The first unit of Bangladesh’s Russian-backed Rooppur power plant, the country’s first nuclear reactor, has achieved criticality, sustaining a controlled nuclear chain reaction for the first time.

Criticality – where a self-sustaining chain reaction occurs – is a major step toward electricity generation and keeps the plant on track to be connected to the power grid by the end of November. The unit reached the milestone at 3:34 PM local time on Sunday, three days after receiving clearance from the Bangladesh Atomic Energy Regulatory Authority.

It followed commissioning delays from technical problems with safety valves. Russian specialists helped resolve the issues, and the reactor underwent further tests before the regulator authorized the controlled reaction.

The reactor successfully initiated nuclear fission, with operating parameters performing according to design, the Daily Star cited Bangladeshi Science and Technology Secretary Md. Anwar Hossain as saying. Further regulatory approvals will be required before electricity generation begins.

Russian state nuclear corporation Rosatom expects the unit to connect to Bangladesh’s grid by the end of the month, CEO Aleksey Likhachev said in Dhaka on Friday.

The Rooppur plant, located about 160 km from Dhaka, comprises two Russian-designed reactors with a combined capacity of 2,400 megawatts. Moscow is financing $11.9 billion in state loans for the project, which is estimated to cost up to $12.8 billion.

The project is expected to diversify Bangladesh’s electricity supplies as the country faces pressure from high energy import costs and disruptions linked to the Middle East conflict.