Toshio Sukegawa

Magnetic nondestructive evaluation of fatigue damage of ferromagnetic steels for nuclear fusion energy systems

An inspection method is proposed for structural metal components of fusion reactors based on magnetic properties measurement. This integrity inspection is very important because structural materials are fully damaged by neutron radiation, heat load, thermal and mechanical stresses. Present work shows that magnetic hysteresis is sensitive to microstructural changes caused by high cycle fatigue, even at a stress level below the yield stress. The fatigue-induced degradation in the microstructure mainly has its fingerprint on the upper part of the hysteresis curve, where the rotation of magnetization is predominant. The changes in magnetic properties are well represented by the decreasing remanent induction with fatigue cycling. A microscopic study was also performed using transmission electron microscopy (TEM) to investigate the microstructural changes in the fatigued specimen. The TEM images show an increase in dislocation density in the ferrite phase.

Study of the plasma-material interaction during simulated plasma disruptions

Abstract During disruptions the plasma facing material of a next-generation tokamak will be exposed to very high heat loads under which the material will show severe erosion. Present estimates which are based on simulation experiments by electron beam or laser predict a very limited disruption lifetime of the component due to excessive thermal erosion. In contrast to the laboratory experiments on which these estimates are based, during a tokamak disruption the eroded material enters the incident plasma and interacts with it. Due to this interaction the actual disruption erosion occurring in a tokamak may be considerably different. The aim of the present work is to investigate the interaction of thermally eroded carbon material with a dense hydrogen plasma by means of quantitative emission spectroscopy. Pyrolytic carbon specimens were subjected to plasma discharge pulses with a peak energy deposition of 375 J/cm2. The line emission of the eroded carbon species was observed and analyzed. The density of C+ ions was found to be strongly peaked in the plasma directly in front of the specimen surface whereas the density of C2+ ions increased monotously over 10–15 mm in front of the specimen. The results indicate that strong radiation is emitted by the eroded material and that a large part of the eroded carbon neutrals is ionized within 10 μs duration.

Effect of impurity content in stainless steel on resolidified surface condition after disruption load

Plasma disruptions in tokamaks may cause melting of the surface layer of first walls. If the layer resolidifies without any movement, the same layer repeatedly melts and protects the rest of the wall. A number of stainless steel samples were irradiated by a neutral beam injector. After irradiation tests with the same condition, some had wavy resolidification surfaces with dents, while some had smooth surfaces. In the former samples, the amplitude of the roughness increased by repetitive heat loads. The melt layer moved to the convex areas, which deepened the dents on each occasion. Sulphur was found to be one of the harmful elements which promotes the formation of the wavy surface. It is recommended to be kept below 0.005 wt% for obtaining a uniform resolidification surface. Even if samples have a low sulphur content, a high oxygen content also makes the surface rough. Other impurities have some effects, but they have not been clarified yet.

High heat load experiments for first wall materials

Abstract Plasma disruptions in tokamaks lead to high energy deposition for short times on the first wall. Melting and evaporation processes on the first wall decrease the structural integrity of this component because of reduction of thickness and initiation of cracks, leading to a shortening of the life time of the first wall. This experimental study is focussed on the melting, evaporation and resolidification behaviour dependence on energy density, using a neutral beam injector (beam energy: 120 keV) on a 10 MW test stand. Specimens tested were SUS 304, SUS 316. PCA (primary candidate alloy): Al, Cu. C and Mo-coated copper alloys, which are expected to be used for components. The pulse length was varied from 20 ms to 250 ms (1.6 MJ/m 2 to 20 MJ/m 2 ) at a constant heat flux of 80 MW/m. Metallographic analyses of the melted zone were carried out by SEM and XMA. Effects of one shot and multi shots on the melt layer thickness and evaporation loss were also studied.

Magnetic non-destructive evaluation of accumulated fatigue damage in ferromagnetic steels for nuclear plant component

We performed the measurement of magnetic properties of ferromagnetic steels that were degraded by tensile plastic deformation and cyclic loading, where lattice imperfections were produced in the steels. Magnetic hysteresis curves were changed depending on the load conditions. The changes in coercive force, residual magnetic flux density and permeability were obtained as functions of the magnitude of residual strain for the tensile tests and the number of loading cycles for the fatigue test. In the case of the tensile deformation, the coercive force increased and the residual magnetic flux density decreased with increasing the residual strains. In the case of fatigue damage accumulated by the cyclic loading, the residual magnetic flux density decreased with increasing the number of cyclic loading, while the coercive force remained constant. The changes in hysteresis curves were well consistent with the transmission electron microscopy (TEM) observation results of microstructural changes in the steels. We concluded that the magnetic property was enough sensitive to microstructural changes caused by mechanical deformation.

