ROSATOM’s Atommash Branch in Volgodonsk has carried out the final heat treatment of the VVER-1000 Reactor Vessel for Kudankulam NPP Unit No 5. After welding the finishing weld and carrying out inspections, the Reactor Vessel weighing 320 tons was moved to a gas furnace and six thermocouples were installed. The equipment was in the furnace for four days at a temperature of 650 degrees Celsius for 8-10 hours. Specialists recorded thermocouple readings for four days. This is necessary to ensure a continuous process of thermal tempering of the item. Heat treatment of the Reactor Vessel is necessary to relieve the stress of welds and obtain the required mechanical properties of the metal.Afterwards, the item was moved to machining for the manufacture of 54 threaded holes for the studs of the main joint for the Reactor Vessel.Reactor is an item of the first safety class, it is a vertical cylindrical vessel with an elliptical bottom. The active zone and internals are located inside the vessel. From above, the Reactor is hermetically sealed by a Top Head with drives of mechanisms and units for regulating and protecting reactors and nozzles for outputting cables of in-reactor control sensors installed on it.Kudankulam NPP situated in the state of Tamil Nadu in the south of India is supposed to include 6 Power Units with VVER-1000-type reactors with an installed capacity of 6000 MW. The first phase consisting of Units No 1 and No 2 was included in India's national electricity grid in 2013 and 2016, respectively. Currently, the construction of Power Units Nos 3, 4, 5 & 6 is underway, which are the second and third phases of Kudankulam NPP.