28 Février 2017
February 27, 2017
OKUMA, Fukushima -- In an attempt to minimize the risk to humans during the search for melted nuclear fuel at the Fukushima No. 1 Nuclear Power Plant, robots have also been deployed to help out with the task.
However, the robots have also encountered some problems. For instance, a Toshiba Corp. robot that was sent in to clear away deposited material inside the containment vessel of the No. 2 reactor failed to clear away much material, and within approximately two hours, its camera had broken.
According to Takahiro Kimoto of plant operator Tokyo Electric Power Co. (TEPCO), "The radiation inside the containment vessel was so intense that the images transmitted back from a camera attached to the robot were pitch black." This was somewhat disappointing for the team working at the No. 2 reactor because by losing their robotic "eye" inside the containment vessel, they were unable to make the progress they were hoping for.
On Feb. 16, a "scorpion robot" was sent into the containment vessel. The intention of the mission was to locate melted nuclear fuel. However, deposited materials inside the vessel meant that the robot became stuck and was unable to move any further.
In the end, images from directly underneath the nuclear reactor were obtained not from the robot, but by "human means," on Jan. 30. By using a pipe and a camera, the team was able to confirm the presence of holes in the platform. They also discovered brown and black deposited material, which appeared to be melted nuclear fuel. Therefore, some might say that "human methods" are more effective than robots in a mission of this nature.
According to TEPCO, "This was the first probe of its kind in the world. We were able to collect sufficient data." However, critics would argue that six years have passed since the outbreak of the Fukushima nuclear disaster in 2011, and yet the exact situation regarding melted nuclear fuel at the site is still unclear.
Looking ahead, further difficulties are anticipated at both the No. 1 and No. 3 reactors, where in the past, there have been hydrogen explosions. This is mainly because there are several meters of contaminated water underneath the containment vessels, and the radiation levels are stronger than at the No. 2 reactor.
There are plans to insert a robot inside the No. 1 reactor in March, but a date has not yet been set for the No. 3 reactor. Satoshi Okada of the nuclear power plant maker Hitachi-GE Nuclear Energy, which oversees the search at the No. 1 reactor, states, "In order to deal with the problem of melted nuclear fuel, we must first ascertain exactly how and where the melted fuel has been scattered inside the reactors."
In summer 2017, TEPCO and the government will look into ways of withdrawing the melted nuclear fuel from the site, with the aim of commencing extraction work in 2021 -- exactly 10 years after the initial disaster.
The Three Mile Island Disaster in the U.S. in 1979 will provide some kind of reference for TEPCO and the government, because in that particular case, the removal of melted nuclear fuel started 11 years after the initial accident. However, the situation at Fukushima appears to be more complicated than at Three Mile Island, because in the case of the latter accident, melted nuclear fuel was retained within pressure containers. Conversely, in the case of Fukushima, some of the material has seeped through the pressure containers.
With regard to the government and TEPCO's decommissioning work, Nuclear Regulation Authority Chairman Shunichi Tanaka states, "It is still early to talk in such an optimistic way. At the moment, we are still feeling around in the dark."
Time will tell as to whether the current plan for removing melted nuclear fuel from the No. 1 power plant is a realistic possibility or just a pipe dream.
February 27, 2017
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)