Photo/Illutration Melted fuel debris retrieved from the Fukushima No. 1 nuclear power plant’s No. 2 reactor in November after it was crushed into pieces for analysis (Provided by the Japan Atomic Energy Agency)

Melted fuel debris retrieved from a crippled reactor at the Fukushima No. 1 nuclear power plant was found to be more brittle than expected, which experts said may offer new ways to tackle the decommissioning process.

The fragment weighs just 700 milligrams. It was removed from the No. 2 reactor containment vessel last November.

Plant operator Tokyo Electric Power Co. and the Japan Atomic Energy Agency said July 31 the surprise findings could lead to development of new methods and tools to remove the rest of the melted fuel from the plant’s hobbled reactors in years to come.

The melted fuel, the first to be removed from any of the crippled reactors at the plant, has been under analysis at the JAEA’s Oarai Nuclear Engineering Institute, in Ibaraki Prefecture, and elsewhere.

TEPCO and JAEA officials said the fragment broke into smaller lumps and grains when it was whacked with a stainless-steel rod.

A compositional study showed that uranium, the key ingredient of the nuclear fuel, makes up the bulk of the sample.

The fragment contained zirconium alloy, commonly used in nuclear reactors as fuel cladding, and iron and nickel, typically found in structural components. The materials were likely caught up in the mixture when the molten fuel broke through the bottom of the reactor core.

Experts had assumed the piece of fuel debris, with its strong metal content, would be hard to break.

The unexpected brittleness is partly due to the porous structure of the sample material, the officials said.

PULVERIZATION AS AN OPTION

On July 29, TEPCO released a provisional plan to retrieve fuel debris from the plant’s No. 3 reactor, starting in fiscal 2037 or later. It aims to accomplish that by pulverizing the melted fuel and removing it from the side of the reactor.

TEPCO said it is weighing the option of using a waterjet or laser to pulverize the debris.

However, a JAEA official who analyzed a sample of the fragment said there was no way of telling whether the fuel debris buried deeper is of similar consistency.

Pulverization has the advantage of reducing debris grain size with little force being applied only if the material crumbles easily, the JAEA official explained.

An estimated 880 tons of melted fuel debris remain inside the No. 1, 2 and 3 reactors at the plant. The facility went into a triple meltdown after being swamped by tsunami triggered by the Great East Japan Earthquake of 2011.

A second batch of fuel debris, weighing a total of 200 milligrams, was retrieved in April, also from the No. 2 reactor.

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