Talking to reporters, Mr Sinha, along with Nuclear Power Corporation of India Chairman K C Purohit and Executive Director Nageswara Rao, said four operators were monitoring the "Boron dilution process", which would allow neutron concentration to go up and start nuclear fission, generating heat.
The heat would start operating the turbine to generate power, he said.
"The boron dilution process will be over by mid-night and the reactor will go critical," he said adding the entire process was well in compliance with the established principles.
Experts from NPCIL and Russian Kursatov Institute of Atomic energy and AERB observers were monitoring the process.
Mr Sinha and Mr Purohit said the process of criticality was started at 11:45 pm on Thursday, when the control rods were removed to allow boron dilution.
They said work on the "first phase of power generation in Unit-I is systematically proceeding."
The turbine would 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.
"Each and every phase of the work from construction of the plant to the commissioning of the plant is being monitored meticulously, and every process is well-documented. The scientists and workers are working with team spirit," Mr Purohit said.
Describing Unit-I as "technically radical", Mr Sinha and Mr Purohit said that reports regarding the plant had been filed by four different agencies including External Affairs Ministry, Tamil Nadu Pollution Control Board, AERB, and NPCIL to the Supreme Court.
NPCIL gave the order to go ahead with the process of criticality only after the report was filed.
Site Director R S Sundar said that neutron multiplication in the reactor core was started around noon on Saturday, in order to criticality at midnight today.
"It is the 21st nuclear power reactor and the country's first pressurised water reactor under the light water reactor category," he said.
"KKNPP-1 is the largest single power generation unit in the country," he added.
Noting that the power of the reactor would be increased to generate 1,000MW as per the procedures, he said, the first stage of the plant will be synchronised to the southern grid with about 400MW power within expected period of 30 to 45 days from criticality.
The first unit of KKNPP would add 1000 MW electricity to the southern grid and increase the contribution from nuclear power to 5,780MW.
KKNPP Units one and two, are VVER-1000 type reactors set up at Kudankulam in Tamil Nadu with technical cooperation with the Russian federation.
KKNPP reactors have enhanced safety features in line with the international standards, he said, adding the combination of multi-layered safety features ensures safety of the plant, public and environment.
The fuel loading in the first unit of the reactor was done in October 2012 and reactor was loaded with fuel assemblies containing about 80 tonne of Uranium Oxide, he said.
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.