One slip-up in the latest step to decommission Japan's crippled Fukushima nuclear plant could trigger a "monumental" chain reaction, experts warn.

Within days, Fukushima nuclear plant operators will begin what is being described as the most dangerous phase of the decommissioning process so far.

In an operation never before attempted, the Tokyo Electric Power Company (TEPCO) will start removing 1,331 highly radioactive used fuel assemblies from a deep pool which sits high above the ground in a shattered reactor building.

The Fukushima nuclear plant's reactors were sent into meltdown by an earthquake and tsunami in 2011 in the world's worst nuclear accident since Chernobyl.

Experts around the world have warned ever since that the fuel pool is in a precarious state - vulnerable to collapsing in another big earthquake.

Yale University professor Charles Perrow wrote about the number 4 fuel pool this year in the Bulletin of Atomic Scientists.

He said one pool contains 10 times the amount of radioactive caesium present in the Chernobyl disaster and warned one slip-up with the removal could trigger a chain reaction.

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"This has me very scared," he told the ABC.

"Tokyo would have to be evacuated because [the] caesium and other poisons that are there will spread very rapidly.

"Even if the wind is blowing in the other way, it's going to be monumental."

TEPCO says it's prepared for delicate operation

It has taken TEPCO more than two-and-a-half years to clear away debris and get the number 4 reactor ready for the delicate operation.

TEPCO's Yoshimi Hitosugi insisted the company's engineers were prepared.

"We are going to transfer the fuel into containers while it's under water," Mr Hitosugi told the ABC through a translator.

"Then we'll use a crane to remove the containers and take them to a new pool."

Even Japan's nuclear watchdog is urging TEPCO to exercise the utmost caution.

Earlier this week the Nuclear Regulation Authority chief told TEPCO's president to proceed very carefully, warning that if TEPCO hits a problem, the risks will grow.

Mr Hitosugi said TEPCO engineers had reinforced the shattered building, propped up the fuel pool, and installed a new crane.

He said there was nothing to worry about.

"We believe it's not dangerous," Mr Hitosugi said.

"The reactor building is structurally sound.

"We don't believe there'll be any accident."

The operation is a test of TEPCO's technical prowess ahead of what will be an even more challenging task in the years to come - removing the piles of nuclear fuel at the bottom of reactors 1, 2, and 3.