Researchers at Russia’s NRNU MEPhI (National Research University – Moscow Power Engineering Institute) are investigating the accumulation of helium and deuterium in promising "smart" W-Cr-Y alloys for use in thermonuclear reactors. The results of the study were published in the Journal of Nuclear Materials. The interaction of plasma with the inner wall of a fusion reactor are a major obstacle to achieving fusion energy, explained Zori Arutyunyan, Candidate of Physical & Mathematical Sciences, and Junior Researcher at MEPhI’s Department of Plasma Physics.

“The fact is that the material of the inner wall of the reactor is exposed to intense flows of particles of isotopes of hydrogen, helium and neutrons. This leads to changes in the operational characteristics of the material and reduces the life of the reactor,” he said.

Currently tungsten is considered the best material for the inner walls in view of its physical properties, such as its high melting point and thermal conductivity, as well as low sputtering under the action of plasma. However, one of the potential problems of using pure tungsten for the inner reactor walls may be the formation of volatile tungsten oxide and its possible release into the environment in emergency situations.

“According to calculations made for the DEMO reactor [the planned successor to the International Thermonuclear Experimental Reactor under construction in southern France], in the event of an accident with loss of coolant and air penetration, the rate of sublimation of tungsten oxide into the atmosphere can reach several hundred kg/h from a surface area of ​​1000 quare metres,” Arutyunyan said. “This must be minimised. Therefore, it is necessary to consider other materials for the inner wall of a thermonuclear reactor, in addition to pure tungsten.”

Today, scientists around the world are developing new alloys based on tungsten with improved properties. Stable oxide-forming light elements such as chromium, titanium, yttrium, and silicon are added to the tungsten. During normal operation of a fusion reactor, the plasma sputters out the added light elements, leaving a nearly bare tungsten surface. In an accident, when the temperature of the inner wall rises, the elements added to the tungsten enrich the surface and, in the presence of oxygen, create their own oxide layer on the surface. This protects the tungsten from further oxidation and sublimation. These innovative alloys are called "smart" alloys.

NRNU MEPhI scientists are now looking into the accumulation of helium and deuterium in promising "smart" W-Cr-Y alloys, which will help determine their suitability for use in fusion reactors. “Using thermal desorption spectroscopy, we studied how deuterium accumulates in the W-11.4Cr-0.6Y and W-10Cr-0.5Y alloys obtained, respectively, by spark plasma sintering and hot isostatic pressing, as well as in pure tungsten, Arutyunyan explained. “The accumulation was carried out by introducing deuterium ions into the material at an ion-beam facility at temperatures expected on the inner wall of the DEMO reactor under stationary plasma operation conditions.”

The scientists also studied the accumulation of deuterium in alloy samples with the successive introduction of helium and deuterium ions. They were able to detect an increased accumulation of deuterium in W-Cr-Y alloys compared with pure tungsten. More deuterium accumulated in W-11.4Cr-0.6Y than in W-10Cr-0.5Y. The general dynamics of the influence of helium on the accumulation of deuterium in W-Cr-Y alloys did not differ much from pure tungsten.

“The results obtained can be used in assessing the accumulation of hydrogen isotopes, in particular, radioactive tritium, the accumulation of which in the reactor walls must be controlled for safety reasons. Also, these data can be used to determine the operating modes of thermonuclear installations using "smart" W-Cr-Y alloys for the material of the inner wall of the reactor," he concluded.

The study was supported by the Russian Science Foundation as part of the Priority 2030 programme.

Image courtesy of Rus Education