International Atomic Energy Agency (IAEA) Director General Yukiya Amano attends a news conference at the United Nations headquarters in Vienna.

Vienna - The reactors at Japan's crippled Fukushima atomic power plant are now “essentially stable”, the United Nations nuclear chief said on Monday, six months after the world's worst nuclear disaster in a quarter of a century.

Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), said the expectation was that a “cold shutdown” of all reactors would be achieved as planned.

“The plant operator and the Japanese authorities have been working hard to regain full control of the situation and have made steady progress in the past six months,” he told the 35-nation governing board of the Vienna-based UN agency.

“The situation at the site remained very serious for many months. The agency's assessment now is that the reactors are essentially stable,” he added.

Fuel rods in three reactors at the Japanese complex started melting down when power and cooling functions failed after it was hit by an earthquake and a huge tsunami, causing radiation leakage and forcing the evacuation of 80 000 people.

It was the world's worst nuclear accident since Chernobyl in 1986.

Last week, Tokyo Electric Power (Tepco) edged another step closer to its near-term goal of bringing the reactors at its Fukushima Daiichi plant to a state of cold shutdown by January, as the temperature at the second of three damaged units fell below boiling point.

Cold shutdown is when water used to cool nuclear fuel rods remains below 100 degrees Celsius, preventing the fuel from reheating. - Reuters