Photo/Illutration Debris taken from the No. 2 reactor of the Fukushima No.1 nuclear power plant in April (Provided by Japan Atomic Energy Agency)

Melted fuel debris retrieved from the Fukushima No. 1 nuclear plant differed in several ways from an earlier sample and could reveal further details on how the 2011 disaster unfolded there, researchers said.

The Japan Atomic Energy Agency (JAEA) and Tokyo Electric Power Co. on May 29 released their analysis of the debris that was removed from the No. 2 reactor at the stricken plant in April.

TEPCO has been analyzing melted nuclear fuel samples as part of efforts to determine the best way to decommission the nuclear power plant.

An estimated 880 tons of melted fuel debris remains inside the No. 1, 2 and 3 reactors at the plant.

The latest debris--11 pebble-sized fragments--may have become attached to wet substances when they were being retrieved inside the reactor with a robotic arm, the analysis said.

The 11 fragments weigh a total of 0.187 gram, while the debris removed from the same reactor in November last year weighed 0.693 gram.

The largest fragment of the latest batch was 5 millimeters in height and 4 mm in length. The smallest was less than 1 mm long.

Radioactive europium and americium, generated through the nuclear fission of uranium, were detected in the debris, confirming the fragments are derived from nuclear fuel, the researchers said.

The radiation dose was 0.3 millisievert per hour at a distance of 1 to 2 centimeters, significantly less than about 8 millisieverts per hour in the previous sample.

The latest debris was dark brown, lighter than the earlier sample, contained more nuclear fuel and featured more internal spaces where gas likely escaped from.

“Melted fuel dropped on the floor with strong stickiness and solidified as it cooled while gas was trapped inside,” the JAEA said.

The debris removed in April came from an area closer to the center of the reactor containment vessel than the sample taken in November.

Both samples have been analyzed at the JAEA’s Oarai Nuclear Engineering Institute in Ibaraki Prefecture.

TEPCO plans to retrieve a third sample from the No. 2 reactor this fiscal year.

“The robot arm has the ability to reach wide areas,” said Akira Ono, president of Fukushima Daiichi Decontamination & Decommissioning Engineering Co. “We want to use it to reach out and take debris from other locations to compare.”

(This article was written by Fumi Yada and Yusuke Ogawa.)

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