TOKYO -- Tokyo Electric Power Co. (9501.TO), the operator of Japan's crippled Fukushima nuclear plant, said Wednesday record amounts of radiation had been detected in the basement of reactor No. 1, further hampering clean-up operations.

Tepco took samples from the basement after lowering a camera and surveying instruments through a drain hole in the basement ceiling.

Radiation levels above radioactive water in the basement reached up to 10,300 millisievert an hour, a dose that will kill humans within a short time after making them sick within minutes.

The annual allowed dose for workers at the stricken site is reached in only 20 seconds.

"Workers cannot enter the site and we must use robots for the demolition," said Tepco.

The Fukushima operator said that radiation levels were 10 times higher than those recorded at the plant's two other crippled reactors, No. 2 and 3. This was due to the poor state of the nuclear fuel in the reactor compared to that in the two others.

The meltdown at the core of three of Fukushima's six reactors occurred after the March 11, 2011, earthquake and ensuing massive tsunami shut off the power supply and cooling system.

Demolition of the three reactors, as well as the plant's No. 4 unit, is expected to take 40 years and will need the use of new technologies.