Workers at the crippled plant found an oil spill on Tuesday in the sea near reactors 5 and 6, which were shutdown when the earthquake and tsunami struck on 11 March, the Tokyo Electric Power Company said. The spill was contained by an oil fence, a Tepco spokesman, Taichi Okazaki, said.
An explosion workers heard at reactor 4 on Tuesday was likely to have come from a gas tank and did not cause any additional radiation leaks, Okazaki added. The cause was being investigated.
The main problems at Fukushima Daiichi involve reactors 1,2 and 3, where the fuel cores have largely melted. Scientists and government officials say the reactors are short of a full meltdown, in which the fuel breaks through the bottom of the outer container.
Workers have been trying to get the reactors under control after the tsunami destroyed backup power generators, halting crucial cooling systems that managed the fuel temperature.
In the immediate days after the tsunami, several explosions – larger than Tuesday’s – at the plant scattered highly radioactive debris into the environment. The plant has also leaked tonnes of radioactive water, which officials are promising to clean up.
Tepco has promised to bring the plant under control by January, but fears are growing that is too optimistic.
Concerns about the risks workers face at the plant surged this week when Tepco said two workers might have exceeded a radiation exposure limit. The government had raised the limit for men soon after the earthquake and tsunami set off the crisis at the plant.
Tepco has been instructed to check internal exposure levels of all workers who may have worked closely with the pair and remove all of them from plant duties until the checks are made, the health minister, Ritsuo Hosokawa, said.
“Workers who had engaged in similar plant work also might have been internally exposed,” the minister added.
Further testing is being done on the two men who were responsible for central control rooms of two reactors, and the company has said they do not show immediate health problems.
More than 2,000 workers tested so far did not have exposure levels beyond the limit, but hundreds more are waiting to be tested, another Tepco spokesman, Takeo Iwamoto, said.
Also on Tuesday, the company said it has finished its promised payment of preliminary compensation of ¥1m (£7,442) for 50,000 households affected by the nuclear crisis adding that it had also started temporary compensation payment for farmers to cover their crop damages.