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RACE has been developing a Blanket Transporter concept for the replacement of tritium breeding blankets within the EUROfusion DEMO tokamak. This operation will be challenging due to the scale of the blankets (˜10 m & 80 tonnes) and the current vessel design, which necessitates complex blanket kinematics within the vessel prior to a high vertical lift through the upper port. The blanket transporter is a key high technical risk system and break-down rescue and recovery has been highlighted as a significant unaddressed risk. This paper outlines the processes used to identify the key hazards resulting from failure scenarios and the improvements made to the concept design to mitigate them. Failure modes were identified via industrial expert input, failure and hazard analyses. Mitigation of the failure scenarios was achieved by providing secondary load paths in the main structure, redundancy in key components and the inclusion of additional features that allowed secondary rescue equipment to engage to assist rescue.

DEMO is a key part of the EU fusion roadmap, and the programme reaches the end of the pre-conceptual phase with a gate review in 2020. As part of the work to complete this phase, eight Key Design Integration Issues (KDII’s) have been identified as critical to the programme. Two of these KDII’s identified a requirement for a more detailed architectural study. Within KDII#3 (advanced divertor configurations) a double null configuration has been developed; in parallel KDII#4 (vertical segment architecture) identified a need to evaluate a split breeding blanket architecture. These two requirements were combined into a single study that assesses the in-vessel architecture for a double null DEMO featuring split breeding blankets. This paper presents the configurations developed and an evaluation of their feasibility with respect to integration of key in-vessel components (breeding blankets, divertors), port hardware (service pipes, shielding, vacuum pumping), and maintenance strategy. Furthermore, wider considerations such as the impact of the port orientation on the building architecture are also included. Finally, this paper will identify the risks and further work required to advance the double null configuration.

A study into the viability of a double null concept for DEMO has been undertaken as part of the work to address Key Design Integration Issues within the DEMO powerplant. The objective of this study was to evaluate the feasibility of a double null architecture featuring split blanket segments to enable a wider range of maintenance options. One of the main challenges found was the integration of the service pipes to provide connections to the cooling and tritium breeding systems for the divertor and breeding blanket. By understanding how these pipes might be integrated, the constraints on key details such as the length of the lower port could be understood. This allowed for progression of other areas of the design and investigations into the remote handling of these pipes during maintenance periods. The key requirements identified for the service pipe routing included determining the removal sequence for the blanket and divertor segments during maintenance; access for the cutting and welding tool at the pipe/blanket interface; pipes grouped as modules per component; and optimisation of port space for the vacuum pumps. Two proposals were considered to investigate their impact on the insertion location for the cutting and welding tool: pipes with large radii, and pipes with smaller radii. The former was found to be preferable due to the remote maintenance advantages, versus the limited space gains offered by the latter. This paper will describe the development of these requirements, both proposals, and discuss the challenges uncovered during the study in more detail.

For ITER or the future DEMO remote maintenance system (WPRM), several types of special tailored automatic manipulators are needed for vacuum vessel (VV) component transportation, inspection, and removal from and replacement to the VV wall. Due to the extreme work conditions such as big size, big payload and high environment temperature, both the static and dynamic error sources should be considered when design a controller. In this paper, the accuracy improvement issues regarding the static and dynamic error sources are taken into account. The paper proposed a plane constraint calibration method for static error calibration, whilst the dynamic errors are calibrated using a linear inverse dynamic error model. Differential Evolution global optimization method was used to generate optimal excitation trajectories, and to identify the static and dynamic parameter errors. A 6-DOF (degrees of freedom) commercial industrial serial robot was employed to carry out the corresponding simulation and experimental studies. This paper mainly focuses on the general case studies, the results found in this research would be extrapolated to support the ITER or the future DEMO remote handling systems.

As part of the conceptual design studies for a European DEMO, different tokamak geometries are being considered. As identified in the EFDA Roadmap to the realisation of Fusion Energy, 2013: “The integration of the Remote Maintenance system within the DEMO plant is an essential task within the DEMO CDA phase. This will involve establishing requirements, functions and interfaces with many other systems to ensure that plant availability and maintainability are considered from the outset.” In order to fulfil this integration requirement different geometries of DEMO have been assessed for their impact on remote maintenance (RM). The aspect ratio and number of TF coils have been identified as the pivotal variables, driving the tokamak geometry, with significant effects on remote maintenance both in terms of technical feasibility and speed of operation. Tokamak geometries with aspect ratios from 2.6 to 4.0 and 16 or 18 TF coils have been compared. The results of this evaluation show that higher aspect ratios and lower numbers of TF coils are beneficial to RM both in terms of technical feasibility and speed of operations. To deliver a maintainable DEMO, efforts must be made to maximize its aspect ratio and minimize the number of TF coils.

