Unit 1 of the Barakah plant in the Al Dhafrah region of Abu Dhabi started producing heat on Saturday, the Emirates Nuclear Energy Corporation said in a statement.
The corporation said the construction of Unit 2 has finished recently, while the other two reactors are still being built -- even though the original schedule called for the plant to become operational by 2017.
Once completed, the four reactors, which are using South Korean technology, should produce 5.6 gigawatts of electricity and supply up to 25% of the UAE's electricity needs, the corporation said.
The new plant is part of the UAE's plan to become less reliant on oil and gas, the current source of the vast majority of its energy.
However, some experts have questioned the need for the nuclear power plant given the country's potential to develop solar energy and the tensions surrounding nuclear power in the Middle East.