To prevent another powerful earthquake from toppling it, the building has been shored up with a reinforcing superstructure. Tepco said it could withstand a quake of similar power to the one of three years ago, which registered magnitude 9.
So far, Tepco has removed 462 fuel rod assemblies, just under one third of the total, to the common pool. Of the total, 1,331 assemblies contain spent fuel and the remainder contain new fuel rods.
The Fukushima plant released large amounts of radiation after the disaster knocked out its power and cooling systems, triggering a meltdown.
Radiation levels around the seaside station in northern Japan are still high, with some communities remaining uninhabitable. Tepco has deployed robots such as iRobot's PackBot to monitor the interiors of reactor buildings one and two, that are still too radioactive for workers.
Apart from removing the fuel rods from reactor four, critical tasks include keeping the other reactors cool, a process that has generated hundreds of thousands of tons of radioactive water. Dealing with the melted-down cores will be an especially difficult engineering challenge.
The fuel rods being transported are roughly 4 meters long and contain pellets of uranium fuel.
The gantry crane is designed to retain its hold on the cask, which can weigh as much as 90 tons, if there's a power failure. It also has double wiring to prevent the cask from falling if one wire were to snap.
That alarming scenario is addressed in Tepco's safety protocol document, which calls for an immediate evacuation of workers if a fuel rod assembly or the cask is dropped.
Meanwhile, the radiation around the pool near reactor four is another danger.
"The (Nuclear Regulatory Authority) conducted an independent survey of its operation and found high radiation levels measuring 81 microsieverts per hour above the fuel handling machine and 90 microsieverts per hour above the operation floor at Unit 4," Tatsujiro Suzuki, vice chairman of the Japan Atomic Energy Commission, wrote in Bulletin of the Atomic Scientists.
"The NRA believes that this high-level radiation may be caused by cobalt 60 contamination in the storage pool, which might have been released during fuel handling before the disaster of March 11, 2011."
Normal background radiation in major cities including Tokyo is usually less than 0.1 microsievert per hour.
If all goes smoothly, the fuel assemblies are scheduled to be transferred into the common pool by the end of 2014, and remain there for 10 to 20 years. Tepco has said the pool will be reinforced against tsunamis and earthquakes.
But the fuel removal is just one step in the overall effort to clean up Fukushima Daiichi. The total decommissioning and dismantling process for the plant is expected to take 30 to 40 years.
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