Readings just above the ground near a set of tanks at the Fukushima Daiichi plant showed radiation as high as 2,200 millisieverts (mSv), the Nuclear Regulation Authority (NRA) said on Wednesday. The previous high in areas holding the tanks, 1,800 mSv, was recorded on Saturday.
Both levels would be enough to kill an unprotected person within hours. The NRA has said the recently discovered hotspots are highly concentrated and easily shielded.
The tanks sit on a hill above the Pacific Ocean at the Fukushima plant, which was devastated by a tsunami generated by a massive earthquake in March 2011, triggering the worst nuclear crisis since Chernobyl a quarter of a century earlier.
The disaster created fuel-rod meltdowns at three reactors, radioactive contamination of the air, sea and food and resulted in the evacuation of 160,000 people in the area, north of Tokyo.
The rising radiation levels and leaks at the plant have prompted international alarm and the Japanese government said on Tuesday it would step in with almost $500 million of funding to fix the growing levels of contaminated water at the plant.
The plant operator, Tokyo Electric Power Co, also known as Tepco, said last month water from one of hundreds of hastily built tanks was leaking.
The NRA later raised the severity of the leak from a level 1 “anomaly” to a level 3 “serious incident” on an international scale for radiation releases.
Tepco, Japan’s biggest utility, has been criticized for a series of mishaps including its admission, after repeated denials, that contaminated water was flowing into the Pacific from another area of the plant.
That was followed by the leaks from above-ground tanks used to store contaminated water after its is flushed over the melted uranium fuel rods to keep them cool.
Tepco said on Wednesday the new readings taken at the plant were not related to a 6.9 magnitude earthquake off southern Japan.