Japan's government on 26 September approved a revision of its 30/40-year plan to decommission the Fukushima Daiichi nuclear power plant, delaying by three more years the removal of used fuel assemblies stored at two of the three reactors.

The plant operator, Tokyo Electric Power Co (Tepco) had originally planned to start moving 1573 used fuel assemblies from spent fuel storage pools by the end of fiscal-year 2020. However, under the new scheme fuel removal from the Fukushima Daiichi 1&2 pools has been deferred until 2023. Removal of the 566 fuel assemblies from the unit 3 spent fuel pool is set to start in 2018 and is expected to take two years. Installation of the equipment started at unit 3 in November 2016 and remains underway.

Including delays made in four earlier revisions, the fuel rod removal plan is now up to six years behind schedule. Naohiro Masuda, Tepco's decommissioning chief, said the delay results from previously unknown damage in the storage pool areas and the need for more decontamination.

Fuel debris removal
The removal of melted fuel debris from units 1, 2 and 3 has also been delayed. The method of extraction is due to be decided in fiscal year 2019, rather than the first half of fiscal 2018 as previously anticipated. However, despite the delay in finalising the extraction methods, the roadmap maintains that removal of fuel debris will start in 2021.

Fuel debris removal is seen as one of the most challenging parts of the decommissioning process. The situation inside the reactors needs to be properly understood before the debris can be removed. However, the high radiation dose inside the primary containment vessel is making it difficult to investigate. Robots have been sent inside the damaged reactors to assess the situation, but with mixed results. In July, images identified what is likely to be melted nuclear fuel debris at the bottom of Fukushima Daiichi unit 3 reactor.

Development of remote control technologies and cutting technologies to facilitate the safe removal of fueldebris are being developed in parallel with efforts to survey the situation inside the reactors.

Contaminated water
The new plan sets a goal of reducing the amount of underground water at the plant to address contaminated water buildup. Groundwater, which mixes with accumulated radioactive water generated in the process of cooling the damaged reactors, is to be cut to around 150t a day by 2020 from the current 200t. The plan does not offer a schedule for the disposal of processed water that still contains radioactive tritium.

Tepco president Tomoaki Kobayakawa said: "The Mid-and-Long-Term Roadmap is being revised to reflect conditions in the field that became evident in conjunction with our progress. The revised roadmap will remain the basis for our daily efforts to reduce risk while prioritising safety as we encounter new issues that must be addressed."

"These unprecedented initiatives will continue until decommissioning is complete."

Photo: Installation of the second part of the Fukushima Daiichi unit 3 spent fuel removal cover dome roof earlier this month (Credit: Tepco)