Robots go deep inside Fukushima nuclear plant to map radiation

Robots armed with new radiation-sensing cameras are performing surveys in areas of the stricken plant where radiation levels are too high for humans to enter

IN THE dark abandoned shell of Fukushima Daiichi nuclear power plant, Rosemary and Sakura shoot what looks like a dystopian first-person shooter game. Rosemary scans her environment, while Sakura records every move.

But this is no traditional film crew. Rosemary and Sakura are robots operated by Tepco, the firm running the plant that went into meltdown following the March 2011 earthquake and tsunami. Using radiation-sensing cameras they perform surveys in areas where radiation levels are still too high for humans to safely enter. The data the robots collect will help plan how to decommission the plant.

Tepco has recently had problems with robots. On 10 April, it sent a robot inside the primary containment vessel of reactor 1 to measure radiation levels and investigate the condition of the melted fuel. But the robot stopped working on its first inspection and had to be abandoned. Radiation levels as high as 5150 millisieverts per hour have been detected in the basement of reactor 1.

Developed at the Chiba Institute of Technology, Rosemary and Sakura can climb 45 degree slopes and use gyroscopes and other sensors to navigate inside buildings without the need for GPS. But the standard radiation detector, gamma cameras weighing 150 kilograms, proved too cumbersome for them to use.

So the team has turned to a gamma camera that weighs just 17 kilograms and can rotate 360 degrees. This has been added to the robots as part of a system called N-Visage, which also includes a laser scanner that draws a 3D image of its environment.

The radiation measurements can be combined with a laser scan of the plant's exact layout, says Trevor Craig of UK start-up Createc, which developed N-Visage for Japan's International Research Institute for Nuclear Decommissioning, and reactor maker Hitachi. An earlier version was used in Sellafield in Cumbria.

Rosemary operates N-Visage, while Sakura acts as a wireless transmission station, feeding the data to human operators located in the plant's seismic-proof centre 200 metres away. They also get a fisheye view via cameras attached to the two robots.

In addition to creating real-time maps of radiation levels, the associated software can also predict how they will evolve as the facility is modified or taken apart. "Once we have a model like this we can play scenarios to find out what happens to dose rates if we decontaminate this wall, or put shielding next to highly contaminated fuel handling machinery," says Craig. "It helps work out what the decommissioning plan should be."

To date the N-Visage system has been used in the first three reactors at Fukushima. Createc is adapting the technology for use inside a snake-arm robot to delve even deeper into the reactors in an attempt to discover exactly where the fuel ended up.