Robots’ limitations exposed in search for melted nuclear fuel in Fukushima

since march 2011 2.jpg

 

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.

http://mainichi.jp/english/articles/20170227/p2a/00m/0na/016000c

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