EU-DEMO vacuum vessel port closure plate fastening developments

The EU-DEMO project, which has been in the Concept Design phase since 2021, is already focusing on the integration of the main system components but is also analysing variants to simplify this process. One of these is the fastening and sealing of vacuum vessel closures, known as port closure plates, in order to simplify the execution of the work with the aid of remote maintenance. Recognising the key role of industry, the fusion programme roadmap foresees intensive industry involvement in specific tasks. To provide a starting point for such collaboration, the Fusion Technology Department (FTD) has developed an initial design for a mechanical fastening solution based on the design of ship bulkhead doors. This could be combined with a welded seal, also developed to an early stage in FTD, which could cope with all load cases without additional measures. This system of easy to use fastening and reliable welded seal could be a significant simplification compared to other established technical solutions to this function. This paper presents the above issues and describes the current state of development. It also outlines initial plans to involve experienced industrial partners in these tasks.


Introduction
The EU-DEMO development entered the Concept Design (CD) phase from 2021.This phase will continue to focus on the synthesis of variants for different components or technical solutions.These will be analysed in a subsequent step to find the most suitable concept.The given boundary and environmental conditions and other requirements must always be sufficiently investigated and taken into account.The integration of the various technical components is crucial.The CD phase aims to simplify this, also through organisational adjustments.
Technical research and development (R&D) will continue to be carried out in specific work packages (WPs), while integration work will increasingly be done by the system design managers in the Fusion Technology Department (FTD).This approach is intended to further facilitate close collaboration in the integration of the various systems [1].
The fusion programme roadmap [2] foresees that EU-DEMO will not only be designed by research organisations.Industry should gradually become a driving force in the development of fusion and should therefore be intensively involved through specific tasks at an early stage.Amongst other things, the investigation of possible solutions for the fastening and sealing of the vacuum vessel (VV) closures, the so-called port closure plates (PCP), was considered suitable for early industrial involvement.The planned approach to address these issues with industry has already been described [3].
Of course, PCPs are required for all openings of the VV.This document focuses on the current work and results and refers to the upper port (UP), which is mainly used for remote maintenance of the Breeding Blankets (BB).

EU-DEMO upper port configuration
In order to be fuel self-sufficient, EU-DEMO aims to produce the tritium required for its operation.For this purpose, the BBs are installed inside the VV and cover most of the surface of the inner walls of the VV.Due to neutron irradiation, the BB materials are subject to degradation and must be replaced within the lifetime of EU-DEMO.
In preparation for the removal of a BB segment, several tasks have to be performed.One of the essential early steps is the removal of the UP PCP [4].The sectional drawing (Fig. 1) gives an overview of the whole structure and the relevant parts.A comprehensive concept study has addressed the issues related to the transport of BBs between the VV and the Active Maintenance Facility (AMF) [5].Important findings are: i) Due to the activation of the in-vessel components (IVCs), only remote maintenance (RM) methods can be applied.Therefore, a BB transporter is required to carry the heavy load and to fulfil the complex kinematics to remove the BBs.ii) The transport of the removed IVCs to the AMF must take place in so-called casks (i.e.sealed containers) to avoid contamination of the environment.Although not explicitly considered in this study, it can be assumed that the same requirements apply to the PCPs for cleaning and preparation for reinstallation in the AMF.

Upper port closure plate design considerations
In principle, high vacuum sealing of the VV is not an uncommon problem and is addressed by many mounting and sealing concepts.A certain challenge arises from the following characteristics of the UP PCP: • Size and geometry.In order to maximise the available space between the magnetic coils, the geometry is an elongated trapezoid with the smallest possible radii at the corners.The approximate size is currently thought to be 6.3 metres by 3.9 metres.• Remote maintenance.To achieve high reliability, common sources of error must be eliminated as far as possible.This is a very broad subject and includes phenomena such as cold welding between identical material pairings, thread seizure [6], overstressing of components due to poor manufacturing or improper assembly.The latter is particularly critical when using RM tools without sufficient haptic feedback.Some of these issues are currently being addressed at ITER.It is already possible to carry out extensive investigations at an equatorial port on a test stand there.It is therefore expected that more experience and knowledge will soon be available [7].• Use of suitable materials.According to the current state of materials development, it is unlikely that elastomers or other plastics will be used for the gaskets of the VV.For this reason, it can be assumed that only metallic sealing solutions can be considered.

• Cask and contamination control door (CCD). In order to transport
IVCs in a sealed manner, the cask must be airtight.At the same time, the VV must also be sealed to prevent the exchange of contaminants with the environment.To achieve this, the cask is equipped with a double door, the two individual components of which seal the VV and the cask, respectively, after they are separated from each other.The external surfaces of the VV or cask remain free of contamination until and after separation.The design of the PCP must always be considered to be compatible with the requirements of this cask system [8].

