Operating a full tungsten actively cooled tokamak: overview of WEST first phase of operation

WEST is an MA class superconducting, actively cooled, full tungsten (W) tokamak, designed to operate in long pulses up to 1000 s. In support of ITER operation and DEMO conceptual activities, key missions of WEST are: (i) qualification of high heat flux plasma-facing components in integrating both technological and physics aspects in relevant heat and particle exhaust conditions, particularly for the tungsten monoblocks foreseen in ITER divertor; (ii) integrated steady-state operation at high confinement, with a focus on power exhaust issues. During the phase 1 of operation (2017–2020), a set of actively cooled ITER-grade plasma facing unit prototypes was integrated into the inertially cooled W coated startup lower divertor. Up to 8.8 MW of RF power has been coupled to the plasma and divertor heat flux of up to 6 MW m−2 were reached. Long pulse operation was started, using the upper actively cooled divertor, with a discharge of about 1 min achieved. This paper gives an overview of the results achieved in phase 1. Perspectives for phase 2, operating with the full capability of the device with the complete ITER-grade actively cooled lower divertor, are also described

WEST full tungsten operation with an ITER grade divertor

The mission of WEST (tungsten-W Environment in Steady-state Tokamak) is to explore long pulse operation in a full tungsten (W) environment for preparing next-step fusion devices (ITER and DEMO) with a focus on testing the ITER actively cooled W divertor in tokamak conditions. Following the successful completion of phase 1 (2016-2021), phase 2 started in December 2022 with the lower divertor made entirely of actively cooled ITER-grade tungsten mono-blocks. A boronization prior the first plasma attempt allowed for a smooth startup with the new divertor. Despite the reduced operating window due to tungsten, rapid progress has been made in long pulse operation, resulting in discharges with a pulse length of 100 s and an injected energy of around 300 MJ per discharge. Plasma startup studies were carried out with equatorial boron nitride limiters to compare them with tungsten limiters, while Ion Cyclotron Resonance Heating assisted startup was attempted. High fluence operation in attached regime, which was the main thrust of the first campaigns, already showed the progressive build up of deposits and appearance of dust, impacting the plasma operation as the plasma fluence increased. In total, the cumulated injected energy during the first campaigns reached 43 GJ and the cumulated plasma time exceeded 5 h. Demonstration of controlled X-Point Radiator regime is also reported, opening a promising route for investigating plasma exhaust and plasma-wall interaction issues in more detached regime. This paper summarises the lessons learned from the manufacturing and the first operation of the ITER-grade divertor, describing the progress achieved in optimising operation in a full W environment with a focus on long pulse operation and plasma wall interaction

Different Methods for Assessing System Failure Criticality in the RAMI Approach

International audienceIn the Reliability, Availability, Maintainability, and Inspectability (RAMI) engineering approach used in nuclear fusion research, criticality identifies the failure modes that have the greatest impact on the availability of the studied system. Criticality is expressed as the product of the occurrence level with the severity level of failure modes. The analytical calculation shows that this formulation is equivalent to their availability provided that the duty cycle of basic functions is introduced to adjust the occurrence and the scales of occurrence and severity are homogeneous. To consolidate the results obtained with a Reliability Block Diagram analysis, we performed a probabilistic study using an advanced Monte Carlo simulation code: the Primavera® Quantitative Schedule Risk Analysis. This method associates failure modes with conditional activities in a schedule and provides the density distribution of failures and tornado graphs to identify the highest criticality failures. Statistical tests were performed for two operational systems, and we showed that the criticality evaluated with the RAMI approach was in good agreement with the results of the other methods. Thus, in many cases, the analytical formulas can be used during the Failure Mode, Effects, and Criticality Analysis to quickly assess availability by using a spreadsheet

Dependability assessment of ITER cask et plug remote handling system during nuclear maintenance operations

International audienceSince the ITER Casks will be not shielded, human access will be forbidden in trajectory zones and, in the event of failure of Cask and Plug Remote Handling System (CPRHS) functions, rescue operations will have to be remotely conducted. As potential failure modes could be severe in terms of time to repair, an inventory of CPRHS function outages and a risk analysis have been made at each stage of a diagnostic port plug maintenance process. CPRHS availability has been calculated in the framework of a RAMI analysis and the resulting time to perform the CPRHS operations for the maintenance of 2 Equatorial Port Plugs (EPP) has been confirmed by using a probabilistic Monte Carlo approach

Exploratory risk analysis of ITER Cask et Plug Remote Handling System

International audienceAn exploratory risk analysis of ITER Cask et Plug Remote Handling System (CPRHS) has been performed while using an approach integrating all dimensions to be considered when the CPRHS is in various operational states with the associated possible loads.A Functional Breakdown Structure was developed from the 4 main functions to be fulfilled by the CPRHS to dock, to handle, to transport and to confine. During maintenance operations, various operating states and locations were defined. In regard to the safety function, to confine, specific configurations were considered to capture all relevant loading cases.A Failure Mode Effects et Criticality Analysis could then be made by identifying potential failures of all basic functions to be fulfilled by CPRHS during the maintenance of a Diagnostic Port Plug (PP) and quantifying them in terms of Criticality (C) defined as the product of the failure Occurrence (O) and Severity (S). The Severity rating scale was related to unavailability of the function due to both technical and safety issues.Specific analyses of docking operations in the different cells and traveling in different rooms while taking into account the safety constraints were made leading to recommend actions for mitigating the failures having the highest criticality levels. CPRHS availability and operations time were estimated while considering the impact of the failure modes and the benefit of mitigation actions.In order to statistically estimate the duration of repairing, the failure modes of CPRHS basic functions were introduced in the schedule Primavera Risk Analysis software which uses a probabilistic Monte Carlo approach. The failure modes were considered as task dependent activities with a duration equal to Mean Time To repair (MTTR) and an existence likelihood equal to the product et61548; x DC where et61548; is the failure rate and DC is the Duty Cycle of the failed basic function