Robot surveys Fukushima Daiichi containment vessel

Tokyo Electric Power Company yesterday sent a robot into the primary containment vessel of the damaged unit 2 at the Fukushima Daiichi plant in Japan. Although the robot was unable to reach the part of the vessel directly under the reactor pressure vessel, the company said the information it gathered will help it determine how to decommission the unit.

Tokyo Electric Power Company (Tepco) yesterday sent a robot into the primary containment vessel of the damaged unit 2 at the Fukushima Daiichi plant in Japan. Although the robot was unable to reach the part of the vessel directly under the reactor pressure vessel, the company said the information it gathered will help it determine how to decommission the unit.

A pre-investigation of the area directly below the pressure vessel - known as the pedestal - was carried out in late January at unit 2 using a remotely operated camera on a telescopic probe. Photos taken during that investigation showed a black mass and deposits near a grating in the pedestal area, possibly melted nuclear fuel.

The scorpion-shaped robot (Image: Tepco)

Yesterday Tepco inserted a "scorpion-shaped" robot into a penetration in the wall of unit 2's primary containment vessel in order to take digital images and take temperature and radiation measurements.

About 54cm long, 9cm high and 9cm wide, and weighing about 5kg, the robot was designed to enter the unit's PCV along a pipe about 10cm in diameter. It was remotely operated by a wired cable.

The robot - developed by Toshiba and the International Research Institute for Nuclear Decommissioning - passed along the control rod drive (CRD) rail within the containment vessel towards the pedestal area. However, when it was about three meters away from the pedestal, one of its crawler belts became stuck and the robot could not move. Tepco decided to leave the robot where it was and disconnected the cable. It said the robot is not in such a position as to obstruct further investigations.

"Even though the robot could not reach the pedestal area, which we had initially planned to investigate, valuable information was obtained which will help us determine the methods to eventually remove fuel debris," Tepco said.

Readings indicate the temperature within the area of the containment vessel where the robot stopped was around 16.5 degrees Celsius and the dose rate was about 210 Sieverts per hour.

Tepco said it would "continue to evaluate the information, such as deposits on the CRD rail and conditions inside the pedestal, obtained from this entire investigation".

Robots are frequently used in nuclear and other industries to access difficult-to-reach or highly contaminated areas. At Fukushima, robots have been used to survey the damage inflicted on the plant after the 2011 tsunami and are being developed to undertake more complex tasks.

Researched and written
by World Nuclear News