Due to the limited irradiation lifetime of the structural material used for in-vessel components in DEMO, it will be necessary to replace all breeding blankets within the given planned maintenance window in order to meet DEMO availability targets (Romanelli, 2012; Ellis, 2017) [ 1 , 2 ]. It is assumed that failure of in-vessel components cannot be excluded, whilst in-situ repair is unrealistic. Hence the replacement of individual breeder blankets must be technically feasible. As such, remote maintenance replacement of the breeding blankets is a mission critical operation. The baseline concept utilises vertical segment architecture to aid in the removal of the blankets (Iglesias, 2013) [ 3 ]. This choice impacts on the tokamak and plant architecture and also affects operational maintenance strategy. Within the EUROfusion PPPT program efforts have been made to perform cross work package investigations on eight Key Integration Issues needed to show the feasibility of the DEMO pre-concept design (Bachmann, 2017) [ 4 ]. Key Design Issue 4 is an investigation into the feasibility of the Vertical Segment Architecture blanket feasibility The present work documents the approach, current progress and developments within this investigation. This includes the strategy, identified risks and proposed solutions.

The EU fusion roadmap defines as one of its goals the development of a Demonstration Fusion Power Reactor (DEMO) to follow ITER. This device shall be tritium self-sufficient, produce net electricity, and acts as a component test facility to demonstrate fusion power plant relevant technologies, e.g. those of the breeding blanket. This article identifies the main DEMO requirements, introduces the rationales for the selected architecture of the DEMO tokamak, and describes how the configuration of the main tokamak components has been derived. This includes (i) the DEMO shielding concept, (ii) the segmentation of the in-vessel components and their maintenance strategy, (iii) an overview of the vessel and in-vessel component technologies, and (iv) a description of the integrated design of the breeding blanket.

In Europe (EU), in the frame of the EUROfusion consortium activities, four Breeding Blanket (BB) concepts are being developed with the aim of fulfilling the performances required by a near-term fusion power demonstration plant (DEMO) in terms of tritium self-sufficiency and electricity production. The four blanket options cover a wide range of technological possibilities, as water and helium are considered as possible coolants and solid ceramic breeder in combination with beryllium and PbLi as tritium breeder and neutron multipliers. The strategy for the BB selection and operation has to account for the challenging schedule of the EU DEMO, the ambitious operational requirements of the BBs and the still large development needed to have a BB qualified and licensed for operating in DEMO. In parallel to the continuous design efforts on the four blanket concepts, their integration in-vessel and ex-vessel has started. On the one hand it has become clear that despite the numerous systems to be integrated in-vessel the protection of the blanket first wall has to be addressed with highest priority. On the other hand the ex-vessel interfaces and the requirements imposed by the blanket to the primary heat transfer system and to the PbLi loop components have a considerable impact on the whole DEMO Plant layout. The aim of this paper is: to present the strategy for the DEMO BB down selection and BB operation in DEMO; to describe the status of the design evolution of the four EU BB concepts; to provide an overview of the challenges of the in-vessel and ex-vessel integration of the main systems interfacing the BBs and describe their design status.

The joint deformation contributes significantly to final end-effector displacement of a manipulator, especially when the manipulator is in the form a serial kinematics with long links. When the manipulators employed in the DEMO are under the heavy payload, the deformation of manipulator is inevitable and the magnitude is significant. In order to maneuver the large object through several via points with high positioning accuracy in the remote handling process of DEMO, the real-time computation of manipulator deformation has to be conducted, which is crucial to the DEMO adaptive position control system for the displacement compensation. Three computation-effective deformation modeling methods are proposed in this paper, which are parametric modeling method, nonparametric deterministic artificial neural network (ANN) modeling method, and nonparametric Bayesian ANN modeling method, respectively. A specific joint in a boom equipped in a telescopic articulated remote mast is taken as the study object in this paper. A nodal deformation in the joint is investigated by three modeling methods, respectively. The parametric deformation model is derived by using the structural mechanics, whose parameters are identified by using the Markov chain Monte Carlo (MCMC) method; the deformation model of deterministic ANN is trained by using the Levenberg–Marquardt method; and the deformation model of the Bayesian ANN is trained by using the MCMC method. The results show that the parametric model from the structural mechanics is linear and is incompetent in the deformation modeling when the nonlinearity presents; both the deterministic and Bayesian ANNs are capable of model the nodal deformation of joint. The performance of both the deterministic network and Bayesian network cannot rival for one another in the application scenario of this paper. The training of the Bayesian network can provide the criterions for estimation of possible ranges of the modeling outputs from its probabilistic distribution curves, and the judgment of proper size of network.

