TOKYO -- A joint venture between Japanese and American high-technology power houses Hitachi and General Electric is developing special robots for removing nuclear debris from the Fukushima Daiichi nuclear power plant, the most difficult task in decommissioning the plant's six reactors, three of which suffered core meltdowns in the March 2011 accident.
The machines under development by Hitachi-GE Nuclear Energy are called "muscle robots," as their hydraulic springs operate like human muscles. The company, based in Hitachi, Ibaraki Prefecture, is stepping up efforts to complete the development project in time for the start of debris removal in 2021.
Hitachi-GE is testing the arms of the robots at a plant of Chugai Technos, a Hiroshima-based engineering service company, located a 30-minute drive from the center of the city. The testing is taking place in a structure with a life-size model of the primary containment vessel of the No. 1 reactor at the Fukushima plant. The robots awkwardly move about, picking up concrete lumps standing in for fuel debris.
"The robots are based on a concept completely different from those of conventional robots," said Koichi Kurosawa, a senior Hitachi-GE engineer heading the development project. Hydraulics are being used because electronics cannot survive in the extreme environment inside the reactors.
"Asked if the robots are applicable to other nuclear power plants, I would say the possibility is low," Kurosawa said, noting that the robots are designed to work amid intense radiation.
While Hitachi-GE has built many nuclear reactors, it is encountering a variety of new challenges in developing the muscle robots simply because of the tough work required to retrieve fuel debris.
In the nuclear accident caused by the March 2011 earthquake and tsunami, cooling the fuel rods became impossible, and melted uranium fuel dropped from them. Some of the fuel broke through nuclear reactor pressure vessels and solidified as fuel debris containing uranium and plutonium.
The debris is estimated to weigh more than 800 tons in total. The insides of the PCVs at the Fukushima plant are directly exposed to the debris and are emitting radioactivity strong enough to kill a human within a few minutes.
The International Research Institute for Nuclear Decommissioning, a Tokyo-based research institute for decommissioning nuclear plants, and three reactor makers -- Hitachi-GE, Toshiba and Mitsubishi Heavy Industries -- have been attempting to ascertain conditions inside the reactor buildings at the Fukushima plant by means of camera- and dosimeter-equipped equipment.
Hitachi-GE developed a flexible snakelike robot, while Toshiba produced a scorpion-type robot with a camera at its tail, which is designed to rise on its own after falling.
Although six years have passed since the accident, little is known about where and how much fuel debris is inside the reactor buildings because the extremely intense radiation inside hampers even simple filming.
Hitachi-GE developed five types of muscle robot for handling fuel debris, including a four-footed robot and a crawler robot that looks like a military tank. Though the five robots look different from each other, they all work via hydraulic pressure and springs, rather than electronics.
Hitachi-GE adopted the simple structure because electronic substrates and sensors are sensitive to radioactivity. Robots that operate via hydraulic pressure and springs can work long hours under intense radiation.
The five robots have a common system in which a rubber tube that sends water to a cylinder, if it is cut, is replaced by another one. The system, similar to an aircraft's "redundancy" -- the duplication of critical components or functions for safety -- is aimed at avoiding a situation in which a robot becomes unworkable inside a reactor, resulting in the need for much labor and time to recover it.
Experiments in Hiroshima are underway not only to remove fuel debris but also to carry out many other tasks, such as installing metal plates to stem radiation-contaminated water leaks and checking underwater movements in a water-filled pool.
Muscle robots currently simply repeat the same moves many times because they are still "at the level of elementary school pupils or the experimental stage," Kurosawa said.
For actual use, Hitachi-GE engineers envision erecting a building adjacent to a nuclear reactor building and digging a tunnel from there into a PCV to send robots into it.
For the removal, furthermore, the first-floor portion of a foothold, called a "grating," used for reactor maintenance, will be eliminated. Then chunks of fuel debris accumulated in the basement will be cut into smaller pieces with a diameter of less than 20cm, and removed.
A machine to dry the debris will be also installed to prevent a hydrogen explosion, before the pieces are placed in containers for removal from the reactor building.
One estimate says that if the muscle robots are used 24 hours a day, they can complete the removal of fuel debris within a year, compared with the initial projection of nearly eight years.
In contrast with industrial multiaxial robots, which require much time to master, the operation of simply-structured muscle robots can be readily learned.
Decommissioning nuclear reactors has become a market valued at an estimated 3 trillion yen ($27.4 billion), drawing strong interest from not only reactor manufacturers but other companies as well.
The work at the Fukushima Daiichi plant is entirely different from conventional decommissioning services due to the extreme difficulty. Therefore, the three reactor makers have a chance to demonstrate their skills, as they are intimately familiar with the reactors.
The reactor that caused the catastrophic accident at the Chernobyl Nuclear Power Plant in the then Soviet Union in 1986 has been enclosed in a huge concrete sarcophagus, but is still emitting radiation.
The Fukushima Daiichi accident was the worst nuclear disaster in history. The three reactor manufacturers, which form the foundation of Japan's nuclear power industry, are poised for the unprecedented difficulty of decommissioning the Fukushima reactors.