A probe inserted into reactor two at the Fukushima Daiichi nuclear plant revealed lethal doses of radiation and that the level of cooling water inside was far lower than expected.
But operator Tepco says the plant remains in a cold shutdown.
The plant was severely damaged by the March 2011 earthquake and tsunami.
Since the crisis began in Fukushima prefecture, the operation to contain it has been hampered, reports the BBC’s Roland Buerk in Tokyo. Tsunami damage to instruments has made it impossible to know what is happening inside the reactors.
On Tuesday workers managed to insert a probe into reactor number two for only the second time and found damage worse than expected.
Radiation was up to 10 times the fatal dose, the highest yet recorded at the plant. The level of water cooling the melted-down nuclear fuel was also far lower than expected.
The other two melted-down reactors, which are yet to be examined closely, could be in an even worse state, our correspondent adds.
Tokyo Electric Power Co (Tepco) says the finding does not affect their assessment that the nuclear fuel is being safely cooled.
But it could make decommissioning harder, as special equipment will have to be designed to withstand the radiation.