Study of ITER Equatorial Port Plug Handling System and Vacuum Sealing Interface

Abstract

In the field of the ITER port plug engineering and integration task, CEA has contributed to define proposals concerning the port plugs vacuum sealing interface with the vessel flange and the equatorial plug handling.

The 2001 baseline vacuum flange sealing consisted of TIG welding of a 316L strip plate on to U shapes. This arrangement presented some issues like welding access, implementation of tools, lip consumption, complex local leak test, continuous leak checking. Therefore, an alternate sealing solution based on the use of metallic gaskets is proposed. The different technical aspects are discussed to explain how this design can simplify the maintenance and deal with safety and vacuum requirements.

The design of the mechanical attachment and vacuum sealing of the plug has constantly evolved, but the associated remote handling equipment was not systematically reviewed. An update of the cask and maintenance procedure was studied in order to design it in accordance with the last generic plug flange design. This includes a concept of a gripping system that uses the plug flange bolting area and, to help the remote handling process, a cantilever assisting system is suggested to increase the reliability of the transfer operation between vacuum vessel and cask.

Introduction

The main structure of ITER is composed of a vacuum vessel surrounded of several ports at its 3 levels. These large windows are filled by components called plugs at the equator and upper level that can provide different kind of functions (heating antennas, diagnostics, limiters or Test Blanket Modules). These systems are mechanically attached in cantilever from their rear main flange that ensures also the function of vacuum vessel boundary.

In the field of the ITER port plug engineering and integration task, CEA has contributed to define proposals concerning the plugs vacuum sealing interface with the vessel flange and the equatorial plug handling.

Equatorial plugs rear flange sealing

State of the design of the rear flange

The 2001 baseline vacuum flange sealing [1–3] consisted in the TIG welding of a 316L strip plate on to U shapes (2 welded lips) (see Fig. 1). This arrangement presented some issues like welding access, implementation of tools, lip consumption, complex local leak test, continuous leak checking. Therefore, an alternate sealing solution based on the use of metallic gaskets is proposed.


Fig. 1. Equatorial port flange 2001 design

Working conditions and ITER specific requirements

As the sealing flange is far from the plasma, the neutronic flux is rather low (about 10 W for the whole flange), nevertheless, this region can be submitted to 100° C (normal functioning) to 240° C during backing. The sealing boundary must withstand the Ultra High Vacuum (10−6 Pa) inside the vessel and atmospheric pressure in the Port Cell, in case of in vessel steam leakage, the absolute over-pressure can reach 0.2 MPa. The required helium leak rate level is 10−8Pams−1 at 200° C[4].

Metallic gasket alternative

The CEA, with the support of ITER, proposed to study a new option for the sealing of the flange based on metallic gaskets for improving, at least, two major factors: safety and maintenance.

Using a double gasket mounting, in-service helium leak test is achievable and it is also possible to maintain the interspace between two gaskets at 0.5 bar in order to check internal and external leaks. The intermediate volume can be pumped in case of small leak in order to continue allowing plasma operations. This could be also possible with TIG welded lips, but this should consume more space.

The 2 mm thick welded lips impose to consume matter at each maintenance operation. This is rather different with a gasket assembly: there is not limit for the number of possible replacements. Also, no complex cutting and welding tools are required and it avoids the management of lip cutting dust and welding smoke. But this sealing technique also has its difficulties, because the bolting tools for gasket compression have to be developed and the cleaning of the sealing surfaces when replaced could be needed.

The study was based on the use of the Helicoflex seals that relies on viscoplastic deformation of a metallic lining. This lining is selected based on its plasticity, which as to be better than the flange material plasticity. Such deformation is obtained by compressing a helicoidal spring. The spring gives to the Helicoflex seal its remarkable elasticity.

Concerning the port plug flange design sealing surfaces, the long experience on the tokamak Tore Supra of the use of Helicoflex seals has shown interesting performances. Even if small scratches appear on the sealing surface (depth superior to 0.13 mm), it can be easily repaired using sandpaper because in the case of a spread hollow, the seal can accept an important depth (0.56 mm) mostly due to the Helicoflex spring action.


Fig. 2. Gasket flange design vertical cross-section


Gasket sealing flange design

The main constraint was to propose a double gasket mounting design that could fit in the available space in order to maximise the area of the internal frame of the plug and minimize the external size of the gasket flange to ensure compatibility with maintenance process.

The design gasket flange is composed of two stiff halves and a compliant part in the middle to allow misalignments between the port duct and the port plug (see Fig. 2). This flexible welded plate provides structural vacuum barrier and allows misalignment between the port flange and the vessel. This part would be adjusted and welded during first assembly. Spacers are included in the gasket assembly in order to ensure the metal-to-metal contact for optimal sealing and Helicoils inserts would be used for better maintenance of the threads.

