Radiation levels recorded inside Fukushima's crippled nuclear power station are at the highest levels since its catastrophic meltdown in 2011.

Tokyo Electric Power Co. Holdings Inc (Tepco) said the radiation level in the containment vessel of Reactor 2 in the Fukushima No 1 power plant had reached a maximum of 530 sieverts per hour, Japan Times reports.

The "unimaginable" radiation levels were assessed by the National Institute of Radiological Sciences.

According to the institute, just 4 sieverts of radiation exposure would be enough kill a handful of people.

A two-metre hole has been found in the metal grate near the reactor. (Tepco)

Tepco said it had found a two-metre hole in the metal grating under the reactor that would need to be patched after a robot was sent in to assess some of the damage.

The hole which appears to be warped is believed to have formed from searing hot fuel escaping the pressure chamber during the accident.

Early next month a new specially designed robot will be sent inside the reactor to start a detailed assessment of various damaged systems, however it will only be able to operate for a few hours before it is destroyed by the extreme levels of radiation inside the plant.

A reactor probe robot PMORPH that will be sent inside the Fukushima reactor early next month. (Source: AFP)

The company also discovered pools of a black mass they believe are melted uranium rods that were once used to power the core.

Due to the extraordinarily high level of radiation – which the company assures the public is still confined to the plant – there are doubts that workers will be able to enter the site anytime soon to start the major works needed to decommission the reactor safely.