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

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

Study of high frequency MHD modes from ECE radiometer in Tore Supra

Tore Supra ECE diagnostic has been recently upgraded to study MHD modes driven by energetic particles up to 400 kHz. To improve the measurement sensitivity, the ECE signals of the 32 channels radiometer were amplified just below the saturation limit and sources of noise were investigated in order to keep it as low as possible. With such an improvement, fast particle driven modes with frequencies up to 200 kHz were detected. A 4-channel correlation ECE system using YIG filters with tuneable frequency was also installed. It allows fine radial scans of MHD modes and correlation length measurements. For the two kinds of YIG filter in use, the minimum frequency separation between two ECE channels that could be achieved was established measuring the correlation coefficient between the respective radiation noises. Finally, by modelling the ECE radiometer taking into account the antenna radiation pattern and the vertical position of the ECE beam relative to the plasma centre we improved the data analysis tools, thus giving a better determination of the phase radial structure of ECE oscillations. The poloidal structure of MHD modes can then be identified from ECE data and, for off axis ECE lines of sight, the direction of the plasma rotation can also be determined. This method allows identifying the occurrence of an inverse cascade of electron fishbone modes ranging from m/n=4/4 to 1/1 (m and n are the poloidal and toroidal mode numbers, respectively) which appears in lower hybrid current drive plasmas

Study of high frequency MHD modes from ECE radiometer in Tore Supra

Tore Supra ECE diagnostic has been recently upgraded to study MHD modes driven by energetic particles up to 400 kHz. To improve the measurement sensitivity, the ECE signals of the 32 channels radiometer were amplified just below the saturation limit and sources of noise were investigated in order to keep it as low as possible. With such an improvement, fast particle driven modes with frequencies up to 200 kHz were detected. A 4-channel correlation ECE system using YIG filters with tuneable frequency was also installed. It allows fine radial scans of MHD modes and correlation length measurements. For the two kinds of YIG filter in use, the minimum frequency separation between two ECE channels that could be achieved was established measuring the correlation coefficient between the respective radiation noises. Finally, by modelling the ECE radiometer taking into account the antenna radiation pattern and the vertical position of the ECE beam relative to the plasma centre we improved the data analysis tools, thus giving a better determination of the phase radial structure of ECE oscillations. The poloidal structure of MHD modes can then be identified from ECE data and, for off axis ECE lines of sight, the direction of the plasma rotation can also be determined. This method allows identifying the occurrence of an inverse cascade of electron fishbone modes ranging from m/n=4/4 to 1/1 (m and n are the poloidal and toroidal mode numbers, respectively) which appears in lower hybrid current drive plasmas

Transfer Cask, a key System for Nuclear Maintenanceand Operational Availability of Iter

International audienceThe operational availability of Iter is strongly dependent on the duration of the maintenance of internal components as divertor cassettes, blanket modules or plugs (diagnostics, heating systems). These plasma facing components will be highly activated by neutrons and/or contaminated with tritium so will have to be moved using the Cask and Plug Remote Handling System (CPRHS) from/to Port Cell (PC) to/from Hot Cell (HC) to be repaired or replaced. The Casks being not shielded, human access will be forbidden in the trajectory zones and, in the event of a failure of CPRHS functions, all rescue operations will have to be remotely conducted.The Iter maintenance systems shall be able of processing, in a six-month maintenance period, several tasks in both parallel and series. The time to perform online CPRHS operations for 2 equatorial plugs maintenance has been estimated at half of the total time allocated to the long term maintenance activities. The remaining time for PC preparation and cleaning, refurbishment and testing of 2 plugs in HC might be too short especially as the duration of online operations has been estimated assuming 100% availability of CPRHS and without taking into account all the regulatory operations (control, test) to do because of the nuclear environment.Insofar as the potential failure modes of CPRHS functions could be severe in terms of time to repair, an inventory of function outages and an analysis of conventional and nuclear risks have been made at each stage of the diagnostic plug maintenance process using CPRHS. Finally, based on the probability of occurrence of abnormal events and the estimated time for potential rescue operations, the CPRHS availability (RAMI approach) and the duration for the transfer of a plug using a probabilistic approach (Monte Carlo analysis) have been assessed to verify that the time for maintaining two diagnostic plugs is in line with the Iter operational availability requirement