Crane, sensor systems are key to removing Fukushima fuel rods
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.
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.