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Tokyo Electric Power Company Holdings Inc., however, stressed that the investigation was worthwhile because its robot was able to take underwater images in the pool of water at its bottom and gauge its radiation level, which will help it estimate where the melted fuel lies.

The monstrous tsunami of March 11, 2011, tipped reactors 1, 2 and 3 into core meltdowns. The molten fuel rods then penetrated their pressure vessels before apparently dropping to the bottom of the giant containment vessels.

There is about a 2.5-meter deep water pool at the bottom of the primary containment vessel of reactor 1, and Tepco believes most of its melted fuel rods fell into it. Thus the main mission of the robot investigation this time was to capture underwater images.

The robot traversed gratings set up several meters above the vessel’s bottom and lowered a wire with a camera and dosimeter on its tip at 10 locations in the water.

Yet none of the images disclosed by Tepco showed anything resembling fuel debris, while parts of machinery, such as a valve, were captured.

When the robot dangled the camera on spots where Tepco thought there was a higher probability of locating the fuel, it instead found a 90-cm pile of sediment.

Tepco spokesman Yuichi Okamura said the sediment is probably not fuel debris, given the relatively low radiation readings, which ranged from 5.9 to 9.4 sieverts per hour.

Although the readings indicate extreme danger to people, Okamura said the readings would have been much higher had they been melted fuel rods. He said Tepco had no idea what the sediment is but added that there was a possibility it was covering the fuel.

According to Okamura, radiation readings get weaker by a hundredth if blocked by a meter of water. Since the robot detected readings from 5.9 to 9.4 sieverts per hour about 90 cm above the pool’s bottom, there might be something down there emitting strong radiation.

Tepco plans another investigation this month to pick up samples of the sediment.

While no fuel debris was recognized, Okamura said Tepco would review the data and analyze it further. By comparing radiation readings from various locations, the utility might be able to roughly pinpoint where the melted rods lay, he said.

He added that it was an achievement that the robot lasted for five days in the deadly radiation and that Tepco was able to retrieve it.