Decommissioning operations at Tokyo Electric Power Co.'s Fukushima No. 1 nuclear power plant, which will take around 40 years to complete, are still in the earliest stages.
Over the past year, progress has been made in the decommissioning work, including removal of spent nuclear fuel from the storage pool at the No. 4 reactor.
However, the leakage of water contaminated with radioactive materials from storage tanks also developed into a serious problem. The government and TEPCO continue to work together to tackle such problems.
The issue regarding the contaminated water has two key facets. The first is that radioactive substances continue to flow into the sea. It appears that high-level radioactive contaminated water from tunnels extending from the No. 1 to No. 4 reactor building is flowing into the sea.
TEPCO is now proceeding with construction work to seal connections between the tunnels and buildings, in an effort to stop the water from leaking. After TEPCO stops the leakage, it will then start collecting the water beginning in June. The utility also will solidify sea floor soil in April to prevent radioactive substances in the soil from spreading.
The second facet is that groundwater continues to flow into the reactor buildings, and the volume of contaminated water keeps increasing.
Currently, TEPCO collects contaminated water from the building, treats it to remove radioactive cesium, then stores it in tanks. The amount of contaminated water in storage has reached about 350,000 tonnes.
There are now, all told, 1,000 storage tank — far too many to manually inspect. Last summer, tanks in the H4 sector leaked large quantities of contaminated water, as did tanks in the H6 sector last month.
The government and TEPCO have jointly worked out measures to reduce the flow of groundwater into the reactor buildings.
These include groundwater bypass wells to pump up groundwater and discharge it to the sea before it flows into the buildings; installing subsurface frozen soil dams surrounding the buildings to block the flow of groundwater into the buildings; and paving the ground to prevent rainwater from sinking into the soil.
Construction of some groundwater bypass wells has already been completed.
The government and TEPCO are working together to explain the situation to local governments concerned in an effort to achieve understanding among local residents concerning discharges of water into the ocean.
Technical feasibility studies for the proposed underground frozen soil dams are scheduled to start soon.
Total capacity of the contaminated water tanks will be increased to about 800,000 tonnes by the end of fiscal 2015. TEPCO said that stronger, welded-construction tanks have been under construction, instead of the currently used hand-assembled tanks.
TEPCO is also speeding up purification of the contaminated water using a new technology called an Advanced Liquid Processing System (ALPS), seeking to minimise adverse effects in the event of any future leaks.
At the Fukushima plant's No. 4 reactor building, the removal of spent nuclear fuel rods from a storage pool began last November.
At the time the nuclear crisis occurred in March 2011, the No. 4 reactor, still undergoing routine inspection, had not only spent fuel rods, but also those still in use housed temporarily in the storage pool. The total of 1,535 rods were in the pool — the largest number among the six reactors at the plant.
By Monday, 442 rods have been removed and transported to a safe storage facility away from the reactor building, which was destroyed by a hydrogen explosion. The rest of the removal work is scheduled to be completed within the year.
While the removal of spent fuel rods from the storage pools of the No. 1 to 3 reactors is scheduled to start sometime from fiscal 2015 to 2017, the radiation level inside these three reactors where the core meltdowns occurred remains high. Robots are therefore being used in decontamination work. TEPCO said, "We cannot remove [the rods] until we can thoroughly analyse the contamination situation."
The processes and challenges for rod removal differs among the three reactors. For the No. 2 reactor, where a hydrogen explosion has not occurred, the main concern is whether specialised equipment, including a crane to remove fuel rods, can be restored. At the Nos. 1 and 3 reactors, reactor buildings damaged by explosions will be resealed, reinforced and equipped with new cranes. A temporary shield used to prevent exposure to radioactive materials must also be removed at the No. 1 reactor.
The biggest hurdle facing the decommissioning plan is to collect the melted nuclear fuel inside Nos. 1 to 3 reactors. TEPCO and the government aim to begin removing the nuclear fuel between fiscal 2020 and 2024 after the containment vessels are filled with water to keep the fuel submerged, so as to reduce worker exposure to radiation.
To submerge the fuel, water leakage at the three reactors must be stopped. Major progress has been made since last November thanks to leakage points identified through robotic inspection.
Prof. Tadashi Narabayashi of Hokkaido University, specialising in nuclear reactor engineering, said: "The key to the decommissioning process is determining to what extent the damaged conditions of the containment vessels can be confirmed."
The Yomiuri Shimbun
Image: Yomiuri Shimbun