Development of High-performance Coating Technology inside a Pipe for an Advanced Blanket
In the future generation of fusion energy we will take out the kinetic energy of high-speed particles as heat, and utilizing that heat we will produce high-temperature high-pressure gas and generate electricity by turbines. The device that stops the high-speed particles and removes that heat to outside by means of a coolant is a “blanket” which encompasses the plasma. The term “blanket” derives from the plasma seeming to be wrapped in a blanket.
At the National Institute for Fusion Science we are conducting research which aims at developing an advanced blanket that circulates lithium alloy and lithium molten salt, which become liquids at high temperature, as coolant. The lithium alloy and the lithium molten salt, should they by chance leak outside, will harden and, being different from sodium, there will be no worry regarding explosion and fire. Inside this advanced blanket are numerous pipes, and coolants flow through these pipes. However, in a magnetic field confinement system like the Large Helical Device (LHD), because a strong magnetic field is present in order to confine the plasma, certain problems occur. Because these coolants have good conductivity, under a strong magnetic field, a force that hinders the flow of coolants is at work. The most effective approach to weakening that effect is affixing a thin insulation coating to the inner side of the metal pipe. At the National Institute for Fusion Science, we have solved this problem through joint research with universities and research institutes both in Japan and abroad by affixing erbium oxide (Er203) to the inner side of the pipe. Further, we have developed a method that enables homogeneous and broad coating of Er203 on metal surfaces. Here, we introduce the success in developing the Er203 coating through the chemical vapor deposition (CVD) method, which is a coating method that uses the chemical reaction of a gas that includes materials.
The Er203 layer composed by CVD continuously grows (epitaxial growth) on a metal base by combining the gaseous Er complex molecules and oxygen. Evaluating the physical properties of the generated coating, we achieved the assumed electrical insulation at the operating temperature of the blanket. Further, we achieved the result in which the flow of the lithium alloy is not blocked and the effect that the amount of permeating hydrogen declined from 1/100 to 1/1000. Furthermore, from the perspective of practical use, we attempted to homogeneously deposit Er203 layer in a broad area, and we succeeded in generating an Er203 coating on all areas of the inner walls of stainless steel pipes and elsewhere. In this way, we are demonstrating steady technological progress in physical properties evaluation and broad-area deposition technology for an advanced blanket.
At present, we are advancing energetically in joint research with universities and research institutes in Japan and abroad in developing a lamination-layer-style thick film process which aims at further enhancing the performance of Er203 coating and in evaluating physical properties that include durability and chemical reactions. Moreover, we are planning for integrated examinations of actual blankets in simulated actual environments, and seek to deepen further our knowledge of Er203 coating.