In March 2011, a magnitude 9.0 earthquake and ensuing tsunami combined to cause a nuclear disaster at the Fukushima power plant in Japan. As part of the ongoing clean-up operation, an underwater robot has been used to locate radioactive material for the first time.
On July 19th, Tokyo Electric Power Company Holdings (TEPCO) launched a robotic expedition to explore the remains of Fukushima’s Unit 3 reactor. The aim was to determine the conditions inside the primary containment vessel and gain a better understanding of the steps required to decommission it.
During the accident back in 2011, three of the Fukushima power plant’s reactors went into meltdown after the tsunami flooded the back-up power generators and cooling systems. Ever since, the search has been on to find the molten metal and radioactive fuel lost in the wreckage.
Inspection of Unit 3 suggests that fuel assemblies melted from the excess heat and dropped down into the pedestal area. In the process, the area is believed to have been damaged. A team of underwater robots has been attempting to gather images and data to provide a clearer picture of exactly what happened.
A hostile environment
The ‘Little Sunfish’ robot has undertaken a number of missions into the depths of Unit 3. Developed by the International Research Institute for Nuclear Decommissioning (IRID), the vessel is equipped with thrusters and five propellers. It can navigate through the water, has front and rear cameras and is controlled using a tether.
The Little Sunfish had to be inserted into the primary containment vessel through a pipe to protect it from any radioactive gas from the reactor. It went on to be the first-ever robot camera to capture what is believed to be the melted fuel.
Discovering the fuel debris from the disaster in each of the three reactors is a vital part of the decommissioning process. In Units 1 and 2, the hunt has been frustrated due to the amount of damage and dangerously high radiation levels. In fact, more than seven robots have already been lost or abandoned during the search.
A lengthy mission
TEPCO spokesman Takahiro Kimoto said that while the findings so far have been beneficial to the decommissioning project, the challenging process looks set to carry on for many years. Technological and logistical solutions still need to be developed.
“It’s still just the beginning of the (decades-long) decommissioning. There is still a long way to go, including developing the necessary technology,” he said. “But it’s a big step forward.”
The data the Little Sunfish probe has gathered will give the decommissioning team a vital insight into the damage inside the reactor and help them determine how best to extract the melted fuel. That process is expected to start after the 2020 Tokyo Olympics.