TOKYO - "Extremely high" levels of radiation were detected inside the No 2 reactor at the crippled Fukushima Daiichi Nuclear Power Plant, according to the results of an investigation published by Tokyo Electric Power Company Holdings (TEPCO), the plant's operator, on Thursday.

This marks the first time since the 2011 nuclear accident at the Fukushima Daiichi plant that TEPCO has measured radiation inside a reactor that suffered a core meltdown.

According to public broadcaster NHK, the investigation was conducted on April 16. A fiberscope equipped with measuring instruments was inserted into the reactor through piping. At a point approximately five meters above the bottom of the reactor, radiation levels were measured at around 4.7 sieverts per hour, classified as "extremely high."

TEPCO stated that the findings indicate a certain amount of nuclear debris remains inside the No 2 reactor. The company will further analyze the data and study methods for removing the debris.

The No 1 through No 3 reactors in the Fukushima Daiichi plant all suffered core meltdowns after the six-reactor facility was hit by a 9.0-magnitude earthquake and an ensuing tsunami in 2011, leaving behind an estimated 880 tonnes of nuclear debris.

The highly radioactive material poses significant risks, and its removal is widely regarded as one of the most formidable challenges facing the decommissioning of the plant.