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[KFE NEWS VOL.26] ITER Blanket Shield Block successfully developed to protect a nuclear fusion reactor from ultra-high temperature plasma


Date of registration 2020-11-24

 - ITER Korea DA succeeded to manufacture the first product of a blank shield block

The first product of the International Thermonuclear Experimental Reactor (ITER) "Blanket Shield Block" has been successfully manufactured in Korea. It is designed to protect nuclear fusion reactor devices from plasma whose neutrons and more than 100 million °C temperature can damage the parts.

"The ITER Blanket Shield Block is a primary barrier to protect the ITER devices from plasma’s neutron damages and enormous energy set from ultra-high temperature over 100 million °C. Like a blanket, it protects all the main components such as vacuum vessel, magnet and so on" said Sa-Woong Kim, the team leader. In other words, it is the part to shield major ITER devices from plasma and neutrons from nuclear fusion, and is to be installed in puzzle-like connections, surrounding the inner wall of the vacuum vessel. Total 440 blanket shield blocks are going to be installed at ITER, procured by South Korea and China each to supply 220 units.

This achievement is remarkable in that the team has successfully resolved technical issues which they have encountered in every stage, including design, manufacture and testing. They have successfully met the high standards required by ITER and have established mass production system for the blanket shield blocks.



First product of ITER blanket shield block

ITER Korea Project Blanket Technology Team members including Byung-Il Park(senior engineer), Sikun Chung (senior engineer), Hee-Jin Shim(principal researcher), Sa-Woong Kim(principal researcher, team leader), starting from left

Mission 1. Finding the best material and design: Stainless Steel

ITER put forward strict standards in selecting and manage the block materials. This was the first to meet for the production of blanket shield blocks.

"Good quality is essential for shielding from neutrons and for cooling plasma. It must be a low-radiation material with a short half-life when exposed to neutrons. On top of that, drilling and welding must be made possible to serve as a part to prevent plasma heat," commented Hee-Jin Shim, the principal researcher of the team.

Accordingly, the researchers first of all worked on developing a special stainless steel (i.e. 316L(N)-IG) for the blanket shield block to withstand extreme environments, which succeeded in satisfying all the strict criteria of ITER. Next, the design was completed in such a complex form considering, on the one hand, the shape of plasma inside, and on the other hand, all coils and pipes outside to tightly adjoin vacuum vessel.

"We had to wait till finishing the design of ITER vacuum vessel to start designing blanket shield blocks in earnest. This is because we had to consider the blanket shield blocks’ location which is internally adjacent to plasma and externally to the vacuum vessel. The shape of blanket shield blocks was decided based on ITER’s idea of the most stable plasma form and by the location of coils and pipes inside the vacuum vessel,” said Dr. Kim.

Mission 2. Carving ‘Stainless Steel 316L(N)-IG’ as elaborately as if it were made of clay

In the production stage, a technique to delicately process difficult-to-machining materials of large size was developed to enable complicated shapes.

"First of all, we introduced a precision drilling equipment to make a path of cooling water into the stainless steel 316L(N)-IG which weighed around 5 to 6 tons. And the cover plate was welded precisely by welding craftsmen. Then, we finished machining the external into complicated shapes by a large precision-machining-device which can move both horizontally, vertically and tilting as well" said Sikun Chung, a senior engineer of the team. It was a challenge of carving very huge, sturdy surface with a very fine knife.

Drilling was another challenge to create a path for coolant. One shield block, whose size is 1m in height, 1.4m in width and 0.4m in thickness, requires as many as 220 drillings to create coolant passage. For the smooth flow of coolant, a hole of 1.4m length must be drilled through by one single drilling without mistake. If the way is drilled from both ends of the shield block towards the middle point, even a tiny miss of the point will lead to slow down the water flow on the spot, causing turbulence and drop in cooling performance. Therefore, it must penetrate 1.4 m with one single drilling, not to mention all the 220 coolant paths drilled inside a blanket shield block must meet each other accurately within 1mm tolerance.

Senior engineer Byung-Il Park explained the difficulty of drilling as follows:

"The 1.4 m should be drilled in one way within a 16-32mm diameter. Stainless chips from drilling may interfere with the way, and the drill may go astray due to the collision of power between the stainless steel and the drilling. We gathered initial mistakes and errors to come up with the best processing method. The ways in and out for coolant were finished by welding approximately 55 pieces of cover plate. Another trial and error was inevitable in welding as well to minimize distortion while welding total 160 meters.”

In addition to the initial trials and lessons, the partnership with domestic companies also enabled engineers to find the best processing method.

The blanket shield block passed a non-destructive test designed to fully inspect for all welds. By using the world's first developed ultra-high temperature helium leakage test facility, it also proved its performance by completing ITER-like operation test under high-temperature, high-vacuum conditions.

Dr. Kim recalled that the various performance tests of the blanket shield block were like a series of endeavors trying to find something invisible. In particular, regarding the high temperature helium leakage test, the lack of both such a device and such experience demanded countries participating in ITER to gather for workshops to find a breakthrough together. It is notable that ITER Korea DA successfully passed the stringent testing standards of ITER for the first time among ITER members, with some of the technologies developed during the tests waiting for their patents to be approved.

For more KFE NEWS: KFE NEWS Vol.26

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