"At 23:05, the first reactor attained criticality and all the parametres are normal," a jubilant site director R S Sundar said.
"After a long time, the mood here is good," he added.
Top officials of the Indian nuclear establishment, including Atomic Energy Commission (AEC) Chairman R K Sinha, and chairman and managing director of Nuclear Power Corporation of India (NPCIL) K C Purohit, were at Kudankulam to see the first of the two Russian-made units attain criticality in a smooth manner.
NPCIL, India's atomic power plant operator, is setting up two 1,000 MW reactors with Russian technology and equipments at Kudankulam, around 650 km from Chennai.
The Rs 17,000 crore project, which generated widespread protests from the locals, has started generating heat and steam from the 163 uranium fuel bundles loaded in the reactor.
The reactor was loaded with fuel assemblies containing about 80 tonnes of uranium oxide.
After receiving clearance from the AERB, the process of criticality was started on Thursday.
According to officials, several low power tests will be carried out in order to verify the conformance of the reactor characteristics to design objectives.
If the reports are satisfactory, then the AERB will give its clearance for the next stage, which is phase-wise increase in reactor power level.
At the first stage, the plant will be synchronised with the southern grid when power generation touches 400 MW. That is expected to happen in 30-45 days. After necessary regulatory clearances the power generation will be increased gradually to 50 percent, 75 percent, 90 percent and finally 100 percent.
When that happens, the total installed nuclear power capacity in the country will go up to 5,780 MW.
KNPP is India's first pressurised water reactor belonging to the light water reactor category.
While the power from KNPP will be shared by the southern states, the lion's share will be for the home state Tamil Nadu, which is suffering from power deficit.
"Tamil Nadu's share of the 1,000 MW will be 463 MW. As and when the power comes to our grid, it will certainly ease the power shortage to some extent," a senior official at Tamil Nadu Generation and Distribution Corporation Ltd (TANGEDCO) said.
"The utility sources power from various central power generating units at varied rates but less than Rs 3 per unit whereas the power from KNPP will be over Rs 3 per unit," he added.
According to Mr Sinha, the total outlay for the third and fourth units would be Rs 40,000 crore.
The KNPP is an outcome of the inter-governmental agreement between India and the erstwhile Soviet Union in 1988. However, construction began only in 2001 but was delayed mainly due to non-sequential supplies of components from Russian vendors.
Fearing for their safety in the wake of the nuclear accident at Fukushima in Japan in 2011, villagers in the vicinity of the Kudankulam plant, under the banner of People's Movement Against Nuclear Energy (PMANE), have been opposing the project.
City-based environmental activist G Sundarrajan had filed a case in the Supreme Court demanding that the project be scrapped. The court dismissed the case in May and laid down 15 directions for the NPCIL, the AERB, the Union Environment and Forest Ministry, the Tamil Nadu government and the state pollution control board to follow.