The country's first 1,000 MW pressurised water reactor at Kundankulam in Tamil Nadu has attained criticality, setting the stage for power generation before end of August.
The milestone was achieved at 11.05 PM last night after the "Boron dilution process" allowed neutron concentration to go up and start nuclear fission, generating heat.
Experts from Nuclear Power Corporation of India Limited, Russian Kursatov Institute of Atomic energy and Atomic Energy Regulatory Board observers monitored the process.
Atomic Energy Commission Chairman S K Sinha had stated on Saturday that the process of criticality was started at 11:45 PM on Thursday, when the control rods were removed to allow boron dilution.
He said the neutron multiplication in the reactor core was started around noon on Saturday to attain criticality and that the boron dilution process would be over by midnight after which KNPP would attain criticality.
"It is the 21st nuclear power reactor and the country's first pressurised water reactor under the light water reactor category, he had said.
The turbine will start generating power meant for consumption in 40 days.
In the first stage, the unit would produce only 50 per cent of its capacity, followed by 70 per cent in the second stage and 90 per cent in the third stage.
Power from the reactor would be increased to generate 1,000 MW as per procedures and the first stage of the plant would be synchronised to the southern grid with about 400 MW within expected 30 to 45 day period from criticality, he said.
The first unit will add 1000 MW to the southern grid and increase the contribution from nuclear power to 5,780 MW.
KKNPP Units one and two are VVER-1000 type reactors set up with technical cooperation with Russia.
The project, dogged by delays since December last following protests by locals over safety concerns, was given the go-ahead by the Supreme Court on May 6.