One of the most important point that requires a special care in the dimensioning of a gasket sealing is the bolting effort that allows and ensures a good sealing tightness quality. A parametric study was performed by GARLOCK (metallic gasket manufacturer) to analyse the best metallic seal choice between silver or aluminium coating and rectified or not.

Following this analysis, the result of the dimensioning led to the use of silver coating with a not rectified gasket in the vacuum side, and a rectified one outside. This difference allows balancing the tightening cantilevered effort on the two seals; that is a consequence of the division of the flange two stiff halves. The number and size of bolts have been determined for the two sides: 160 M20 for the inner half and 180 M20 for the outer half (see Fig. 3).

Even if that study has proven the feasibility of the bolting in the given space, for the detailed design the gasket provider will have also to make recommendation on bolting effort and pattern according to the material choices. The first design that was proposed to ITER was approved by GARLOCK.

The design also proposes an other feature that reinforces its compliance from the machine life cycle point of view: the design was considered in order to have always the fallback option of going back to a welded configuration. The tokamak will have different phases in its life and the gasket should be particularly interesting during the first years of the whole plant commissioning. It is quite obvious that the gasket mounting flexibility will be useful in the first years and, if for safety or lifecycle reasons, it is necessary to use welds, the proposed hybrid design allows reconfiguring the sealing interface (see Fig. 4).


Fig. 4. Hybrid concept, fallback option



Fig. 5. Gasket flange maintenance equipment

Maintenance equipment and procedure

A previous study introduced the concept of the removable docking flange for the transfer cask interfacing [5]. This was in order to allow having enough room around the U shape to proceed to the welding/cutting of the lips. This component is also needed in the case of the gasket mounting, thus it is part of the maintenance equipment components.

The design phase of the gasket flange has shown that this would be a 1 ton weight equipment and around 300 M20 bolts are required to provide an efficient tightness. As the maintenance is foreseen hands-on in this area of the tokamak until the plug is installed, at least, a flange carrier will be necessary. A preliminary study of a semi-automated maintenance equipment was done including more functionalities as positioning and bolting the flange and prepare the sealing surfaces in order to allow automation of long tasks to minimize (suppress) operators exposure during flange mounting task (if applicable) (see Fig. 5). The design of the gasket flange maintenance equipment uses a modified cask envelope to carry a 6 actuated DOF positioning carriage for positioning the flange. The bolting tools are equipped with hardened cameras and embedded on 4 travelling modules able to follow the rectangular shape of the flange. This modular equipment tool could also be adapted in order to transport and mount the removable docking flange, or also positioning bolting/pretensioning tools in the plug flange area (see the port plug gripping system).

An installation duration estimation considering the use of those designed maintenance tools was performed following the As Low As Reasonably Achievable principles (ALARA – minimisation and rationalisation of the dose to workers). The total estimated time is between 12.5 and 22.5 h. The reason why the difference is so important is that some operations could take much more time considering normal maintenance, or disturbed if some operation could not run as fast as expected. This first assumption has shown that even if the number of bolt is rather important, the bolting task is not consuming so much time with regard to the whole sequence (2–3 h to bolt the 300 × M20).


Fig. 3. Gasket flange with bolts (300 × M20)


Port plugs gasket flange development

The design of the new flange has been approved by ITER IO, even if the sealing concept is still under discussion: a preferred option has to be selected and some R&D has to complete this integration study. The port duct interface will occur in several places of the tokamak, at least it will concern 14 equatorial ports, 18 upper ports and 5 divertor ports. With regard to the size and number of components that will be concerned, a prototype to qualify the sealing solution that will be retained could help to fix all the remaining uncertainties of this sealing concept.

Port plug remote handling and maintenance

Transfer equipment overview

A system of transfer casks is used to move in-vessel component and tools between the ITER machine and the Hot Cell [6] (see Fig. 6). This transfer equipment is mainly composed of a tight container (but not shielded) closed by double doors systems. It is designed to contain the port plug plus the associated handling tools. The cask is mounted on a pallet system that allows positioning accommodation. The whole equipment is supported by an air cushion system module remotely controlled.

Transfer cask system adaptation to port plug flange

The original design of the equatorial port interface connecting flange consisted of a mechanical flange using M36 bolts and 8 tangential keys. Since 2001, the design constantly evolved without formal approval, and now, the alternate bolted flange design is based on M52 bolts linking by friction the 2 flanges to be consistent with the current Vacuum Vessel configuration and electromagnetic loads requirements. This leads to an increase of the flange size that impacts onto the equator plug maintenance equipment (see Fig. 7).

An update of the transfer cask has been proposed to avoid any clash with the new port plug flange. As the flange is quite now occupying all space in the vertical direction, it is necessary to push all the functions on the sides of the cask (see Fig. 8). These modifications propositions have been designed with respect of the original geometry: no fundamental function or section have been radically modified. Nevertheless, this new layout will need to be further analysed in detail. As an additional remark, it is important to mention that, as it was previously the case, the tractor cannot go through the door. This has to be taken into account for the further steps of the study.