In the EU DEMO design (Romanelli, 2012; Federici et al., 2014), due to the large number of complex systems inside the tokamak vessel it is of vital importance to address the in-vessel integration at an early stage in the design process. In the EU DEMO design, after a first phase in which the different systems have been developed independently based on the defined baseline DEMO configuration, an effort has been made to define the interface requirements and to propose the strategies for the mechanical integration of the auxiliary heating and fuelling systems into the Vacuum Vessel and the Breeding Blanket. This work presents the options studied, the engineering solutions proposed, and the issues highlighted for the mechanical in-vessel integration of the DEMO fuelling lines, auxiliaries heating systems, and diagnostics. (C) 2017 The Author(s). Published by Elsevier B.V.

As part of the EUROfusion DEMO programme, RACE has been developing a set of concept designs for remote maintenance systems. The tritium breeding blankets will require periodic replacement via the upper vertical ports at the top of the vacuum vessel. This operation will be challenging due to the scale of the blankets (∼10 m tall, and presently assumed to weigh up to 80 t). Concepts have been developed for the blanket replacement process. Analysis of this activity has identified the blanket transporter as a key system, carrying a high technical risk. This will manoeuvre the blanket segments between the mounts and fixations within the vacuum vessel, and a position that allows vertical lifting through the upper port. This paper outlines a conceptual study to develop a feasible design for the blanket transporter. Requirements were obtained via functional analysis and kinematic analysis of the breeder blanket replacement allowing development of concepts for the main kinematic mechanism. Evaluation lead to down-selection of a concept for further development: A Hybrid Kinematic Mechanism with the first three of its degrees of freedom as a parallel mechanism. The proposed concept demonstrates potential for developing and integrating technologies within the blanket transporter to produce an engineering design that can validate the blanket replacement strategy and hence viability of the DEMO concept.

In the inside engineering of DEMO, the robotic machines and manipulators are envisaged to be widely employed, which often have to deal with the demanding working conditions. The construction of the dynamic model of the robot and manipulator can be incorporated into the control system design to gain the adaptive control performance. A method of constructing the dynamic model with the unknown parts is proposed. The method can identify the unknown parts of the dynamic system by incorporating a BP neural network that will eventually substitute the unknown parts in the system dynamics after the well training. A modified Levenberg-Marquardt algorithm is developed for the training of BP neural network, which can back propagate the errors between entire actual system and the constructed dynamic model into the training process of the neural network. An example of constructing the dynamic model for a general Stewart structure mechanism is presented, in which the unknown parts, as well the entire dynamics are successfully identified. The proposed method can be extrapolated to the dynamics modeling of the blanket transporter in DEMO design, as well as the other general manipulators.

A design integration study (KDII4) was conducted in the DEMO Pre-Concept Design Phase with the primary goal to develop a pre-concept feasibility design and concept of operation for the Vertical Blanket Segment Architecture. The primary goal was to develop two workable variants for removal of large in-vessel components. This, prompted by two earlier studies, highlighting the integration challenge finding a self-consistent DEMO design point (KDII) and secondly the complexity of operation required to ensure compatibly with the proposed port-based maintenance schemes. Initially, only a single null (SN) divertor configuration was considered, but due to additional identified issues and technical challenges centred on the extraction of the BB segments, a double null (DN) alternative variant has also been investigated. This was prompted by other KDII's studies, especially KDII1 (Design, performance and feasibility of wall protection limiters during plasma transients) and KDII3 (Advanced Magnetic Configurations). Alternative forms of vertical maintenance architecture have been investigated by breaking down the study into Ports, In-vessel Components, Operations and Safety. SN and DN configurations were studied with split and full blankets numbering seven variants in total. Each variant was studied in some detail, comparatively assessing each on its merits. However, no viable solution has been found for down-selection. Nevertheless, the question of vertical maintenance is now better understood. This paper will describe the findings of KDII4 in conjunction with the Remote Maintenance (RM) technology work package, which suggests that the current ‘reference’ design envelope available for RM is too constrained.