Record levels of radiation have been recorded at the damaged Fukushima Daiichi plant reactor, just months after the nuclear accident resulting from the earthquake and tsunami in March.
The Tokyo Electric Power Company (TEPCO) reported that Geiger counters - a hand-held device used to measure radiation - registered their highest possible reading at the site on Monday.
TEPCO said that radiation exceeding 10 sieverts [10,000 millisieverts] per hour was found at the bottom of a ventilation stack standing between two reactors.
Al Jazeera's Aela Callan, reporting from Japan's Ibaraki prefecture, said the level recorded was "fatal to humans" but that it was contained just to the plant's site. However, scientists are planning to carry out more tests on Tuesday.
"Authorities are working on the theory though that it has come from those initial hydrogen explosions that we've seen at the plant in the days after the earthquake and tsunami," she said.
"It is now looking more likely that this area has been this radioactive since the earthquake and tsunami but no one realised until now."
On Tuesday, TEPCO said it found another spot on the ventilation stack itself where radiation exceeded 10 sieverts per hour, a level that could lead to incapacitation or death after just several seconds of exposure.
The company used equipment to measure radiation from a distance and was unable to ascertain the exact level because the device's maximum reading is 10 sieverts.
While TEPCO said the readings would not hinder its goal of stabilising the Fukushima reactors by January, experts said that worker safety could be at risk if the operator prioritised hitting the deadline over radiation risks.
"Radiation leakage at the plant may have been contained or slowed but it has not been sealed off completely," Kenji Sumita, a professor at Osaka University who specialises in nuclear engineering, said.
"The utility is likely to continue finding these spots of high radiation.
"Considering this, recovery work at the plant should not be rushed to meet schedules and goals as that could put workers in harm's way.
"We are past the immediate crisis phase and some delays should be permissible."
Workers at Daiichi are only allowed to be exposed to 250 millisieverts of radiation per year.
TEPCO, which provides power to Tokyo and neighbouring areas, said it had not detected a sharp increase in overall radiation levels at the compound.
"The high dose was discovered in an area that doesn't hamper recovery efforts at the plant," Junichi Matsumoto, a TEPCO spokesman, said on Tuesday.
Although it is still investigating the matter, TEPCO said the spots of high radiation could stem from debris left behind by emergency venting conducted days after the March 11 earthquake and tsunami that crippled the plant.
Source: Al Jazeera and agencies