18 Novembre 2013
November 18, 2013
Tokyo Electric Power Co. started a yearlong operation Monday to remove hundreds of nuclear fuel assemblies stored atop reactor 4 at the Fukushima No. 1 power plant to prevent the rods from causing another radiation catastrophe.
The building housing reactor 4 was hit by a hydrogen explosion in the early stages of the triple meltdown triggered by the March 11, 2011, Great East Japan Earthquake and giant tsunami it spawned.
The explosion blew off the roof, exposing the spent-fuel pool on the fifth floor to the sky and falling debris. Tepco has since built a steel-framed cover to protect it from the elements, but getting the fuel out of the damaged building will allow the utility to monitor and safeguard it more easily and safely.
The pool contains 1,533 fuel rod assemblies, 202 of which are fresh. The utility plans to remove the fresh ones first.
The 4.5-meter-long assemblies will be lifted out of their racks individually by using a fuel handling machine similar to a hoist. They will then placed into special transport casks waiting inside.
“Today we reached an important milestone in our work at the Fukushima No. 1 plant,” Tepco President Naomi Hirose said in a video message on the utility’s website.
Hirose said this would be one of the biggest tasks in the plant’s decades-long decommissioning process.
“The success of the extraction process therefore represents the beginning of a new and important chapter,” he said.
Tepco said it did some preparation work in the morning and began extracting the fuel at 3:18 p.m. The first one entered the transport cask at 3:57 p.m., and a total of four had been inserted by 6:45 p.m.
Hirose said that although the beleaguered utility has performed more than 1,000 fuel assembly removals in the past, it is well aware that special care must be taken this time.
One transport cask can hold 22 assemblies. Once full, it will be loaded onto a trailer to be moved to an undamaged building where a common pool will keep them secure.
Experts have warned that the removal task should be handled very carefully to avoid dropping and damaging the fuel, which might cause a release of radioactive materials, although it would be below the legal limit.
The pool still contains small amounts of debris from the explosion that might get entangled with the assemblies and racks, causing the hoist to jam.
Tepco says it has taken several precautions. For instance, the wires for the fuel handling machine and the crane for raising the transport casks have been doubled. And the hooks have been designed to hold the assemblies and casks in place even if the power is cut off.
While the fuel removal operation is seen as a milestone, it is expected to take 30 to 40 years to decommission the plant.
The spent-fuel pools in reactor buildings 1, 2 and 3 are also filled with assemblies, but removing them will take a few years.
It will take even longer to remove the molten fuel in the three reactors hit by meltdowns.