Photo/Illutration The No. 2 reactor of the Fukushima No. 1 nuclear power plant in October (Asahi Shimbun file photo)

Tokyo Electric Power Co. started collecting melted nuclear fuel for the second time at the crippled Fukushima No. 1 nuclear power plant, this time closer to the center of a reactor.

The telescopic retrieval device was inserted into a passageway leading to the containment vessel of the No. 2 reactor on April 15.

The retrieval mission is expected to take about 12 days to complete.

In November, TEPCO collected nuclear fuel debris on an experimental basis for the first time after three reactors melted down at the plant during the Great East Japan Earthquake and tsunami in March 2011.

The sample taken from the No. 2 reactor weighed about 0.7 gram.

An analysis detected uranium, which is contained in nuclear fuel, zirconium, which is used for fuel cladding tubes, and other materials.

However, it remains unclear whether the sample’s composition represents the entire scope of the nuclear fuel debris.

In the latest attempt, TEPCO plans to collect a sample from a location one to two meters closer to the center of the No. 2 reactor’s containment vessel than in November.

The utility hopes that greater knowledge of the characteristics and the distribution of debris could lead to better understanding of the harsh conditions inside the containment vessel.

It also plans to use the findings to study methods to extract melted nuclear fuel on a full scale and for storing it. 

TEPCO will collect debris from the same location as in the previous work if the retrieval device cannot reach the bottom of the containment vessel due to obstacles.

The remote-controlled device, which acts like a fishing rod, can extend up to about 22 meters. Cameras as well as a metal instrument to hold debris are installed at the far end.

Samples will be analyzed at a facility of the Japan Atomic Energy Agency in Ibaraki Prefecture.

The previous operation, which used the same device, faced many problems.

TEPCO initially planned to start the project in August, but it was postponed because five pipes designed to thrust the device into the containment vessel had been placed in the wrong order during preparations.

The work finally started in September but was suspended for more than a month after it became impossible to view camera images.

To prepare for the second attempt, TEPCO conducted training for workers to check the order of the pipes in late March.

The retrieval device has also been improved to increase stability when the tip section is lowered.

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