A swimming robot that was sent into one of the flooded reactors of Japan's Fukushima plant has returned its first images and footage, a key step towards cleaning up the site.

The robot is called "Little Sunfish", or "mini manbo", and we first learned about it back in June. The Tokyo Electric Power Company (TEPCO), who is running the clean-up operation, have now revealed the first footage from the robot.

Little Sunfish was sent into Fukushima’s Unit 3 on Wednesday, one of three reactors that went into meltdown in March 2011 after a tsunami struck the Fukushima Daiichi Nuclear Power Station. About 160,000 people were evacuated from the nearby area as a result of the radiation leak, with little prospect of ever returning.

“This was the first time that a robot has entered the containment vessel of reactor 3,” said Takahiro Kimoto, a Tepco spokesman, reported the Japan Times. “We think this is a big step.”

In video footage recorded by the robot, scattered equipment can be seen inside the reactor. Some steel gratings that would have acted as scaffolding were also in the wrong place, something engineers could not explain.

However, the robot did not locate any melted fuel debris in this first excursion, which lasted three hours. This was merely a test of the robot. The next step will be to send the robot even deeper into Unit 3 today, where the water is about 6 meters (20 feet) deep. The ultimate goal is to locate all of the melted radioactive fuel from the reactor, so that the clean-up process can begin.

The robot, called an underwater ROV (Remotely Operated Vehicle), measures just 30 centimeters (11.9 inches) long and 13 centimeters (5.1 inches) wide. It has front- and rear-facing cameras and LED wires. Five thrusters, four at the rear and one at the front, help steer it through Fukushima.

It entered Unit 3 through a borehole drilled by engineers, with a long cable used to supply power and keep track of the robot. This borehole measured just 14 centimeters (5.5 inches) across, just wide enough for Little Sunfish to fit through.

The robot can handle radiation levels of up to 200 Sieverts per hour. But some parts of the plant have levels of up to 530 Sieverts per hour, more than enough to kill a human. Other robots that have been sent into Fukushima before have failed.

The process of cleaning up Fukushima will be long and expensive. Engineers do not expect to find all of the melted fuel from the three reactors until 2021. It will then take four decades to clean-up this fuel and decommission Fukushima, at a cost of $188 billion.