Work to remove all the used nuclear fuel from the storage pool at unit 3 of the Fukushima Daiichi NPP has been completed after two years of work, Tokyo Electric Power Co (Tepco) said on 28 February. This was the first of the storage pools to be emptied at the three damaged reactors which suffered core meltdowns following the March 2011 earthquake and tsunami. The reactor building at unit 3 had been severely damaged by a hydrogen explosion during the meltdown leaving the fuel pond exposed and covered with debris, including twisted steel beams.
The unit 3 storage pool is situated on an upper floor of the building.
Tepco had planned to move the fuel from the pool to a shared storage pool at the plant site. Tepco initially aimed to start removal of the fuel in late 2014 but repeatedly postponed the work due to high radiation levels in the reactor building. Work finally began in April 2019 after rubble and other debris were cleared away. A special crane with a robotic arm was used to lift the fuel from the pool. Operators had to work remotely from an operational centre 500 meters away because of high radiation readings inside the reactor building.
The work was delayed in the early stages by malfunctions in the equipment and the crane. The problem was further complicated by rubble and debris in the pool that had distorted the handles of some of the used fuel assemblies. Recently operators were working in shifts around the clock to complete the work. The last six of the 566 assemblies were transferred to the shared pool on 28 February, some three years after the government and Tepco announced an initial roadmap for the work in December 2011.
Tepco said removal of fuel from the storage pool comprised a series of preparatory tasks including: removal of rubble from, and decontamination of, the uppermost floors of the reactor building; construction of a fuel removal cover; and, the installation of a fuel handling machine. All these tasks had to be performed in a high-dose environment using remotely operated equipment. “This was the first time such methods were attempted and there were various problems that had to be overcome," Tepco said.
Tepco installed a dome-shaped cover over the reactor building to prevent the spread of radioactive materials before beginning fuel removalusing a remotely-operated crane. This 54-metre-tall structure includes a steel frame, filtered ventilation and an arched section at the top to accommodate the crane. It is not fixed to the reactor building, but supported on the ground on one side, and against the turbine building on the other.
Tepco said the "knowledge and lessons learned" from unit 3 "will be leveraged during the upcoming fuel removal from the unit 1 and unit 2 used fuel pools as we continue to steadily move forward with decommissioning whilst prioritising safety."
The removal of used fuel from the reactor 4 building was completed in late 2014. Unit 4 had been shut down for maintenance when the earthquake struck and so avoided a meltdown but the storage pool suffered serious structural damage. The pool contained a total of 1533 fuel assemblies (1331 used and 202 unused). The operation involved moving the assemblies from the pool into a transport cask, which was transported across the site and unloaded at the shared pool storage facility. This process was repeated 63 times.
As to units 1 and 2, a total of 1,000 used fuel assemblies remain in their storage pools. In unit 1 there is a lot of debris that needs removal and in unit 2 radiation levels are particularly high. Under the roadmap for decommissioning the complex compiled by Tepco and the government, work to remove the 615 fuel assemblies from the unit 2 reactor's building will start between fiscal 2024 and 2026 and the 392 assemblies from unit 1 between fiscal 2027 and 2028. As well as the used fuel, 800-900 tons of melted nuclear fuel remain inside the three damaged reactors.