The Fukushima nuclear accident – 7 years after
Seven years after the nuclear accident in Fukushima on March 11, 2011, about 5,000 workers are employed on the plant site every day. At present, the operator TEPCO is making structural preparations for the removal of fuel assemblies from the fuel pools at Units 1 to 3. Another focus of the work is the exploration of the interior of the containments of Units 1 to 3. The knowledge gained by this will be used for the removal of the melted nuclear fuel.
Preparations for the removal of the fuel assemblies from the spent fuel pools
TEPCO plans to start removal of the 566 fuel assemblies from the fuel pool of Unit 3 in 2018 and has mounted a cylindrical cover on top of the reactor building in which the device for removing the fuel assemblies and a crane were installed (see Fig. 1).
According to current plans, TEPCO intends to start removal of the fuel assemblies from Units 1 and 2 in the fiscal year 2023. In preparation for it, pillars and beams of the reactor building cover were already renewed in Unit 1 and wind screens were installed. At Unit 2, the upper part of the reactor building is to be dismantled and a new structure to be built. From there, both the removal of the 615 fuel assemblies and later also the removal of the melted fuel is to take place. The fuel assemblies of Unit 4 have already been transferred to the storage facility in 2014.
Preparing the removal of the melted fuel
How the removal of nuclear fuel from Units 1 to 3 is to take place depends on how the fuel and the melt is distributed inside the plants. To gain more insight, TEPCO has been conducting inspections inside the containments for several years. Due to the high radiation values, remote-controlled robots are used.
For example, an investigation inside the containment of Unit 3 in July 2017 using a robot provided images from the control rod drive housing below the reactor pressure vessel. They show damaged and melted parts from the control rod drive housing and parts of the control rod drives in the pedestal area. Images of investigations which took place in Unit 2 in January 2018, show, among other things, the handle of a fuel assembly (see Fig. 2).
Another technique TEPCO uses to explore the interior of the reactor building is called muon tomography, or muon scan for short. Here, a kind of X-ray image is created from the inside of the reactor building by means of a technical process.
The most recent muon scan to date was performed in Unit 3 in 2017. Based on the images taken, TEPCO suspects a massive accumulation of highly contaminated material outside the reactor pressure vessel. Nevertheless, according to TEPCO, there is the possibility that melt has also remained inside the reactor pressure vessel. The results of the muon scans in Units 1 to 3 essentially confirm the computational results of the accident analyses (see Fig. 3). The results of the accident analyses conducted by TEPCO are consistent with those obtained by GRS.
According to TEPCO, a fuel removal plan for one of the units is to be developed by 2019. TEPCO has commissioned the International Research Institute for Nuclear Decommissioning (IRID) to develop and investigate possible scenarios, which is a merger of various Japanese organisations such as the Japan Atomic Energy Agency, manufacturers, TEPCO and other operators.
Handling contaminated water at the plant site
Another focus of work is the handling of contaminated water, which accumulates at the site and has to be treated. Depending on the amount of precipitation, 50 to 100 m³ of new contaminated water is produced each day, which is pumped out of the reactor buildings and cleaned and finally stored in tanks (see Fig. 4). Today, more than 1 million tonnes of water are stored at the site, of which around 10,000 tonnes are highly concentrated liquid waste.
Except for tritium, all radionuclides contained in the water can be removed using the cleaning systems. TEPCO, the Japanese regulator NRA, the government and the regional fisheries associations are currently discussing whether the tritiated water can be discharged into the sea. According to media reports in early 2018, the NRA advocates dumping of the water into the sea.
Due to the nuclide concentrations, measured for example in the port basin of the plant, TEPCO assumes that the measures taken at the site to protect seawater and groundwater will be effective. The measures include, inter alia, the underground ice wall, completed in August 2017, which encloses the reactor buildings and is to prevent the ingress of groundwater.
Radiological situation at the site
The local dose rates determined by TEPCO at the plant site and in the area of the reactor buildings have not changed significantly in the last few months (until November 2017).
The development of the radiation values also has an impact on occupational health and safety at the plant. In September 2017, the regulations for wearing protective clothing at the site could be adjusted again. Since then, workers only have to wear a dust mask and regular work clothes on 95 percent of the plant site. However, more stringent protective measures, such as wearing a full face mask and protective suit, still apply in the immediate vicinity of the reactors. In addition to the operator TEPCO, the Japanese Ministry of Health, Labour and Welfare (MHLW) also documents the radiation exposure of workers at the plant site.
Information on the radiological situation at the site and in the vicinity, on the sea and air monitoring data and on the levels of radioactive contaminants in foods and drinking water by the MHLW is regularly published by GRS in its monthly reports.