The plight of Fukushima's robots

Photo shows the 30-cm, 2 kg submersible robot "mini mambo" which is set for deployment in unit 3 next month (Image: Toshiba-IRID)

First there was the octopus, then the snake and the scorpion. Now it's the turn of the “mini mambo,” or Little Sunfish.

Several of the robots used for cleanup and inspection operations inside Fukushima Daiichi Nuclear Power Plant have been likened to various creatures, the latest being named after the heaviest known bony fish in the world, albeit one with some pretty illuminating features.

The 30 cm long, 2 kg underwater robot will be deployed in July to inspect the damage at Fukushima Daiichi’s No. 3 reactor in order to to investigate the damage and locate parts of melted fuel believed to have fallen to the bottom of the chamber following the meltdown that occurred there in 2011.

The third reactor is one of three of Fukushima Daiichi’s six reactors that went into meltdown, triggered by mega earthquakes and tsunami in the vicinity in March 2011, leading to the evacuation of more than 150,000 residents. Jointly developed by Toshiba and the International Research Institute for Nuclear Decommissioning (IRID), the swimming robot will collects data using two cameras, located at the front and rear of the device, and a dosimeter.

Experts believe it can access areas below the reactor's melted core in order to locate where the melted fuel ended up — something that is still unknown. Even if this mission is a success, the data collected could take a significant time to assess and utilize in a plan to remove the melted fuel.

Dale Klein, a former US Nuclear Regulatory Commission chief told the Associated Press it would take at least six months to a year to collect data and decide on how to remove the fuel.

"The fuel debris will be a challenge," said Klein. "No one in the world has ever had to remove material like this before. So this is something new and it would have to be done carefully and accurately.”

According to Japan’s decommissioning plan, removal of fuel debris will commence in 2021. Some experts believe this may be an ambitious goal, especially due to the somewhat dubious results of previous robotic attempts at the plant.

In the early stages following the disasters, unmanned robots were employed to monitor the radiation levels around the mangled reactor 3, among them the US-made PackBot and TALON robots, both of which were developed for tactical reconnaissance.

Like many others that would follow, including the scorpion and the snake, they ultimately failed, some due to poor onsite operational skills, others due to the radiation levels, which were so high that they rendered the robots’ electronics inoperable.

“After about 30 feet they [the robots] were completely useless,” one Fukushima worker who was involved with assisting in the deployment of the robots told me while researching "Yoshida's Dilemma." 

​Dozens of so-called “diagnostic” robots developed by companies such as Toshiba, Hitachi and Mitsubishi since have been deployed at the Fukushima plant to assist in the decommissioning process. One or two have given insights into the state of two of the stricken reactors, though invariably operators are unable to retrieve the multi-million-dollar devices due to high contamination levels. One device developer commented that the insides of the stricken reactors surveyed thus far must resemble a "robotic graveyard."

As of mid-2016, however, none of those deployed had definitively located any melted fuel, though in February 2017 some progress was made in this area.

On Feb. 2, 2017 plant operator TEPCO reported that cameras inserted inside reactor 2 had located a 1-sq.-meter hole where the fuel had apparently melted through steel grating directly beneath the reactor pressure vessel. The utility estimated that the radiation levels there were 530 Sv/hr – sufficient to cause death in less than a minute. The highest radiation level that had been recorded inside the ruined reactors until then was 0.73 Sv/hr. TEPCO had previously claimed that muon tomography inspections had confirmed their belief that no fuel had leaked out of the pressure vessel.

The latest robot is reportedly better equipped to deal with the testing conditions at reactor 3. It is small enough to be able to swim into the now-flooded primary containment vessel, where engineers have created a 14 cm-wide penetration hole. What's more, Little Sunfish can handle radiation levels of up to 200 Sieverts per hour — still 2.5 times lower than the levels recorded in February, but an improvement on previous devices deployedt.

"We have already developed remotely operated robots for inspections at Fukushima," commented Goro Yanase, general manager of Toshiba's nuclear energy systems & services division in a statement that was released on June 15. "In this case, we had to meet the specific challenges of limited access and flooding, in a highly radioactive environment. Working with IRID, we succeeded in developing a small robot with high level radiation resistance, and through its deployment we expect to get information that will support the advance of decommissioning.”

However, even if they succeed, retrieving that melted fuel is a long way off, Kazuhiro Suzuki, former executive director of IRID, told me while researching “Yoshida’s Dilemma”. “The safest way to remove the fuel is to fill the primary containment vessel with water and … under such a condition maybe the workers can access the fuel debris underwater,” said Suzuki.

But to realize this first requires locating and fixing “possibly hundreds” of damaged components of the containment vessels, he added. “With the high radiation levels, just to fix all of these is very difficult,” he said.