Scientists Model Tungsten Behavior in Thermonuclear Reactor

Specialists at NSU and the G.I. Budker Institute of Nuclear Physics SB RAS (INP SB RAS) have developed technology to conduct optical diagnostics of the tungsten surface that makes it possible for real-time observation of cracking from powerful pulsed heating. This method helps predict this metal’s reaction under a thermal load to the first wall of the vacuum chamber in an ITER thermonuclear reactor. The results of this research are published in the journal “Physica Scripta”.

This technique makes it possible to study the dynamics of the impulse action - heat shock and cracking of the material. Applying this technique scientists obtained fundamentally new data on the behavior of materials under extreme conditions. The delay between the action on tungsten and the reaction to it that was detected in these experiments, can influence ideas about the mechanisms for the brittle fracture of solids.

Traditional methods of analysis are applied after thermal exposure so they only provide an indirect idea of ​​what happened to the metal during pulsed heating. Scientists are forced to reconstruct the course of events in the wake of the devastation left on the surface of the material. This new method provides real-time diagnostics.

Alexander Vasiliev, NSU postgraduate student and Senior Laboratory Assistant INP SB RAS described their work,

At our experimental stand BETA (Beam of Electron for Material Test Applications) of the GOL-3 complex, we developed on-site optical diagnostics. We used a powerful electron beam to create a thermal shock because it gives relatively little background light that usually interferes with diagnostics. We monitored the state of the surface according to the structure of its thermal emission and the radiation scattering from the diagnostic laser on it. The combination of the pulse heating method and the diagnostics allows for real-time monitoring of the surface modification. We discovered that with uniform heating, hot areas with increased deformation can form.

According to Vasiliev, the experiments demonstrated that the cracking process is much more complicated than previously thought. It turns out that cracks may not appear during heat exposure but after an unexpectedly long delay.   "With a pulse duration of less than one thousandth of a second, when tungsten has time to heat several thousand degrees, we observed the formation of cracks a few seconds after the exposure, when the material has cooled to room temperature," the scientist noted.

Many laboratories around the world are engaged in research on the effects of high-power plasma flows on materials. The stability of the first wall materials of the vacuum chamber is one of the key problems in creating an energy source based on controlled thermonuclear fusion. The temperature of the plasma in the ITER tokamak is expected to be 150 million degrees. In a quiet state, it is held by a magnetic field and does not touch the surface. However, the reactor will presumably operate in a system in which uncontrolled releases of plasma are unavoidable.

Today, the most suitable material for a thermonuclear reactor is tungsten-metal because it is resistant to thermal and radiation loads. During pulsed heating, the material greatly expands and then contracts as it cools. Thermal shock is dangerous because it is very powerful and destroys the surface most intensively. This new technology provides scientists with the ability to predict the behavior of tungsten under such powerful loads. The parameters for the beam used in the experiments are similar to the presumed plasma pulses in the ITER reactor (duration - up to 300 microseconds, power - 10 GW / m2).