Atommash, an engineering subsidiary of state nuclear corporation Rosatom, said the 340-tonne component is being shipped 21,000 km from its facility in Volgodonsk via St Petersburg and the Suez Canal.
There are two Russia-procured VVER-1000 pressurised water reactor units under construction at Kudankulam.
Construction of Kudankulam-3 began in June 2017 and of Kudankulam-4 in October 2017. Two more units are planned for the site.