Built by the central government at a cost of 35 billion yen, or some $320 million, the ice wall is intended to seal off the reactor buildings within a vast, rectangular-shaped barrier of man-made permafrost. If it becomes successfully operational as soon as this autumn, the frozen soil will act as a dam to block new groundwater from entering the buildings. It will also help stop leaks of radioactive water into the nearby Pacific Ocean, which have decreased significantly since the calamity but may be continuing.

However, the ice wall has also been widely criticized as an expensive and overly complex solution that may not even work. Such concerns re-emerged this month after the plant’s operator announced that a section that was switched on more than four months ago had yet to fully freeze. Some also warn that the wall, which is electrically powered, may prove as vulnerable to natural disasters as the plant itself, which lost the ability to cool its reactors after the 45-foot tsunami caused a blackout there.

The reactor buildings are vulnerable to an influx of groundwater because of how the operator, Tokyo Electric Power Co., or Tepco, built the plant in the 1960s, by cutting away a hillside to place it closer to the sea, so the plant could pump in water more easily. That also put the buildings in contact with a deep layer of permeable rock filled with water, mostly rain and melted snow from the nearby Abukuma Mountains, that flows to the Pacific.

The buildings managed to keep the water out until the accident on March 11, 2011. Either the natural disasters themselves, or the explosive meltdowns of three of the plant’s six reactors that followed, are believed to have cracked the buildings’ basements, allowing groundwater to pour in. Nearly 40,000 gallons of water a day keep flooding into the buildings.

Once inside, the water becomes highly radioactive, impeding efforts to eventually dismantle the plant. During the accident, the uranium fuel grew so hot that some of it is believed to have melted through the reactor’s steel floors and possibly into the basement underneath, though no one knows exactly where it lies. The continual flood of radioactive water has prevented engineers from searching for the fuel.