Melted Nuclear Fuel Search Proceeds One Dead Robot at a Timeby and
Third robot abandoned in search of fuel at Japan’s Fukushima
Melted fuel still needs to be located before removal begins
The latest robot seeking to find the 600 tons of nuclear fuel and debris that melted down six year ago in Japan’s wrecked Fukushima Dai-Ichi power plant met its end in less than a day.
The scorpion-shaped machine, built by Toshiba Corp., entered the No. 2 reactor core Thursday and stopped 3 meters (9.8 feet) short of a grate that would have provided a view of where fuel residue is suspected to have gathered. Two previous robots aborted similar missions after one got stuck in a gap and another was abandoned after finding no fuel in six days.
After spending most of the time since the 2011 disaster containing radiation and limiting ground water contamination, scientists still don’t have all the information they need for a cleanup that the Japanese government estimates will take four decades and cost 8 trillion yen ($70.6 billion). It’s not yet known if the fuel melted into or through the containment vessel’s concrete floor, and determining the fuel’s radioactivity and location is crucial to inventing the technology needed to remove it.
“The roadmap for removing the fuel is going to be long, 2020 and beyond,” Jacopo Buongiorno, a professor of nuclear science and engineering at the Massachusetts Institute of Technology, said in an e-mail. “The re-solidified fuel is likely stuck to the vessel wall and vessel internal structures. So the debris have to be cut, scooped, put into a sealed and shielded container and then extracted from the containment vessel. All done by robots.”
To enter a primary containment vessel, which measures about 20 meters at its widest, more than 30 meters tall and is encased in meters of concrete, outside air pressure is increased to keep radiation from escaping and a sealed hole is opened that the robot passes through. Three reactors at the plant suffered meltdowns, and each poses different challenges and requires a custom approach for locating and removing the fuel, said Tatsuhiro Yamagishi, Tokyo Electric Power Co. Holdings Inc. spokesman.
The machines are built with specially hardened parts and minimal electronic circuitry so that they can withstand radiation, if only for a few hours at a time. Thursday’s mission ended after the robot’s left roller-belt failed, according to Tokyo Electric, better known as Tepco. Even if it had returned, this robot, like all others so far designed to aid the search for the lost fuel, was expected to find its final resting place inside a reactor.
No. 1 Unit
Hitachi Corp. in the next two months plans to send a machine into the No. 1 reactor core that scientists hope can transmit photos of the fuel and measure radiation levels.
The snake-like robot will lower a camera on a wire from a grate platform in the reactor to take photos and generate 3-D models of the bottom of the containment vessel. This will be the third time Hitachi sends in this robot design.
While the company is hopeful this robot will find some of the fuel, it will likely be unable to find all of it, according to Satoshi Okada, a Hitachi engineer working on the project. The company is already planning the next robot voyage for after April.
“We are gathering information so that we can decide on a way to remove the fuel,” said Okada. “Once we understand the situation inside, we will be able to see the way to remove the fuel.”
No. 2 Unit
On Thursday, Toshiba’s scorpion-like robot entered the reactor and stopped short of making it onto the containment vessel’s grate. While Tepco decided not to retrieve it, the company views the attempt as progress.
“We got a very good hint as to where the fuel could be from this entire expedition” Tepco official Yuichi Okamura said Thursday at a briefing in Tokyo. "I consider this a success, a big success."
Tepco released images last month of a grate under the No. 2 reactor covered in black residue that may be the melted fuel -- one of the strongest clues yet to its location. The company measured radiation levels of around 650 sieverts per hour through the sound-noise in the video, the highest so far recorded in the Fukushima complex.
A short-term, whole-body dose of over 10 sieverts would cause immediate illness and subsequent death within a few weeks, according to the World Nuclear Association.
The Hitachi and Toshiba robots are designed to handle 1,000 sieverts and no robot has yet been disabled due to radiation.
“Radiation levels near the fuel are lethal,” said MIT’s Buongiorno, who holds the university’s Tepco chair, a professorship based on an initial donation by the company 10 years ago. There are no formal affiliations or obligations for the faculty who receive the chair, he said.
Because the No. 2 unit is the only one of the three reactors that didn’t experience a hydrogen explosion, there was no release into the atmosphere and radiation levels inside the core are higher compared to the other two units, according to the utility.
No. 3 Unit
Tepco’s balance sheet has been strapped by ballooning Fukushima cleanup costs and slumping national power demand. All of the company’s nuclear power plants remain shut since it halted the No. 6 reactor at its Kashiwazaki Kariwa station in March 2012. The company is seeking drastic changes in top management in consultation with the Japanese government, TV Asahi reported Friday, without attribution.
The utility has focused on removing spent fuel in the upper part of the reactor building, which Toshiba aims to extract with a claw-like system. This fuel didn’t melt and is still in a pool that controls its temperature.
The used-fuel in No. 3 is scheduled to begin removal before the end of the decade, the first among the three reactors that melted down. Toshiba is developing another robot to search for melted fuel, planned to enter sometime in the year ending March 2018. The company hasn’t announced yet the design or strategy.