Explained | The nuclear challenges that await Japan after Fukushima water release

Twelve years after the Fukushima nuclear disaster, Japan has started to release treated radioactive water into the sea, a key step in the process of decommissioning the stricken plant, but much tougher tasks lie ahead, such as molten fuel removal.

Here are the challenges facing the government and plant operator Tokyo Electric Power Co (Tepco) as they try to draw a line by the middle of the century under the world's worst nuclear accident since Chornobyl.

Molten fuel remove

Tepco has described the effort to remove highly radioactive fuel debris from reactor cores as an "unprecedented and difficult challenge never attempted anywhere in the world".

Trial-based retrieval at the No.2 reactor, the first at the plant to go through such a step, has been delayed twice from an initially scheduled date of 2021, and is now set for a six-month period starting in October.

At Three Mile Island (TMI), the U.S. nuclear plant in Pennsylvania that partly melted down in 1979 after a failure, fuel debris was kept under water during retrieval work, providing a shield against radiation.

That was the worst nuclear plant accident before the 1986 Chornobyl tragedy in Ukraine, then part of the Soviet Union.

Japan and Tepco plan to remove molten fuel while it is exposed to air because it is difficult to fill the badly damaged reactor cores with water.

But that will also make it hard to protect workers and retrieval gear from strong radiation.

The Fukushima plant suffered triple meltdowns, compared to the single fuel core meltdown at Three Mile Island, which means the debris retrieval operation will be much larger and more complicated this time around.

The retrieval will be done by a remotely controlled, 22-metre-long (72-foot) robot arm. The initial stage aims to extract only a few grammes of fuel debris, although the total molten fuel at the plant is estimated to be 880 metric tonnes.

Radioactive soil

The 2011 accident spewed radiation into the air, which eventually contaminated the soil. Part of that tainted soil is stored at an interim site more than four times as big as New York's Central Park.

But the law requires the soil stored at the interim site, located next to the tsunami-wrecked power plant, to be moved out of Fukushima within 30 years from when it began operating in 2015.

More than a quarter of that interval has elapsed with no clear sign the government is nearer to securing permanent storage, though the environment ministry says the earliest the search for specific locations will start is 2025.

Ballooning costs

In 2016, the government doubled to 21.5 trillion yen ($148.60 billion) its estimate of the costs of responding to the Fukushima disaster, including compensation, decommissioning and decontamination efforts.

About 12.1 trillion yen had been spent on such activities by March 2022, Japan's audit panel, which reviews government expenditures, has said.

That represents an expenditure of more than half of the government's estimate, even before really tough tasks such as fuel debris retrieval have begun, in turns raising concerns about cost overruns.

Tepco's continuing payouts to the victims hits its bottom line.

In 2019, a private think tank, the Japan Center for Economic Research, said compensation, decommissioning and decontamination costs were expected to reach 41 trillion yen in a scenario in which Fukushima water was diluted and discharged into the sea.