The operator of the crippled Fukushima Daiichi nuclear power plant has released the results of its latest probe of the site.
A remote-controlled inspection of the Unit 2 reactor containment vessel last month detected a maximum of 8 sieverts per hour of radiation.
Experts say exposure to such radiation for about an hour would be fatal.
Officials from Tokyo Electric Power Company, or TEPCO, released the results on Thursday.
They said the radiation reading was taken near what appeared to be fuel debris, the term used to describe a mixture of molten fuel and broken interior parts.
The finding shows that nearly 7 years after the meltdowns, radiation levels remain so high that they present a major challenge to decommissioning work.
During the probe, 42 sieverts per hour of radiation was also detected outside the foundations of the reactor.
But officials said they have doubts about the accuracy of the reading because a cover had not been removed from the measuring instrument at the time.
They added that they don't know why radiation levels were lower near the suspected fuel debris than around the foundations.
They gave a number of possible reasons, such as that cooling water may have washed radioactive materials off the debris.
TEPCO's Chief Decommissioning Officer, Naohiro Masuda, says the company will develop debris-removal technology based on the outcome of the investigation.