The 160-tonne structure with a diameter of 4.5 metres was assembled in a vertical position and later placed horizontally on a special stand for welding.
Rosatom said welding had been completed. The operation lasted 17 days and included automated welding at constant temperatures between 150 and 300 degrees Celsius.
The RPV is one of the largest reactor components in a nuclear power plant. It is a vessel containing the nuclear reactor coolant, core shroud, and the reactor core and fuel.
The Tianwan nuclear power station in Jiangsu province, east China, has six nuclear units in commercial operation and three under construction.
Russia supplied the VVER-1000 pressurised water reactor technology for Tianwan-1, -2, -3, and -4, while Tianwan-5 and -6 are a domestic CNP-1000 PWR units.
Construction of Tianwan-7 and -8 began in May 2021 and February 2022. Both units will be Russia-made VVER-1200 Generation III+ PWRs.