RAMI analysis for the ITER In-Vessel Coils System
Introduction
The International Thermonuclear Experimental Reactor (ITER) project is the largest international collaboration in the scientific field ever set up. It forms the heart of a unique collaborative agreement to build the first experimental nuclear fusion device designed to prove the scientific and technological feasibility of sustained fusion power generation.
The In-Vessel Coils System (IVCS) has the crucial role in ITER of ensuring and maintaining stable plasma operations. Two aspects of plasma behavior are addressed by the deployment of In-Vessel Coils (IVC). The first concerns “Edge Localized Modes” (ELMs) and the second concerns “Vertical Stabilization” (VS). An ELM is a disruptive instability occurring in the edge region of a tokamak plasma. ELMs result in impulsive bursts of energy deposition on to the “Plasma Facing Components” (PFCs) causing a reduction in their lifetime through processes including erosion, thermal fatigue, and cracking. Furthermore, the elongated plasma of ITER is inherently unstable and requires feedback control to maintain vertical position.
ITER has an ambitious inherent availability target of 60% on a 20-year reference period for plasma production. To be able to achieve this objective, the potential technical risk that may affect the machine operation, was assessed by means of RAMI (Reliability, Availability, Maintainability and Inspectability) approach. The RAMI analysis aims to handle technical risks that have an impact on the availability of the ITER machine operation.
The RAMI analysis is a continuously iterative process that begins during the design & development phase of a system. It consists of four major steps: (1) the functional analysis is the first step of the RAMI analysis, (2) the following step is the identification and evaluation of failure mode following the FMECA (Failure Mode, Effects, and Criticality Analysis) model from a functional point of view. As foreseen for Final design Review level, to obtain results in terms of system reliability and availability the system is modelled through RBD analysis (3). The last step of the RAMI analysis concerns the identification of risk mitigation actions, the calculation of optimal lot of spare parts to be stored on-site and a proposal of component standardization (4).
The main issue of the analysis was to assess a proper failure behaviour of the components of the In-vessel Coils system. The ITER IVC design team provided design and operational data of ITER In-Vessel Coils. Failure rate data was collected from ITER Failure Rate Database and from publications on fusion components failure rate [1], [2], [3] and adjusted for varying environment and service condition through k corrective multiplicative factor [4] according to ITER RAMI approach, resulting in the main goal achievement of failure rate data of In-vessel Coils.
The in-vessel coils consist of one set of coils to mitigate the effect of Edge Localized Modes (ELMs) and another set to provide plasma vertical stabilization (VS). An overview of the In-vessel Coils system is provided in Fig. 1. The ELM coils consist of nine toroidal sectors of three (upper, mid-plane, and lower) 6-turn rectangular “picture frame coils”, total of 27 coils mounted to the vacuum vessel and positioned behind the blanket shield modules. The VS coils consist of one upper and one lower 4-turn solenoidal “ring” coil. The coils are mounted to the vacuum vessel and positioned behind the blanket shield modules. The in-vessel components consist of the coils and their associated feeders that carry current and coolant. The coils and feeders are located behind the blanket shield module. The feeders include ex-vessel terminations that interface with the DC bus bars and with the cooling water supply/return lines at ground potential. The IVCS consists of the following main components: ELM Coils, VS Coils, Feeders, Feedthroughs (FTs), Insulating breaks (IBs), process pipes connecting the FTs with the IBs and instrumentation [6].
According to ITER Stage Approach Configuration document that identifies the systems and subsystems role and define the basic Configuration needed at each phase of the Staged Approach, from First Plasma to DT phase, the ITER Construction and Operation phases are broken down into a number of stages, each comprising sequences of assembly work, integrated commissioning and machine operation. Four stages are foreseen: First Plasma (FP), Pre-Fusion Power Operation I (PFPO-I), Pre-Fusion Power Operation II (PFPO-II) and Fusion Power Operation (FPO). During the different stages, the IVC System will be operational as specified in Table 1.
RAMI requirements, according to ITER Project requirements document where the ITER project-level technical requirements that are needed to establish the suitability of the ITER design for its mission are contained, provide the applicable quantitative requirements referred to Inherent Availability objectives. It is established that the In-Vessel Coils system shall have a target inherent availability of 98,7% and 96,7% for PFPO and FPO stages, respectively.
Section snippets
Functional Breakdown
The first step of the RAMI analysis of a system is the functional breakdown, a top-down description of the system as a hierarchy of functions on multiple levels, from the main functions fulfilled by the system to the basic functions performed by its components.
The Functional Breakdown in Table 2 highlights the functions fulfilled by the IVC System and the basic functions performed by its components. There are three main function, ten intermediate functions and 16 basic functions. The main
FMECA
The identification and evaluation of failure modes has been made following FMECA model and approach [10], [11]. The FMECA is performed from a functional point of view and basing on a top-down approach. The failure modes, with their causes and effects, are identified for each basic function.
According to the ITER RAMI Analysis Program document, which purpose is the definition of a RAMI Analysis Program for ITER together with the description of the engineering processes and basic tools to be used,
Maintainability strategy
The maintainability and reliability assumptions for the IVC system consider the different environment and the different operating conditions characteristics of each phase according to the ITER “Staged approach” configuration.
The strategy defined for the maintenance of the In-Vessel Coils system has been defined in accordance with the In-Vessel coils system maintenance plan. It concerns the definition of preventive maintenance and inspection actions, the strategy for execution of corrective
RBD
The Reliability Block Diagrams (RBD) approach was used to estimate the reliability and availability of the system according to given operating conditions. The RBD is a graphical representation of how the components of a system are reliability-wise connected. The reliability and availability were performed using BlockSim software.
The operational times of ITER machine used for reliability and availability calculation is 18 months for reliability calculation for PFPO-1 stage, 21 months for
Conclusion
The RAMI approach was applied to the In-Vessel Coils System in the Final Design Review phase. The scope of the RAMI analysis was to perform RAMI analysis completed according to ITER RAMI Analysis Program, which includes a Functional analysis modeled in IDEF0 language, Reliability Block Diagrams and Failure Modes, Effects and Criticality Analysis. For the first step, a functional analysis was performed to understand the required functions of the ITER IVC system along with IDEF0 model to
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.
Acknowledgments
The present work was performed under the Vitrociset contract with ITER Organization for provision of specialized Engineering Services. A fruitful collaboration between the Big Science Team of Vitrociset and the ITER In-Vessel Coils Design Team led to the achievement of excellent results from the point of view of the methodology, resources and supervision of the activity. The authors would like to express their thanks to all the members of the ITER IVC and Vitrociset teams who supported the
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