More than three-and-a-half years after four nuclear reactors at Fukushima Daiichi on the east coast of Japan were devastated by an earthquake and tsunami, they still leak.

Radioactivity is spewing into the air, leaching into the soil and seeping into the sea.

The huge scale of the containment problems faced by the plants' operator, Tepco, are difficult to comprehend. Highly radioactive uranium fuel in three of the reactors melted down and wrecked the water-cooling systems essential for preventing overheating.

In order to prevent renewed chain reactions triggering another disaster, Tepco has to pour hundreds of tonnes of water over the fuel every day. But this becomes very radioactive, leaks out of the bottom and mixes with groundwater heading for the Pacific.

Water is collected, filtered to remove the worst of the contamination, then stored in over 1000 huge tanks and ponds on the site. Given the scale of the operations, there have been leaks.

Last year, a series of accidental discharges were reported. In one, it was revealed groundwater next to one of the reactors was contaminated.

One solution stretches the limits of technology. Engineers are trying to create a 1500-metre "ice wall" around the reactors in an attempt to reduce groundwater contamination. They are sinking pipes 30-40 metres into the ground, pumping liquid nitrogen through them to freeze groundwater into a contamination barrier.

Experts point out, however, that this may not work and, if it does, it will require large amounts of power over a long period to keep the water frozen.

At the fourth reactor, the fear has been that some of the 1500 hot and highly radioactive fuel rods stored there would lose their cooling water. If they were exposed to the air, they could start disgorging large clouds of dangerous radioactivity over Japan.

According to Yukiteru Naka, a nuclear engineer at Fukushima for 40 years, work to remove the fuel rods is "almost finished".

The Fukushima Daiichi reactors are also continually discharging radioactivity into the air, though at much lower levels than during the accident.

Naka estimates it will be at least another 30 years before all the reactors are rendered safe. One problem is an acute shortage of skilled clean-up workers.

The nightmare scenario is another earthquake. Water tanks and other facilities installed since the accident in lack foundations and support, Naka adds. "They are unstable if there's another earthquake," he says.

"Piping and tanks systems are usually built to high regulations, but for these no attention was paid to the law. It's only a temporary system."