Tokyo Electric Power Co. (TEPCO) said Monday it had monitored record high radiation at the Fukushima Daiichi nuclear power plant crippled by the March 11 quake and tsunami.
TEPCO said radiation levels reached at least 10 sieverts per hour near the debris left between the number one and number two reactors of the plant at the centre of the ongoing nuclear crisis.
The previous record was three to four sieverts per hour monitored inside the number one reactor on June 3.
"Three plant workers were exposed to a dosage of four millisieverts while they were monitoring radiation," a TEPCO spokeswoman said. "We are still checking the cause of such high levels of radioactivity."
The government and TEPCO say they remain on target to bring the reactors to a safe state of cold shutdown by January at the latest now that a water circulation system has been established.
Efforts to stabilise the worst nuclear crisis since Chernobyl 25 years ago have continued since a 9.0 magnitude earthquake triggered a tsunami, sparking reactor meltdowns at the plant and spewing radiation into the environment.
The government has said radiation levels around the plant, which lies 220 kilometres (136 miles) from Tokyo, had fallen to "two-millionths" of the peak recorded March 15.
Tens of thousands of people remain evacuated from homes, businesses and farms in a no-go zone around the plant.
Explore further: Israeli port city closes 5 factories over cancer fears