Materials effects and design implications of disruptions and off-normal events in ITER

Abstract

Damage to plasma-facing components (PFCs) and structural materials during abnormal plasma behavior such as hard disruptions, edge-localized modes (ELMS), and vertical displacement events (VDEs) is considered a serious life-limiting concern for these components. The PFCs in the International Thermonuclear Experimental Reactor (ITER), such as the divertor, limiter, and parts of the first wall, will be subjected to high energy deposition during these plasma instabilities. High erosion losses on material surfaces, high temperature rise in structural materials (particularly at the bonding interface), and high heat flux levels and possible burnout of the coolant tubes are critical constraints that severely limit component lifetime and therefore degrade reactor performance, safety, and economics. Recently developed computer models and simulation experiments are being used to evaluate various damage to PFCs during the abnormal events. The design implications of plasma-facing and nearby components are discussed, and recommendations are made to mitigate the effects of these events. . IntroductionDamage to plasma-facing components (PFCs) and structural materials due to plasma instabilities in magnetic fusion reactors is one of the most serious concerns for safe, successful, and reliable reactor operation. Plasma instabilities can take various forms such as hard disruptions, whch include both thermal and current quench, edge localized modes (ELMS), and vertical displacement events (VDEs). The intense energy deposition (10-200 MJ/m2) in these events over a short period (0.1-300 ms) may result in severe surface and bulk effects. These include high erosion losses of surface materials, high temperature rise in the structural materials, and high heat flux levels and possible burn-out of coolant tubes.Recently developed comprehensive models have been enhanced and used for comparison with results of several simulation experiments to analyze and evaluate the thermal response and the erosion damage of ITER PFCs resulting from different plasma instabilities [1,2].Models for material thermal evolution and phase change, magnetohydrodynamics models for the developed vapor cloud above the surface, and models to calculate resulting radiation, due to vapor heating, and its transport through the vapor cloud are dynamically coupled in a self-consistent approach to evaluate detailed time-dependent responses of plasma-facing materials (PFMs). The extent of the resulting damage to PFMs, structural materials, and coolant channels depends mainly on the total plasma energy deposited, deposition time, and thickness and the type of armor material.During the short disruption events ( z d 5. 1 ms), initially evaporated material may significantly reduce further PFM vaporization erosion. During longer plasma instabilities, however, such as VDEs ( T~ = 100-300 ms) no significant self-shielding is expected to occur and therefore serious erosion and melting can occur. In addition, hydrodynamic 2 instabilities and other forces will further erode mel...

Plasma-material interactions in current tokamaks and their implications for next step fusion reactors

“…The erosion due to VDEs in a device like ITER has been modelled by various authors, [138,361,866,867], whereas the results of analysis done to quantify the effects on PFCs resulting from runaway electrons can be found in Ref. [868].…”

“…Tungsten and carbon armours of similar thickness usually result in similar and higher copper surface temperature than that of beryllium armour of the same thickness. This is because most of the incident plasma energy is removed by the beryllium's higher surface vaporisation rate, which leaves little energy to be conducted through the structural material [361]. In order to reduce the temperature at the copper interface, thicker tiles would be required.…”

“…The vapour cloud, if well confined, is predicted to greatly reduce the net energy flux to the surface, resulting in a reduction in erosion by evaporation of one to two orders of magnitude [360,361]. The magnetic field lines are initially frozen into the surface of the liquid metal layer because of the liquid's high conductivity.…”

High-Z plasma facing components in fusion devices: boundary conditions and operational experiences

“…The erosion rate during off normal events will be controlled and dominated by the evolution and hydrodynamics of these melt layers and vapour clouds. Although much modelling work has been done on this issue [34,35], there is a lack of experimental confirmation since the present day devices do not have comparable power densities in off normal events. The simulation in other test devices is also difficult, due to the complex dynamics and its interplay with the strong magnetic field of a fusion device.…”

Impact of various plasma instabilities on reliability and performance of tokamak fusion devices

“…These large temperature increases can cause high thermal stresses, melting and detachment of surface coating material, and material fatigue and failure. In addition to these effects, the transport and redeposition of the eroded surface materials to various locations on PFC are of major concern for plasma contamination, safety (dust inventory hazard), and successful and prolonged plasma operation after instability events [5].…”