Workers wearing protective suits and masks work on the No. 2 reactor building at the Fukushima Daiichi nuclear power plant.
TOKYO – A state-backed entity tasked with supporting the decommissioning of the Fukushima nuclear power station proposed Thursday that melted fuel be removed from the side of three of the crippled reactors as part of the process to scrap the complex.
Based on a formal proposal, the government and the plant operator Tokyo Electric Power Company Holdings Inc (TEPCO) will determine specific approaches to carry out the process on each reactor next month and update the plant decommissioning road map.
Under its strategic plan for 2017, the Nuclear Damage Compensation and Decommissioning Facilitation Corp called for the removal of the fuel by partially filling the three reactors with water to cover some of the nuclear debris while allowing access to carry out the work.
The entity also pointed out that the decommissioning work requires phased efforts while maintaining flexibility, as the project still faces many uncertainties.
The extraction work from the Nos. 1-3 reactors at the Fukushima Daiichi nuclear complex, which suffered meltdowns following the massive 2011 earthquake and tsunami disaster, is seen as the most difficult step toward the ultimate goal of decommissioning the entire complex, set to take at least 30 to 40 years to complete.
The government and TEPCO are currently aiming to start the extraction work from 2021.
Under the plan, the Nuclear Damage Compensation and Decommissioning Facilitation body proposed using a remotely controlled apparatus to shave debris from the underside of the lower section of the reactors’ containment vessel while controlling the level of water.
Debris remains not only in the reactors’ pressure vessel but also piled and scattered at the bottom of the containment vessel that houses the reactor vessel.
As for debris left in the reactors’ pressure vessel, the entity will consider removing it from the upper part of the reactors, it said.
The decommissioning body had previously considered a strategy to fill the containment vessel with water as water is effective in containing radiation, but it has shelved the idea as the reactor containers are believed to have been damaged and would leak.
Following a magnitude-9.0 earthquake in March 2011, tsunami inundated the six-reactor plant, located on ground 10 meters above sea level, and flooded power supply facilities.
Reactor cooling systems were crippled and the three reactors suffered fuel meltdowns, while hydrogen explosions damaged the buildings housing the Nos. 1, 3 and 4 reactors.
The Nuclear Damage Compensation and Decommissioning Facilitation entity was established after the Fukushima crisis, the worst nuclear disaster since Chernobyl, to help the utility pay damages. The state-backed entity holds a majority stake in the operator.
Hitachi-GE testing variety of simply structured, radiation-resistant equipment
The Unit 1 reactor building at the Fukushima Dai-ichi nuclear power plant of the Tokyo Electric Power Co. (TEPCO) in Okuma town, Fukushima prefecture, June 21, 2017.
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.
Mainichi Shimbun reporters visited the Fukushima No. 1 Nuclear Power Plant on July 27. While the working environment at the station has improved, plant operator Tokyo Electric Power Co. (TEPCO) still has a mountain of problems to tackle, such as removing melted nuclear fuel from the No. 1 to No. 3 reactors and treating contaminated water.