The unit went critical at 2:51 pm in an operation, the jubiliant scientists said was like clockwork.

Atomic Energy Commission chairman P Chidambaram said the operation went on ``perfectly as planned''. The project director Vinod Kumar Sharma was also present.

The unit, expected to be synchronised with the grid in about four weeks, might start commercial power generation from November, the project scientists said.

According to the scientists, safety has been given priority and the motto was `Safety first, generation next'.

The Kaiga Project cost is estimated at Rs 2,896 crore. Work on the first unit is delayed due to the collapse of a portion of the inner containment dome.