The 160-tonne structure with a diameter of 4.5 metres was assembled in a vertical position but then placed horizontally on a special stand in preparation for welding, a statement said.
Atomenergomash said welding will be automated and is expected to take 17 days at constant temperatures between 150 and 300 degrees Celsius in the zones of the welding seams.
In a nuclear power plant, the RPV is one of the largest reactor components, 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 five 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 is a domestic CNP-1000 PWR unit.
China has been building Tianwan-6 since 2016 using the CNP-1000 design and recently started construction of Tianwan-7 and -8, which will both be Russia-made VVER-1200 Generation III+ PWR units under agreements signed in 2018 and 2019.