Upper port closure plate fastening
To ensure a vacuum-tight seal of the VV, it is essential that the PCP is firmly connected to the VV port.It is essential that the chosen method of connection securely connects the two components, PCP and VV, at the maximum pressures occurring in the VV, even in the event of malfunction, and prevents transverse movement.In the case of the vertically orientated UP, the dead weight of the horizontally mounted PCP supports these requirements.The UP is therefore particularly suitable for the resumption of studies on fastening and sealing solutions.Initial investigations are currently being carried out to determine the requirements for a locking system for the UP's PCP.Essentially, the following criteria have been identified as important: • Minimising the number of bolted joints used.
• Avoidance of threads exposed to vacuum.
• Sufficient rigidity of the PCP construction in relation to the relevant pressure conditions.• Operability of the locks with the use of RM tools.
• Transport of the PCP with the BB transporter.Therefore, the PCP shall be equipped with a compatible docking mechanism for the BB gripper [9].• Provide sufficient preload to minimise any movement between components.
The use of RM tools can be greatly simplified by using an easy to use locking system.Solutions such as those used for bulkhead doors on submarines or for locking large vault doors could serve as a model.Here, a central operating element usually mechanically transmits a movement to a number of locking bolts.These then engage with correspondingly designed counterparts in the frame and can also generate a certain amount of contact pressure.
There are limitations to this design, such as the uneven generation of contact pressure and the use of the PCP surface by the drive unit.In general, a PCP without additional use for feedthroughs of any kind is preferable.However, this cannot be ruled out in the early stages of the CD, as many IVCs need to be supplied with diagnostic signals or coolant [10].
A solution based on the functional principle described above, but without central actuation, was developed as a first design point.In this solution, locking and contact pressure generation is performed by multiple wedge clamps distributed around the circumference of the PCP, as shown in Fig. 2.
The translational drive of the wedge-shaped locking bolt is generated by a scissors gear operated by a counter-rotating lead screw mechanism.By defining the lengths and angles of the levers, the thread diameter and pitch, and the angle of the wedge, it is possible to generate a suitable contact pressure for each lock.In principle, a pneumatic or hydraulic drive could also be used to replace the screw or even the entire gearbox.The drive unit itself is hermetically isolated from the locking pin by a bellows.Fig. 3 gives an impression of this concept.

Welded seal
The hermetically welded lip seal is one of the sealing methods envisaged for ITER [11].Numerous studies have been carried out to qualify it and it is already at a high Technology Readiness Level (TRL).However, to prevent damage from excessive deformation in the event of an in-vessel loss of coolant accident (IV-LOCA), the current design applies additional support clamps.These are likely to require additional RM activities during assembly and disassembly.
To gain a better understanding of the various requirements for a lip seal, the results of the extensive structural analysis of the ITER vacuum vessel [12] and the initial definition of the structural loading conditions in EU-DEMO [13] were used.Taking these results into account, an initial design was created to simplify remote handling by eliminating the support clamps (see Fig. 4).This design was then created in Ansys as a finite element (FE) model to analyse and better quantify the stresses and deformations caused by different loads on the lip seal.Further analysis is required, so only a small aspect is described here: Despite the intended use of dowel pins to align the PCP, it is expected that minor alignment errors may occur during assembly and adjustment due to the large dimensions of the UP PCP.A deviation from the target value of 1 mm has been assumed as an achievable value for positioning.Ideally, the lip seal should be as narrow as possible in order to minimise the amount of space taken up in the opening and to achieve greater stability.
The simulation process in Ansys was continuously optimised and then carried out as follows: The first step was to "simulate" the installation of the PCP itself.To reduce the size of the model, work was concentrated on a single corner.Contact areas between the surfaces of the joint and the lip seal were defined to model the deformation due to misalignment (see Fig. 5).The welding process was then approximated by progressively creating bonded joints between the seal edge and the mating surface.This simulates mechanical clamping of the lip seal to the VV port weld lip prior to actual welding.The material model of the gasket was kept linearly elastic in the model to avoid further complications with the already complex non-linear model.
Although it was possible to obtain some converged solutions, further work or a different approach is needed to explore the lip seal parameters: geometry, sheet thickness, etc.The stresses obtained are still high and may be due to the "welding" process implemented in the model itself, also the use of an elastic-plastic material model may be necessary.

Industry involvement
The involvement of industry partners for the further development of the PCP fastening and sealing started successfully in 2021.One of the first topics has been the investigation of the proven HELICOFLEX® metallic seal with regard to a reduced contact pressure required to guarantee the sealing effect.Now this industry participation is to be extended with the current primary areas of emphasis being: i) Qualification of interlocks as used for doors and gates in shipbuilding, large vault doors, or nuclear  applications.ii) Investigation of relevant issues that arise during the application of a welded seal.This includes, for example: Optimisation of the lip seal design for sufficient elasticity and overpressure resistance.Selection of an appropriate welding process [14], including consideration of the required welding positions.Investigating the opening and re-welding of the seal.Consider the effect of material degradation due to continuous neutron irradiation.iii) The use of leak testing and non-destructive testing (NDT), such as ultrasonic testing or radiography to qualify welds, is considered essential during assembly and operation to obtain approval for such a nuclear facility.This list is not exhaustive but gives a rough idea of the issues to be expected.

Summary
In order to provide a starting point for further industry involvement, developments have already been initiated in two directions within the Fusion Technology Department.i) A mechanical locking mechanism for the port closure plate, based on the operation of a ship's bulkhead door, was developed in a first draft.The application of this solution could significantly simplify the requirements for remote maintenance.By eliminating the bolts used in many other solutions as connecting elements, the risks associated with threads such as cold welding and galling can be significantly reduced.ii) An initial iteration of a welded lip seal design has been created with the aim of simplifying the assembly and resealing process.This design was modelled in Ansys to better understand all the issues involved.Numerous variations in sheet thickness, misalignment and assembly procedure were carried out.It was found that this is a very broad subject.Further investigation and optimisation of design and modelling is therefore required.Separately, it is planned to identify and involve industrial partners with expertise in these specific

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .
Fig. 1.Sectional drawing of the EU-DEMO with the relevant parts BB transporter, port closure plate, VV, Breeding Blankets.

Fig. 2 .
Fig. 2. View of the UP PCP with the locking mechanism around the circumference, the coupling, and the BB Gripper.

Fig. 3 .Fig. 4 .
Fig. 3. Possible design of a PCP locking mechanism.The scissors gear with counter rotating lead screw converts a rotation into a translatory motion.

Fig. 5 .
Fig. 5.Total displacement due to 1 mm radial misalignment between lip seal and upper port after lifted in position in Ansys.