Now there are concerns that the situation is worse than the authorities believed.
Tepco said it has discovered small amounts of xenon, a byproduct of fission, in the number two reactor at the plant and had poured in a mixture of water and boric acid, an agent that helps prevent nuclear reactions, as a precaution.
However, the operator played down the find, saying there was no evidence of a “criticality” in the reactor. The temperature and pressure at the reactor remained stable.
But nuclear expert John Large told Channel 4 News that any evidence of fission was a “concern” which raised problems for Tepco’s plans to make the plant safe by the end of the year.
He added: “It shows there are still considerable problems. It’s concerning because it shows they are not in total control of the nuclear processes in the plant – and even a small amount of fission is a concern because it can become a large amount of fission.”
It has also brought down temperatures at the three damaged reactors.
Following criticism of Tepco, Japan’s nuclear authorities and the government at the time of the incident for releasing information too slowly to the public, they are keen to now show that they are being as open as possible.
On Tuesday, to show that decontamination efforts were progressing, Japanese cabinet official Yasuhiro Sonoda drank a glass of purified water taken from the Daiichi plant after being challenged by journalists to prove it was safe.