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Search for melted nuclear fuel at Fukushima plant's No. 2 reactor faces obstacles

Although nearly six years have passed since the nuclear disaster at the Fukushima No. 1 Nuclear Power Plant in 2011, the search for the melted nuclear fuel inside the plant continues.

The operators of the plant, Tokyo Electric Power Co. (TEPCO), deployed over 800 workers inside the No. 2 reactor at the No. 1 plant between December 2016 and February 2017 -- but so far, they have been unable to identify the location of the melted nuclear fuel.

TEPCO also plans to conduct studies inside the No. 1 and No. 3 reactors, but they are surely headed for a rough road as the search for the melted nuclear fuel continues to be extremely difficult. It is likely that struggles in that search will have a negative effect on the government and TEPCO's target of completing the Fukushima decommissioning work between 2041 and 2051.

Apart from humans, robots have also been involved in the search. In the case of the No. 2 reactor for example, robots have been used in the following way.

The mission to get a good look inside the No. 2 reactor containment vessel had four steps; first, workers would drill a hole measuring 11.5 centimeters in diameter into the containment vessel wall, allowing robots to enter the vessel; then workers would insert a pipe with a camera into the hole so that the situation inside the vessel could be observed; a cleaning robot would then be sent inside the vessel to clear away any sediment in the way for the next robot; and finally a self-propelled, scorpion-shaped robot would travel to the area directly below the nuclear reactor, in search of the melted fuel. However, a number of unexpected problems emerged along the way.

Heavy machinery giant IHI Corp.'s Keizo Imahori, 38, who oversaw the mechanical boring of the containment vessel in December 2016, explains that, "A number of unexpected dents were found on the floor of the nuclear reactor building." This was a surprising discovery for Imahori and his team. The presence of the dents meant that it would be difficult for machines to get sufficiently close to the necessary areas to drill a hole, which in turn has a detrimental effect on the entire search for melted nuclear fuel.

As an emergency measure, 1-meter by 1-meter iron sheets were used to cover the dents, but workers involved in laying the sheets were exposed to extra radiation because of this additional work.

In addition to the dents, the No. 1 to No. 3 reactors at the Fukushima plant, which first started operating in the 1970s, had many parts that have undergone repair work not reflected in their original construction plans. It was impossible to check such changes in the structure beforehand due to high levels of radiation.

There was another problem -- the machines could not be attached to the side of the containment vessel, which meant workers were unable to carry out drilling work. This was caused by the containment vessel's paint peeling away. The problem was solved after workers peeled off the paint by hand, but this also caused them to be exposed to more radiation.

The hole-boring process at the No. 2 reactor took approximately 20 days to complete -- during which, workers involved in the project were exposed to approximately 4.5 millisieverts of radiation on average. Based on national guidelines, many companies involved in decommissioning work set the annual upper radiation dose at 20 millisieverts for their workers. Therefore, workers can only be involved in this project up to five times before their level of radiation exposure exceeds the limit. However, as Imahori points out, "We have no way of knowing the situation unless we actually go in there."

Nevertheless, in order to ensure that highly-skilled professionals with expert knowledge in nuclear power plants continue to be involved in the search for the melted nuclear fuel, it is necessary to use robots as much as possible to reduce the amount of radiation to which humans are exposed.

At the same time, with the Fukushima No. 1 Nuclear Power Plant being somewhat like a "burning house," manpower is also required to make effective progress with the search. Yasuo Hirose, of IHI Corp., states, "If we completely rely on robots for the decommissioning work, they will not be able to deal with any unexpected problems. The decommissioning process is likely to be a very long task." (Mirai Nagira, Science & Environment News Department)

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