Tokyo Electric Power (TEPCO) on Monday will start removing fuel from a storage pool at Japan's Fukushima nuclear plant, the most challenging operation since runaway reactors were brought under control two years ago.
Here are some key facts about the operation.
Q: What's the state of nuclear fuel at the site?
A: Reactors No. 1, 2 and 3 went into meltdown after their cooling systems were knocked out by the March 2011 tsunami. The temperature of the cores and spent fuel pools at all reactors is now stable and water is being used to keep them cool.
Reactor No. 4, whose outer building was damaged by fires and an explosion, has an empty core but a total of 1,533 fuel assemblies -- 1,331 spent fuel bundles and 202 unused ones -- are in its storage pool.
Q: Why does TEPCO have to take fuel from the pool?
A: According to the firm, it is safer to store all fuel in a shared pool that is reinforced against possible future earthquakes and tsunamis.
This will be the first post-tsunami attempt to move any fuel from one part of the plant to another.
Q: How will the operation work?
A: Under normal circumstances, nuclear plants shuffle fuel rods around fairly frequently, often using computer-controlled robotic arms that "know" exactly where each fuel assembly is.
But the damage to the building housing this pool, along with the presence in the pool of debris from explosions, is a wildcard that will complicate this operation considerably.
Workers in heavy protective equipment will use a remote control to direct a specially installed "grabber" into the pool, where it will latch onto fuel assemblies and drop them into a huge cask.
Each 4.5-metre (15-foot) fuel bundle needs to be kept completely submerged at all times to prevent it from heating up.
Once loaded with assemblies and water, the 91-tonne cask will be lifted out by a different crane and put onto a trailer. It will then be taken to another part of the complex and the process will be reversed.
Removing all 1,500-odd assemblies is expected to take until the end of 2014. Getting this done successfully will mean engineers can then start trying to extricate fuel from the reactors that went into meltdown.
But where the fuel pool operation is tricky and contains a few unknowns, removing fuel from the melted and misshapen cores of reactors 1, 2 and 3 will pose a whole new level of difficulty.
Q. What could go wrong?
A: Each rod contains uranium and a small amount of plutonium. If they are exposed to the air, for example if they are dropped by the grabber, they would start to heat up, a process that, left unchecked, could lead to a self-sustaining nuclear reaction - known as "criticality".
TEPCO says a single assembly should not reach criticality and the grabber will not carry more than one at a time.
Assemblies exposed to the air would give off so much radiation that it would be difficult for a worker to get near enough to fix it.
Sceptics say with so many unknowables in an operation that has never been attempted under these conditions, there is potential for a catastrophe.
Government modelling in the immediate aftermath of the Fukushima disaster, which was only subsequently made public, suggested that an uncontrolled nuclear conflagration at Fukushima could start a chain reaction in other nearby nuclear plants.
That worst-case scenario said a huge evacuation area could encompass a large part of greater Tokyo, a megalopolis with 35 million inhabitants.