A simulated plasma disruption experiment using a magneto-plasma-dynamic arcjet

Abstract If a melt layer is expelled by a strong electromagnetic force from some plasma during a plasma disruption, the wall thickness is reduced there remarkably. Although this phenomenon is considered as a very important issue, it has not been studied so far because of its difficulty and complexity. In this study, the phenomenon was simulated using a magneto-plasmic dynamic (MPD) arcjet. The MPD arcjet was used as both a heat source and an electric current source. The current flowed radially in a stainless steel test piece installed in a transverse magnetic field. The circumferential electromagnetic force generated a swirl flow in the melt layer, causing a centrifugal force, which thinned the central part of the round region and formed a circular embankment on the fringe. A numerical code was developed which could calculate the melting, the evaporation and the melt layer movement by the centrifugal force and the beam pressure. The calculational results on the melting depth and the thickness reduction in the central part were compared with experiment.

Surface ripple of resolidified stainless steel after disruption load

The lifetime the first wall in fusion reactor depends on its durability against plasma disruption loads that may cause the melting and evaporation of the surface layer. Irradiation tests for stainless steels were made using a neutral beam injector. It was found that high sulfur and oxygen contents in the steels made the resolidification surfaces rough and wavy, while high calcium, aluminum and titanium contents were revealed to effectively offset the sulfur and oxygen. These impurity contents affect the temperature coefficient of surface tension in liquid iron. The surface temperature in the wider sections declines more slowly than in the thinner parts, therefore, the surface temperature is nonuniform. When the liquid iron has a positive temperature coefficient of surface tension, the surface tension in the denser parts becomes relatively greater than in the thinner parts. Consequently, the former expands and the thinner section contracts.

Cracks in the resolidification layer of stainless steel formed by simulated plasma disruption

Abstract An experimental study was made of the cracks in the resolidification layer of stainless formed by a simulated plasma disruption. A neutral beam injector was employed as heat source, by which a heat flux of about 77 MW/M2 was applied on the test pieces for durations of 40 to 110 ms. The crack density and the crack depth increased with increasing duration until the heat load reached around 5 MJ/m2. At higher energy depositions the crack density decreased with increasing duration, while the crack depth changed little. After repetititive pulsed irradiation, the crack depth increased while the crack density was scarcely affected. A calculation was made using a one-dimensional stress analysis code to determine the stress and strain distribution during the heating up and cooling down processes. The increasing rate of strain in the resolidification layer decreased with increasing duration, which is one reason for the low crack density under the heavy-load condition. The residual stress in the resolidification layer increased under load repetition, which is considered to be one of the reasons for the deep cracks after repetitive irradiation.

Thermal erosion behaviour of carbon and boron-carbon materials during simulated plasma disruptions

Results from disruption simulation experiments are described in which specimens were exposed to 3.8 MJ m−2 energy deposition during about 10 ms from a hydrogen plasma with a density of 3×1020 m−3 and a temperature of about 10 eV. The materials tested were pure graphite, graphite with 3 to 30% boron addition, B4C coat mix material, and plasma sprayed B4C coatings on graphite. The interaction of the thermally eroded material was measured by emission spectroscopy. The results show that the addition of boron to graphite does not significantly change the thermal erosion behaviour, or the formation of a vapour cloud in front of the specimen surface. B4C coat mix material melted and the eroded species show a much weaker interaction with the incident plasma than the graphite materials.

Distribution of ionized carbon during simulated plasma disruption using spectroscopy

In the present study, the behavior of ionized carbons during a simulated plasma disruption is investigated with the magneto-plasma-dynamic (MPD) arc jet. The MPD arc jet was improved to generate the high heat flux. The temporal and spatial distributions of the ionized carbons were measured by emission spectroscopy. The effect of heat load was evaluated using two pulses, i.e. the original and improved MPD arc jet. The C II and C III distributions were obtained for the two pulses. The emission intensities were strongly related to the evaporated flux of carbon.