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Crane, sensor systems are key to removing Fukushima fuel rods

Tim Hornyak | March 12, 2014
While Japan on Tuesday marked the third anniversary of the 2011 earthquake and tsunami that left over 18,000 dead or missing, a huge crane has been quietly moving fuel rods out of a pool at the stricken Fukushima Daiichi nuclear power plant.

While Japan on Tuesday marked the third anniversary of the 2011 earthquake and tsunami that left over 18,000 dead or missing, a huge crane has been quietly moving fuel rods out of a pool at the stricken Fukushima Daiichi nuclear power plant.

The procedure is fraught with radiation risk and could take over a year. But the heavy lifting equipment — consisting of a fuel-handling machine and the crane — has been operating slowly but smoothly so far, removing nearly one-third of the hazardous nuclear fuel rods from the reactor four building.

"We will treat the spent fuel in the common pool in a safe manner, using facilities that have the same structure, design and level of safety as our existing operation methods," a spokeswoman for operator Tokyo Electric Power (Tepco) said, referring to the utility's previous operations to transfer fuel rods at its nuclear power plants.

The assemblies of fuel rods are first lifted out of their underwater storage rack using the large, specially built Hitachi fuel handling machine that has a gripper device. Resembling a piece of construction equipment, the big green machine moves back and forth over the surface of the pool as workers control it in a cockpit.

Its air cylinder-powered gripper has an interlock system that automatically detects whether the gripper has a proper hold of the handle on the fuel-rod assembly. If it's not locked on, the operation stops.

Another sensor system measures the height of the handle. If it's not within guidelines, it may have been hit by debris, suggesting damage to the assembly. That would also trigger an alert and stop the procedure.

Meanwhile, weight sensors on the machine will call a halt if an unusual weight is detected, which may indicate the presence of debris from the explosion.

Moving at a speed of around 1 centimeter per second, the fuel assemblies are slowly transferred to a special protective cask that is also underwater. Workers monitor progress via underwater cameras.

Once the minibus-sized cask is filled with up to 22 assemblies, it is sealed and lifted out of the pool by the large overhead crane that's similar to a gantry crane, which lifts objects with a hoist and moves horizontally on a rail or other system. The cask is then decontaminated and moved to a nearby shaft for lowering onto the bed of a truck for transport to a secure "common pool" for rod storage nearby.

Tepco began to remove 1,533 fuel rod assemblies from the pool last November. At the time, some observers said that any slip-ups that would expose the fuel rods to air could have disastrous consequences.

Reactor four was shut down for inspection when the quake hit and its fuel was stored in the adjacent pool. The building housing the reactor exploded in a hydrogen blast and suffered heavy damage.

 

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