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MSC Seminars
Overview and assessment of electrical arcing faults in ITER superconducting magnets system AND Development and application of integrated ANSYS model for ITER superconducting magnets fault scenarios
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Europe/Zurich
30/7-010 (CERN)
Description
Session 1
Title: Overview and assessment of electrical arcing faults in ITER superconducting magnets system
Speaker: Shanliang Zheng, Applied Physics and Technology Group, Culham Centre for Fusion Energy, UK Atomic Energy Authority (CCFE/UKAEA) Abingdon, Oxfordshire, UK
Abstract
The energy stored in superconducting magnets is generally designed to be safely discharged via dedicated resistances in the event of a loss of superconductivity. However, in extreme accidents when the magnetic energy is not discharged as expected, energy deposition localisation may cause melting and initiate electrical shorts. To ultimately consume all available energy, thermal and electrical interactions would further develop so as to result in damage not only to the superconductor but also to their surroundings. A model has been developed to examine the consequences of partial or full failure to discharge the magnets in case of a quench. The whole fault events propagation is rather complex. To predict the consequence of such fault events and to perform safety assessments, a methodology has been developed; it couples the interactions of the superconductor quench propagation model, electrical network simulations, thermal mechanical analyses and electrical arcs to deliver an integrated analysis. It is important to employ appropriate arc models covering the voltage-current characteristics as functions of certain environmental variables such as pressure to determine the localisation of heat deposition. Although no detailed models has been well established and validated for electrical arcs both within coils with solid insulation and also within the cryostat under conditions of Paschen breakdown, several candidate empirical arc models have been employed in the assessment for analysis as well as to serve as the basis for uncertainty studies.
In the first presentation we will present the background to the arc modelling, describing the different physical processes which have been modelled and the measurements which support the modelling.
Then in the second presentation, as an example, we will describe calculations of arc initiation and propagation which have been carried out for some of the ITER superconducting magnets in which a large energy inventory is present, nominally ~41GJ in TF coils and ~10GJ in PF/CS coils. The uncontrolled fault events evolvement has been captured and simulated in an integrated three-dimensional model within the ANSYS software environment. The damage of ITER TF, PF/CS coils has been estimated in terms of the temperature increases up to materials melting.
Session 2.
Title: Development and application of integrated ANSYS model for ITER superconducting magnets fault scenarios
Speaker: Simon McIntosh, Magnet Division, ITER Organisation, St Paul Lez Durance, France
Abstract
An integrated thermo-electrical model is developed within the ANSYS software environment to simulate faults scenarios in superconducting magnets, and aimed to predict and to assess the potential damage as consequences. The model integrates interactions of superconductor quench propagation, electrical current and voltage evolvement, thermal convection and conduction, electrical arc elements, and thermal and electrical heat deposition in a three-dimensional model.
This integrated model has been applied to simulate unmitigated fault events in ITER superconducting magnet coils, namely TF, PF and CS coils, to assess the consequences if the stored magnetic energy were not dissipated through a bank of resistors in extreme accidents despite the probability of such faults being extremely low. The model transfers energy stored within magnetic fields first to an electrical model and then to a thermal model. Energy is dissipated within the thermal model via Ohmic heating or electrical arcs. An iterative solution method is employed to ensure the conservation of energy flowing from magnetic to thermal fields. Damage to the fault coil is assessed via the calculation of molten material volume generated within the superconducting coils and the assessment of temperatures and voltages developed around the fault magnets exterior. The integrated ANSYS model is benchmarked with the MAGARC code developed by Idaho National Laboratory (INL, US) that previously supported the development of safety case for ITER superconducting magnets. A good agreement is attained.
In the application of the model to ITER superconducting magnets, the largest melt pool is produced by a fault occurring in the TF magnet due to its largest stored energy, ~41GJ. Assessment of the maximum temperature external the TF case indicates that this melt pool does not extend to the edge of the magnet. This pool is however considered mobile as the structural integrity of the TF case under high thermal loads cannot be assumed. Melt volumes for the PF and CS cases are much smaller and the stored energy is ~10GJ in PF/CS coils. The result of melt volume in PF/CS coils is not considered to pose a risk to the integrity of the vacuum vessel.