Fig. 6. Transfer cask system with port plug



Fig. 7. Equatorial port plug flange increase


Fig. 8. TCS update to avoid clashes


Gripping system

Several options were investigated in the port to address vacuum sealing and mechanical attachment aspects, but handling features and procedures for plug assembly were never fully integrated in the studies. The present study tries to address the assembly and maintenance processes. The plug gripping system is designed with the aim to use the mechanical bolting area of the equatorial flange for the following two main reasons: this is designed to afford the load and this interface is common to all different port plugs.

The main principle is to simplify as much as possible the remote handled phase of the maintenance operation and to integrate the gripping slots in the flange bolting area. In the new flange design, the number of bolts (54 M52) was calculated in order to allow to this mechanical connection to withstand the disruption efforts. During maintenance, only the static load must be considered. In that case, very few bolts are required to withstand the load of the plug (6 M52). In order to increase the level of safety of the maintenance process, a preparation phase could consist on replacing a few of the pretensioned M52 bolts by “maintenance bolts” that would support the port plug load temporary and would be unbolted by the port plug mover bolting tools. Then, all remaining pretensioned M52 bots could be removed. This preliminary operation could be made hands-on or by specific automated tool (e.g. based on the gasket flange maintenance equipment). The main benefits of that scheme philosophy is that the number of bolts to be operated by the remote handling system is significantly reduced (roughly divided by 10). An other advantage is that all the pretensioned bolts that could be damaged due to their severe conditions of use will be removed before the cask intervention. That could prevent from undesired scenarios where seized bolts could be problematic to remove remotely with the cask docked on the vacuum vessel.

The plug gripping and bolting method is based on an ITER concept developed by Framatome ANP in a previous study [7]. The original design has been modified to suit the integration of the new gripping system that avoids interferences inside the port plug flange area (see Fig. 9).

All components used in the design are commercial type. The bolting tools are pneumatic torque wrenches (not impact wrenches) that provides a smooth and continuous torque output suitable for this application.


Fig. 9. Gripping system implemented on tractor

Cantilever assisting system

The aim of the Cantilever Assisting System is to increase the safety of the plugs transfer operation between vacuum vessel and cask. The plug support system main design specifications are to provide a low friction support during maintenance and to allow the 20 mm nominal gap between plug and duct during operations. Starting form a study already done in the past [8], the proposed system is based on permanent wheels attached to the plug and a continuous rail between Transfer Cask System and VV.

Two pairs of wheels (equipped with needle bearing rollers) are integrated after the gravity centre (estimated at the geometric centre of the port plug). As the rollers are 20 mm beyond the port plug bottom surface and the flange is 125 mm large, it is necessary to crop 2 holes for the rails in the transfer cask. This lead to suppress two M52 studs but these can be located somewhere else by redistributing the bolting pattern.

Inserting wheels under the port plug will imply to have a suitable surface in regard to perform an efficient supporting guiding. Two tracks could be inserted in the port floor to achieve that, but, at least, it is necessary to have a planar surface for hard wheels. At the end of those tracks, at the final position, two cavities have to be machined in the port floor to allow the 20 mm clearance needed (see Fig. 10). The port plug is inserted using the wheels, but at the end of the introduction, the component is in a full cantilever mode on a 70 mm distance before reaching its final position.

To complete the cantilever assisting system, the transfer cask internal equipment needs to be modified to achieve the guidance continuity. This additional rail system provides guidance during all component translation and ensures a stiff support even in case of accidental event (hydraulic jack out of order, earthquake, etc.) (see Figs. 11 and 12). But it further complicates cask design and operation and it is required check if rails cut outs in the plug flange are acceptable.


Fig. 10. Cantilever assisting system in port rails



Fig. 11. Cantilever assisting system in cask rails


Fig. 12. Rail continuity


Conclusion

A complete preliminary design including gasket flange and integrating cantilever assisting system, gripping system with avoiding clashes has been produced during this equatorial port plugs interfaces study. The hybrid gasket flange and welded lips sealing design have been implemented by ITER IO at upper and equatorial level, but the sealing concept is still under discussion. Some improvements have been proposed for ensuring a better level of reliability of the transfer operation at the equatorial level. The next important step should be the development of prototypes to qualify the whole plug interfaces (mechanical attachment, sealing, maintenance procedure) in order to guarantee a successful integration in ITER.

The work carried out has again demonstrated that a common approach on both maintenance equipments (including procedure) and components design is fundamental to reach the required safety and reliability level of ITER.

Acknowledgements

This work, supported by the European Communities under the contract of Association between EURATOM/CEA, was carried out within the framework of the European Fusion Development Agreement [under EFDA Contract 06-1429]. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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