Manufacturing and assembly of the plasma- and outer vessel of the cryostat for Wendelstein 7-X

Wendelstein 7-X is an advanced helical stellarator, which is presently under construction at the Greifswald branch of IPP. A set of 70 superconducting coils arranged in five modules provides a twisted shaped magnetic cage for the plasma and allows steady state operation. Operation of the magnet system at cryogenic temperatures requires a cryostat which provides thermal protection and gives access to the plasma. The main components of the cryostat are the plasma vessel, the outer vessel, the ports, and the thermal insulation. The German company, MAN Diesel & Turbo SE Deggendorf (former MAN DWE GmbH Deggendorf), is responsible for the manufacture and assembly of the plasma vessel, the outer vessel and the thermal insulation. This paper describes the manufacturing and assembly technology of the plasma and outer vessel of the cryostat for Wendelstein 7-X.     

Thermo-mechanical analysis of Wendelstein 7-X plasma facing components

The stellarator experiment Wendelstein 7-X (W7-X) is designed for stationary plasma operation (30 min). Plasma facing components (PFCs) such as the divertor targets, baffles, heat shields and wall panels are being installed in the plasma vessel (PV) in order to protect it and other in-vessel components. The different PFCs will be exposed to different magnitude of heat loads in the range of 100 kW/m²–10 MW/m² during plasma operation. An important issue concerning the design of these PFCs is the thermo-mechanical analysis to verify their suitability for the specified operation phases. A series of finite element (FE) simulations has been performed to achieve this goal. Previous studies focused on the test divertor unit (TDU) and high heat flux (HHF) target elements. The paper presents detailed FE thermo-mechanical analyses of a prototype HHF target module, baffles, heat shields and wall panels, as well as benchmarking against tests.     

Mechanical analysis of the joint between Wendelstein 7-X target elements and the divertor frame structure

The target elements of the actively cooled high heat flux (HHF) divertor of Wendelstein 7-X are made of CFC (carbon fiber-reinforced carbon composite) tiles bonded to a CuCrZr heat sink and are mounted onto a support frame. During operation, the power loading will result in the thermal expansion of the target elements. Their attachment to the support frame needs to provide, on the one hand, enough flexibility to allow some movement to release the induced thermal stresses and, on the other hand, to provide enough stiffness to avoid a misalignment of one target element relative to the others. This flexibility is realized by a spring element made of a stack of disc springs together with a sliding support at one of the two or three mounting points. Detailed finite element calculations have shown that the deformation of the heat sink leads to some non-axial deformation of the spring elements. A mechanical test was performed to validate the attachment design under cyclic loading and to measure the deformations typical of the expected deformation of the elements. The outcome of this study is the validation of the design selected for the attachment of the target elements, which survived experimentally the applied mechanical cycling which simulates the thermal cycling under operation.     

Analysis of the accident with the coolant discharge into the plasma vessel of the W7-X fusion experimental facility

Fusion is the energy production technology, which could potentially solve problems with growing energy demand of population in the future. Starting 2007, Lithuanian Energy Institute (LEI) is a member of European Fusion Development Agreement (EFDA) organization. LEI is cooperating with Max Planck Institute for Plasma Physics (IPP, Germany) in the frames of EFDA project by performing safety analysis of fusion device W7-X. Wendelstein 7-X (W7-X) is an experimental stellarator facility currently being built in Greifswald, Germany, which shall demonstrate that in the future energy could be produced in such type of fusion reactors. In this paper the safety analysis of 40 mm inner diameter coolant pipe rupture in cooling circuit and discharge of steam–water mixture through the leak into plasma vessel during the W7-X no-plasma “baking” operation mode is presented. For the analysis the model of W7-X cooling system (pumps, valves, pipes, hydro-accumulators, and heat exchangers) and plasma vessel was developed by employing system thermal-hydraulic state-of-the-art RELAP5 Mod3.3 code. This paper demonstrated that the developed RELAP5 model enables to analyze the processes in divertor cooling system and plasma vessel. The results of analysis demonstrated that the proposed burst disc, connecting the plasma vessel with venting system, opens and pressure inside plasma vessel does not exceed the limiting 1.1 × 10⁵ Pa absolute pressure. Thus, the plasma vessel remains intact after loss-of-coolant accident during no-plasma operation of Wendelstein 7-X experimental nuclear fusion facility.     

Lessons learned from the design and fabrication of the baffles and heat shields of Wendelstein 7-X

The baffles and heat shields of the wall protection of the Wendelstein 7-X stellarator are actively water cooled components based on the same technology. Fine grain graphite tiles are clamped onto a CuCrZr heat sink, which is vacuum brazed to a stainless steel tube. The baffles are part of the divertor and improve the divertor pumping efficiency. The heat shields protect the plasma vessel wall, water piping, cables and the integrated diagnostics. The 170 baffles with 25 variants and 162 heat shield modules with 85 variants comprise a total surface of 33 m² and 51 m², respectively. Design guidelines enabled as much as possible the standardization of the fabrication to allow for a more efficient work organization. Individual jigs have been manufactured for each variant in order to weld, bend and mill the different parts of the baffles and heat shields to the required 3D accuracy. At the end of the manufacturing process, each component has been checked and documented according to a detailed quality plan.     

Gate valve and shutter control system of the fusion experiment Wendelstein 7-X

The plasma vessel of the fusion experiment Wendelstein 7-X (W7-X) is a plasma vessel covering a plasma volume of about 30 m³. The vacuum conditions for plasma experiments inside the plasma vessel are supposed to be in a range of 1 × 10⁻⁸ mbar (ultra high vacuum conditions) after evacuation and conditioning. The 254 ports of the plasma vessel allow an external access to the inner space of the plasma vessel. Ports for heating and diagnostic systems are equipped with gate valves or with shutters. The vacuum gate valves are used as a controllable mechanical and a vacuum disconnection point between diagnostics and heating systems on the port side and the inner plasma vessel on the other side. The shutters are responsible for an optical and thermal protection for port windows or installed equipments inside the ports. After an overview of the main requirements for the control of the huge number of gate valves and shutters for the operational phases 1 and 2 of W7-X the design and realization of a centralized control system for controlling and observing all shutters and the majority of gate valves of the machine Wendelstein 7-X will be introduced and discussed.