FUTABA, Japan — For the wrecked Fukushima Daiichi nuclear plant, managing the ever-growing volume of radioactive wastewater held in more than 1,000 tanks has been a safety risk and a burden since the meltdown in March 2011. Its release marks a milestone for the decommissioning, which is expected to take decades.

But it’s just the beginning of the challenges ahead, such as the removal of the fatally radioactive melted fuel debris that remains in the three damaged reactors, a daunting task if ever accomplished.

Here’s a look at what’s going on with the plant’s decommissioning:

WHAT HAPPENED AT FUKUSHIMA DAIICHI?

A magnitude 9.0 quake on March 11, 2011, triggered a massive tsunami that destroyed the plant’s power supply and cooling systems, causing three reactors to melt and spew large amounts of radiation. Highly contaminated cooling water applied to the damaged reactors has leaked continuously into building basements and mixed with groundwater. The water is collected and treated. Then, some is recycled as cooling water for melted fuel, while the rest is held in tanks that cover much of the plant.

WHY RELEASE THE WATER?

Fukushima Daiichi has struggled to handle the contaminated water since the 2011 disaster. The government and the plant operator, Tokyo Electric Power Company Holdings, say the tanks must be removed to make way for facilities needed to decommission the plant, such as storage space for melted fuel debris and other highly contaminated waste.

WILL THE WASTEWATER RELEASE PUSH DECOMMISSIONING FORWARD?

Not right away, because the water release is slow and the decommissioning is making little progress. TEPCO says it plans to release 31,200 tons of treated water by the end of March 2024, which would empty only 10 tanks out of 1,000 because of the continued production of wastewater at the plant.

The pace will later pick up, and about 1/3 of the tanks will be removed over the next 10 years, freeing up space for the plant’s decommissioning, said TEPCO executive Junichi Matsumoto, who is in charge of the treated water release. He says the water would be released gradually over the span of 30 years, but as long as the melted fuel stays in the reactors, it requires cooling water, which creates more wastewater.

Emptied tanks also need to be scrapped for storage. Highly radioactive sludge, a byproduct of filtering at the treatment machine, also is a concern.

WHAT CHALLENGES ARE AHEAD?

About 880 tons of fatally radioactive melted nuclear fuel remain inside the reactors. Robotic probes have provided some information but the status of the melted debris remains largely unknown.

Earlier this year, a remote-controlled underwater vehicle successfully collected a tiny sample from inside Unit 1’s reactor — only a spoonful of the melted fuel debris in the three reactors. That’s 10 times the amount of damaged fuel removed at the Three Mile Island cleanup following its 1979 partial core melt.

Trial removal of melted debris using a giant remote-controlled robotic arm will begin in Unit 2 later this year after a nearly two-year delay. Spent fuel removal from Unit 1 reactor’s cooling pool is set to start in 2027 after a 10-year delay. Once all the spent fuel is removed, the focus will turn in 2031 to taking melted debris out of the reactors. But debris removal methods for two other reactors have not been decided.

Matsumoto says “technical difficulty involving the decommissioning is much higher” than the water release and involves higher risks of exposures by plant workers to remove spent fuel or melted fuel.

“Measures to reduce radiation exposure risks by plant workers will be increasingly difficult,” Matsumoto said. “Reduction of exposure risks is the basis for achieving both Fukushima’s recovery and decommissioning.”

HOW BADLY WERE THE REACTORS DAMAGED?

Inside the worst-hit Unit 1, most of its reactor core melted and fell to the bottom of the primary containment chamber and possibly further into the concrete basement. A robotic probe sent inside the Unit 1 primary containment chamber found that its pedestal — the main supporting structure directly under its core — was extensively damaged.

Most of its thick concrete exterior was missing, exposing the internal steel reinforcement, and the nuclear regulators have requested TEPCO to make risk assessment.

CAN DECOMMISSIONING END BY 2051 AS PLANNED?

The government has stuck to its initial 30-to-40-year target for completing the decommissioning, without defining what that means.

An overly ambitious schedule could result in unnecessary radiation exposures for plant workers and excess environmental damage. Some experts say it would be impossible to remove all the melted fuel debris by 2051 and would take 50-100 years, if achieved at all.