Russia's integrated nuclear power player ROSATOM on Tuesday said one of the critical components for the upcoming 1,000 MW atomic power unit at Tamil Nadu's Kudankulam is getting ready in Russia.

ROSATOM said its group company has started welding the upper semi-vessel of VVER-1000 nuclear reactor for the fifth unit under construction at Kudankulam.

India's atomic power plant operator Nuclear Power Corporation of India Ltd (NPCIL) has two 1,000 MW plants (Units 1 and 2) at Kudankulam, while four more (Units 3, 4, 5 and 6) are under construction.

All the six units are built with Russian technology and equipment supplied by that country's integrated nuclear power operator, ROSATOM.

According to ROSATOM, after vertical assembly of two shells and flange, the 170-tonne upper semi-vessel was moved by crane to the turner and then to the welding station, where specialists started welding of two circumferential welds.

Welding is performed under heating at a temperature of 150-300 degrees for 22 days.

After the operation is completed, weld areas of the semi-vessel will be heated to 300 degrees.

Then the semi-vessel is transferred to a heat treatment furnace to obtain the required mechanical properties of the metal.

The heating and holding process takes place at a maximum temperature of 620 degrees for three days.

Specialists will carry out a number of necessary inspections of welded joints after heat treatment.

The semi-vessel will then be prepared for internal corrosion-resistant overlaying in the welded areas.

The reactor is a vertical cylindrical vessel with an elliptical bottom, inside of which the core and internals are located.

The top of the reactor is hermetically sealed with a cover with drives of mechanisms and controls and protection elements of reactors installed on it and nozzles for outputting cables of in-reactor control sensors.

In the upper part of the vessel, there are nozzles for supplying and removing of the coolant, as well as nozzles for emergency supply of the coolant when the circuit is depressurised.