The operator of Japan's Fukushima Daiichi nuclear power plant says it has retrieved a small amount of nuclear fuel debris from the containment vessel of one of the reactors that melted down 13 years ago.

Tokyo Electric Power Company said on Saturday that the debris was taken out of the containment vessel of the No.2 unit earlier in the day. This is the first-ever debris retrieval from a containment vessel since the 2011 nuclear accident triggered by a powerful earthquake and tsunami.

A trial attempt to remove molten fuel mixed with parts of the reactor's structure began in September.

TEPCO is using a pipe-like retrieval device to collect debris accumulating at the bottom of the containment vessel.

On Wednesday, the device successfully grabbed a small piece of debris, measuring about 5 millimeters and presumably weighing several grams.

TEPCO says the device holding the debris was pulled out of the containment vessel a little before 10 a.m. on Saturday.

The radiation level of the collected sample will be measured early next week while it is still inside a storage box holding the retrieval device.

If the radiation reading is below a certain level, the sample will be placed in a dedicated container for transportation, ending the process of the trial retrieval.

There are an estimated 880 tons of debris in the containment vessels of the No.1, 2 and 3 reactors.

Removing it is considered the most challenging step in the decommissioning of the plant due to the extremely high levels of radiation.

TEPCO plans to utilize data obtained from analysis of the collected sample to work out a method for full-fledged debris retrieval in the future.