March 21, 2022, Büyükeceli, Mersin province, Turkey – Welding of the main circulation pipeline (MCP), a key assembly operation scheduled for 2022, has started in the reactor building of Akkuyu Nuclear Power Plant Unit 1. MCP connects the main equipment of the reactor plant - reactor, steam generators, and reactor coolant pumps, thus shaping the NPP primary circuit.
"Welding of the main circulation pipeline is one of the key stages in the power unit construction. While reactor vessel is often compared to the heart of a nuclear power plant, the main circulation pipeline is its main artery. During the NPP operation water at a temperature up to 330 degrees C continuously circulates through it. Only special pipes with 70 mm wall thickness can withstand such loads, therefore pipeline welding is a complex and high-tech operation, which is subject to the most stringent requirements. The completion of the welding operation will be the starting point for open reactor circulation tests. At this stage we will check the free flow of the pipelines, perform their post-installation cleaning, and confirm the readiness of the equipment for the next stage – reactor start-up," – said Akkuyu Nuclear JSC Chief Executive Officer Anastasia Zoteeva.
Installation of the main circulation pipeline includes not only assembly and welding of pipes, but also high-temperature treatment of joints followed by a special build-up welding on the inside of the pipeline. It provides corrosion resistance to the pipeline, increases durability of welded joints and ensures the pipeline's operating lifetime of at least sixty years, as stipulated by the project design. During the installation, specialists will have to weld 28 pipeline joints, assemble about 260 tons of pipes and steel structures, as well as to perform welded joints quality control.
Aleksey Mezhiritsky, Program Director of TİTAN2 IC İÇTAŞ İNŞAAT ANONİM ŞİRKETİ Joint Venture, the main contractor of Akkuyu NPP construction, commented on the thorough quality control of welding operations: "The welded joints of the MCP are subject to very strict requirements which ensure the highest level of reliability. Welding quality control is performed at several stages and includes independent testing methods according to accepted guidelines and standards. The quality control service accepts each element prepared for welding in order to make sure that the parts meet the established requirements. During the welding process specialists conduct a step-by-step survey of all operations performed; visual, dimensional, X-ray, dye penetrant and ultrasonic. Employees performing quality control have to meet the highest qualification requirements. Only professionals are allowed to perform such work after passing rigid certification tests."
The length of the MCP is 160 meters. The wall thickness of the special steel pipes is 7 centimetres. At the NPP operation stage the coolant – water with a temperature of up to 330 degrees Celsius at a pressure of 160 atm will be circulating in the MCP.
Heat energy generated in the reactor is transferred from the primary circuit to the secondary circuit through the steam generator heat-exchange tubes, and thus saturated steam is produced. Through the steam pipeline it goes to the steam turbine which drives the generator that generates electric current.