design and integration work is still required. Subsequently, the main objective of this activity is to bring together the results yielded by past and present activities into a well-integrated concept and to address the areas of the design which still need substantial development. These areas include, but are not limited to, refining the design of mechanical structures (spider support, port closure plate), reducing the probe size, describing interfaces, integrating service umbilicals and connectors, designing and positioning the control cubicles, designing vacuum feedthroughs, defining maintenance processes, developing built-in recovery features and proposing a cask-based rescue strategy. This activity is conducted by ENEA for proberelated tasks and by OTL for the broader design and integration work.

3.2. Probe concept validation under environmental conditions Although the IVVS probe concept is already well advanced, the selection of its various components needs to be verified in relation to operation under the expected environmental conditions listed in Table 1. Within this task, literature surveys are supported by experimental evidence where needed. It concerns areas such as optics and motorisation but also cabling and cubicles. In particular, an ultrasonic piezo rotary stage will be tested under gamma rays up to 4–5 MGy (on-going) [2] and prolonged exposure to neutrons. Its immunity to magnetic fields up to 8T will also be checked. At last, the effect of operation at elevated temperature and under high vacuum will be studied. The IVVS probe concept shall also be validated in the presence of low frequency vibrations, including practical verification of the efficiency of a vibration compensation scheme. This includes design, prototyping and testing of

a compact, highly sensitive and rad hard FBG-based accelerometer. 3.3. Nuclear analysis In addition to these design and R&D tasks, a nuclear analysis is being performed with the aim to estimate the neutron fluence and gamma activation in the various parts of the 8-m long plug. The neutron flux spectra will be simulated for different shielding configurations. Then the complete nuclear response (absorbed dose rates, activation, nuclear heating and decay heat power) will be calculated for the layout providing the most adequate shielding. An equivalent dose rate map will also be generated for an isolated IVVS, to simulate the system during maintenance. At last, the neutron map will be extended up to the IVVS front end electronics. This analysis, performed by CCFE, will yield important inputs for several other activities: plug integration, control cubicles design, qualification tests of critical components, maintenance strategy. 3.4. Development of a metrology strategy Being the only inspection/diagnostic tool able to provide global measurements of the in-vessel erosion, the IVVS plays a particular role in the evaluation of the amount of mobilisable dust inside ITER [3]. As a consequence, a robust strategy for the use of IVVS over the lifetime of the tokamak is being developed. It is based on mapping studies predicting the effectiveness of the IVVS in terms of coverage, viewing resolution and metrology data accuracy. To achieve this, a high-speed software tool simulating the system performance inside the VV is currently being developed by SRS ED (Fig. 2). This tool determines the quality of the acquired images and range data, associating quantitative information to both inspected components and probe locations. Eventually, it will allow the user to assess the optimal configuration of the IVVS units to perform a given scan. The basic performance requirements given in Section 1 are currently being reviewed in the light of the latest ITER operational requirements. Consequently, a R&D task has been launched in order to evaluate the potential to implement upgrades to the current IVVS probe design, with the aim of improving the dust quantity estimation. This activity, executed by ENEA, will soon investigate the benefit of introducing a multistep focus system and of increasing

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Fig. 2. Screenshot from the performance mapping software.

both laser power and modulation index. It will be supplemented by a survey on complementary viewing/metrology techniques (pulsed and FM lidar systems). Finally, a study aims to identify feasible solutions for a metrological simulation tool to be used for the IVVS. Indeed, providing high accuracy erosion measurements in the context of an everchanging environment lacking fixed reference points represents a major challenge. A proposal is being developed on a metrological strategy helping to achieve the required accuracy – by making use of reference markers for instance – accompanied by design proposals for its practical implementation inside the tokamak. This study is supported by the experimental validation by ENEA of a procedure for dust quantity estimation from IVVS measurements. 3.5. Definition of a testing programme During the lifetime of ITER there will be a need for testing and/or commissioning of the IVVS in an on-site test stand, after maintenance or upgrade for instance. Several options are envisaged, either in the refurbish-ment area of the Hot Cell Facility (HCF), which is a no-man access area and where all operations shall be carried out entirely by RH methods, or in the RH Test Facility (RHTF), where activities are likely to be carried out hands-on on non-activated components. The objective of this task – to be performed in late 2012 – is to prepare a pre-concept design of such test stands and to establish clear interface definitions with both HCF and RHTF.

4. Alternative design In the event of unfavourable results from the on-going neutronics analysis – a computed neutron fluence greater than what the probe is expected to survive – an alternative plug design has been envisaged. It introduces the use of a vacuum tank situated in the port cell and linked to the current port extension by a double bellow surrounded by two gate valves (see Fig. 3). The bellows – similar to those equipping the port extension – prevent excessive stresses in the vicinity of the valves in case of moves due to thermal expansion or to a seismic event. In this alternative design, the active assembly – functionally equivalent to the cartridge – is resident in the vacuum tank. During plasma operations, the probe is retracted in the tank and totally isolated from the primary vacuum. There, the neutron flux reaching the probe is significantly reduced, which is one major advantage of this alternative design. Another benefit of this concept is the simplification of the maintenance process. It relies on the ability to remove the whole tank without breaking the primary vacuum, by shutting off the gates valves. Before deploying the IVVS probe, the tank pressure must be balanced with the VV pressure. To this purpose, the tank is equipped with its own vacuum pump, pressure control and baking system. Once this operation is complete, the gate valves are opened and the tank itself acts as a port extension. Its alignment is achieved with external actuators arranged in a Stewart platform. Several options remain open for the system main actuation. They include, but are not limited to, using an internal vacuumcompatible motor-gearbox assembly similar to the one described in Section 2.3 or actuating the rigid chain drive sprocket with an external standard motor through a vacuum-tight magnetic coupling.

Acknowledgments

Fig. 3. Vacuum tank located in the port cell.

The authors would like to thank all the contributors taking part to the activities touched upon in this paper. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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References [1] C. Neri, P. Costa, M. Ferri De Collibus, M. Florean, G. Mugnaini, M. Pillon, et al., ITER in-vessel viewing system design and assessment activities, Fusion Engineering and Design 86 (9–11) (2011) 1954–1957.

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[2] P. Rossi, M. Ferri de Collibus, M. Florean, C. Monti, G. Mugnaini, C. Neri, et al., IVVS actuating system compatibility test to ITER gamma radiation conditions, Fusion Engineering and Design (2013). [3] M. Shimada, R.A. Pitts, S. Ciattaglia, S. Carpentier, C.H. Choi, G. Dell Orco, et al., In-vessel dust and tritium control strategy in ITER, Journal of Nuclear Materials (2013).