The plant, the first in the Arab world, had been due to go online in late 2017 but faced a number of delays that officials attributed to safety and regulatory requirements.
"The Nawah Energy Company has commenced loading the first nuclear fuel assemblies as the initial step in the process towards safely and gradually commencing operations and subsequent generation of clean electricity," Nawah said in a statement.
The statement added that Unit 1 will begin commercial operations after a "series of tests" leading to the start-up process.
During the process, the unit will be synchronised with the power grid and the first electricity produced.
"Our teams are trained, certified and ready to safely commence the testing and start-up processes for Unit 1," said Nawah CEO Mark Reddemann.
Abu Dhabi authorities said in January that the plant, on the Gulf coast west of the capital, would start operating within a few months.
The UAE has substantial oil and gas reserves, but with a power-hungry population of 10 million it has made huge investments in developing clean alternatives, including solar energy.
The plant is a regional first -- Saudi Arabia, the world's top oil exporter, has said it plans to build up to 16 nuclear reactors, but the project has yet to materialise.
It is being built by a consortium led by the Korea Electric Power Corporation at a cost of some USD 24.4 billion.
When fully operational, the four reactors have the capacity to generate 5,600 megawatts of electricity, around 25 percent of the nation's needs. The other three reactors are almost ready.