Tokyo Electric Power Co. (TEPCO) today began the delicate process of removing spent fuel from Unit 4 of the crippled Fukushima Daiichi nuclear plant.

The six-unit site has two reactor groups, comprising Units 1–4 and Units 5 and 6. Unit 4 is the only reactor of its group that did not suffer a meltdown following the earthquake and tsunami in March 2011. At the time, the reactor was shut down for maintenance and the fuel rods had been placed in reactor’s spent-fuel storage pool. However, the building suffered an explosion and fire four days after the disaster began, and the extent of damage to the fuel is not known for certain.

Because seawater was used for cooling during the initial emergency, there is concern that some of the fuel rods may be corroded. Also, portions of the building collapsed onto the pool after the explosion, and it is possible some of the rods were damaged. Examination since the disaster has suggested the assemblies have remained intact, however.

More than 1,500 fuel assemblies, each holding 60 to 80 four-meter-long fuel rods, are scheduled to be removed. TEPCO has been planning and preparing for the operation for months, building a specialized structure incorporating several cranes over the reactor building. During the process, the assemblies will be placed inside a cask while still inside the pool to keep them covered with water, and then lifted out in the cask to be transported to a more secure aboveground pool. The cask can hold 22 assemblies at a time, and each operation will take about seven to 10 days. TEPCO is hoping to complete the removal process by the end of next year.

The damaged fuel in Units 1–3, which all suffered meltdowns, will likely not be removed until 2020 at the earliest because of continuing high radiation levels and extensive damage to the reactor buildings.

Thomas W. Overton, JD, gas technology editor (@thomas_overton, @POWERmagazine)