Readings from reactor No 2 at the Fukushima plant are so high they threaten to shut down the robot camera probe being used to monitor the site while it is being dismantled.
Pictures from the inside of the reactor also show a metal grating that contained nuclear fuel, has largely collapsed, causing a hole about a metre wide.
Black debris that could be melted fuel is also seen in the images, taken from the deepest point yet reached in the reactor.
Radiation levels at one spot were estimated at 530 sieverts per hour, far higher than the previous record of 73 sieverts per hour.
Tokyo Electric Power (Tepco) said the reading focused on one single point and levels estimated elsewhere were much lower.
The firm played down concerns the levels could damage the robot probe, which can withstand up to 1,000 sieverts, because it was unlikely to stay for too long at one location.
It hoped to eventually use robots to locate the fuel debris as part of the decommissioning process.
Tepco also pointed out the radiation was not leaking outside the reactor.
Spokesman Tatsuhiro Yamagishi said: “We believe the captured images offer very useful information, but we still need to investigate given that it is very difficult to assume the actual condition inside.”
A massive undersea earthquake on 11 March, 2011, sent a huge tsunami sweeping across Japan’s northeast coast.
More than 18,000 people were killed and the Fukushima plant was badly damaged in the worst such accident since Chernobyl in 1986.
The Japanese government said in December it expects the total costs including compensation, decommissioning and decontamination to reach £150bn.
The process is likely to take decades because high radiation levels have slowed operations.