Tokyo: The Fukushima nuclear plant has activated an underground "wall of ice" built around the reactors, to help contain spills of contaminated water into the Pacific Ocean.

The system comprises underground pipes that will freeze the ground and create a physical barrier around the four units that were damaged by the earthquake and tsunami of March 11, 2011, a spokesperson for the plant's operator, Tokyo Electric Power Company, or TEPCO, has confirmed on Thursday.

The installation of the system, the cost of which was estimated at $US307 million ($400 million), has taken two years and while TEPCO presented the project in 2013, Japan's Nuclear Regulation Authority, or NRA, had not approved it till now as it was analysing the safety of the project.

On Wednesday, the NRA gave the green light to the first phase of the "wall of ice", which flanks the Fukushima No.1 plant, overlooking the sea on the west.