How dangerous is the Fukushima nuclear plant today?

This satellite image provided by Maxar Technologies shows Fukushima Daiichi nuclear power plant and burning reactor in Okuma, Fukushima prefecture, Japan on March 14, 2011. Japan will mark the 10th anniversary of the 2011 earthquake, tsunami and nuclear disaster, on Thursday, March 11, 2021. (Satellite image ©2021 Maxar Technologies via AP)

OKUMA, Japan — A decade ago, a massive tsunami crashed into the Fukushima Daiichi nuclear power plant. Three of its reactors melted down, leaving it looking like a bombed-out factory. Emergency workers risked their lives trying to keep one of history's worst nuclear crises from spiraling out of control.

Proper equipment has now replaced ragged plastic hoses held together with tape and an outdoor power switchboard infested by rats, which caused blackouts. Radiation levels have declined, allowing workers and visitors to wear regular clothes and surgical masks in most areas.

But deep inside the plant, danger still lurks. Officials don't know exactly how long the cleanup will take, whether it will be successful and what might become of the land where the plant sits.

Journalists from The Associated Press recently visited the plant to document progress in its cleanup on the 10th anniversary of the meltdowns and the challenges that lie ahead.

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WHAT HAPPENED 10 YEARS AGO?

After a magnitude 9.0 earthquake on March 11, 2011, a tsunami 17 metres (56 feet) high slammed into the coastal plant, destroying its power supply and cooling systems and causing meltdowns at reactors No. 1, 2 and 3.

The plant's three other reactors were offline and survived, though a fourth building, along with two of the three melted reactors, had hydrogen explosions, spewing massive radiation and causing long-term contamination in the area.

The plant's operator, Tokyo Electric Power Co., says the tsunami couldn't have been anticipated, but reports from government and independent investigations and recent court decisions described the disaster at the plant as human-made and a result of safety negligence, lax oversight by regulators and collusion.

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WHAT'S INSIDE THE MELTED REACTORS?

About 900 tons of melted nuclear fuel remain inside the three damaged reactors, and its removal is a daunting task that officials say will take 30-40 years. Critics say that's overly optimistic.

Separate efforts to remove spent fuel from cooling pools inside the reactor buildings were hampered by high radiation and debris and have been delayed for up to five years. If the plant's pools lose their cooling water in another major quake, exposed fuel rods could quickly overheat and cause an even worse meltdown.

The melted cores in Units 1, 2 and 3 mostly fell to the bottom of their primary containment vessels, some penetrating and mixing with the concrete foundation, making removal extremely difficult.

Remote-controlled robots with cameras have provided only a limited view of the melted fuel in areas still too dangerous for humans to go.

Plant chief Akira Ono says the inability to see what's happening inside the reactors means that details about the melted fuel are still largely unknown.

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ARE THERE UNDERGROUND LEAKS?

Since the disaster, contaminated cooling water has constantly escaped from the damaged primary containment vessels into the reactor building basements, where it mixes with groundwater that seeps in. The water is pumped up and treated. Part is recycled as cooling water, with the remainder stored in 1,000 huge tanks crowding the plant.

Early in the crisis, highly contaminated water that leaked from damaged basements and maintenance ditches escaped into the ocean, but the main leakage points have been closed, TEPCO says. Tons of contaminated sandbags filled with a material used to reduce cesium in the highly radioactive water early in the disaster remain in two basements.

Tiny amounts of radiation have continued leaking into the sea and elsewhere through underground passages, though the amount today is small and fish caught off the coast are safe to eat, scientists say.

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WHAT WILL HAPPEN TO THE STORED RADIOACTIVE WATER?

The 1,000 tanks filled with treated but still radioactive water tower over workers and visitors at the plant.

TEPCO says the tanks' 1.37 million ton storage capacity will be full in 2022. A government panel's recommendation that the water be released into the sea is facing fierce opposition from local residents, especially fishermen concerned about further damage to the area's reputation. A decision on that recommendation is pending.

TEPCO and government officials say tritium, which is not harmful in small amounts, cannot be removed from the water, but all other radionuclides selected for treatment can be reduced to safe levels for release. Even so, they remain in the water, some scientists say, raises a concern about their impact on marine life from a long-term release.

TEPCO has managed to cut the amount of contaminated water to one-third of what it used to be through a series of measures.

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WHAT'S IT LIKE TO VISIT THE PLANT?

The first thing visitors see is a stylish office building that holds the TEPCO decommissioning unit.

In another building, plant workers -- about 4,000 per day now -- go through automated security checkpoints and radiation measurements.

Because radiation levels have fallen significantly following decontamination, full protection gear is only needed in a few places in the plant, including in and around the melted reactor buildings.

On a recent visit, AP journalists donned partial protective gear to tour a low-radiation area: a helmet, double socks, cotton gloves, surgical masks, goggles and a vest with a personal dosimeter.

Full protection gear, which means hazmat coveralls, a full-face mask, a head cover, triple socks and double rubber gloves, was required at a shared storage pool where fuel relocation from the No. 3 reactor pool was recently completed.

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WHAT'S THE ENDGAME?

A decade after the accident, Japan doesn't yet have a plan to dispose of the highly radioactive melted fuel, debris and waste at the plant. Technology also isn't advanced enough yet to manage the waste by reducing its toxicity.

TEPCO says it needs to get rid of the water storage tanks to free up space at the plant so workers can build facilities that will be used to study and store melted fuel and other debris.

There are about 500,000 tons of solid radioactive waste, including contaminated debris and soil, sludge from water treatment, scrapped tanks and other waste.

It's unclear what the plant will look like when the work there is done. Local officials and residents say they expect the complex to one day be open space where they can walk freely. But there's no clear idea if or when that might happen.

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Tokyo correspondent Mari Yamaguchi has visited the Fukushima nuclear plant nine times, starting in 2012.