Chennai, Sep 28 : The final heat treatment of the reactor vessel for the upcoming fifth 1,000 MW nuclear power unit at Tamil Nadu's Kudankulam has been completed, Russian integrated nuclear power major Rosatom said. According to Rosatom, after the finishing welding 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. The heat treatment of the reactor vessel is necessary to relieve the stress of welds and obtain the required mechanical properties of the metal, Rosatom said. 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. An item of the first safety class, the reactor 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. 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 Rosatom. /IANS