The crisis at the Fukushima Daiichi nuclear power plant reached a "highly critical" condition in the first two weeks after a massive earthquake and tsunami crippled its reactors, a key aide to Prime Minister Naoto Kan said Saturday.
"We were in a very critical situation, where we had to consider several worst-case scenarios," premier Kan's special adviser Goshi Hosono said in a television program.
He said that the situation was today much more stable as plant operator Tokyo Electric Power Co. (9501.TO) has managed to slowly bring down the temperatures at the reactors to safer levels.
"Compared with where we were, we have little by little gained control back over the nuclear plant. Now it's been more than a month, and the situation is much stabler," Mr. Hosono said.
The Atomic Energy Society of Japan released a report on its assessment on the Fukushima Daiichi reactors Friday, saying that the nuclear fuel has partially melted and that the molten part apparently has settled in granular form at the bottom of the pressure vessel.
It said this posed less of a threat than if the material had massed together but said that a long interruption of cooling for two or more days could create new serious risks. As major aftershocks continue to hit the region, there are concerns that power supplying the temporary pumps could again be disrupted.
Mr. Hosono said that one of the greatest issues at present is how to deal with the thousands of tons of radioactive water that has been used to cool the reactor cores and is now radioactive -- in some cases 100 million times the radioactivity normally found in a nuclear plant.
Nuclear and Industrial Safety Agency official Hidehiko Nishiyama said setting up a system to purify contaminated cooling water is the key to a long-term solution to the crisis.
When the earthquake and tsunami hit on March 11, the complex was left without electricity. In the initial days, the nuclear fuel was left partially or sometimes even fully exposed, according to regulators, creating dangerously high temperatures and a buildup of hydrogen that led to massive explosions at three of the four main units.
Since then, various work forces including Japan's Self Defense Force have poured tens of thousands tons of waters into the reactors. Temperatures within the reactor now range from 90-200 degrees centigrade, well below the 1,000 plus degrees required for a chain nuclear reaction to take place.
Government and Tepco officials have said there was no choice but to pour water as cooling down the fuel rods was and is the priority. However, this has created a new problem of a large amount of water contaminated with radioactivity.
As the most urgent dangers have receded, operators have begun to face the issue of how to deal with the water, estimated at 50,000 to 60,000 tons, or enough to fill 20 to 24 olympic-sized swimming pools.
The government Friday released an analysis of the 10,000 tons of water it had expelled from the plant over the previous week to make room for the more radioactive waste water. It said that since much of the water had come from the initial tsunami, it had only a low level of radioactivity and after its release had no discernable impact on the marine environment.
This was not the case with the highly toxic water that continues to come from the No. 2 reactor, believed to be the most heavily damaged with an estimated 70% of its fuel rods partly or completely melted. A small leak of this water had caused a spike in radioactivity in the nearby ocean and toxicity levels have slowly fallen since the leak was plugged on April 6.
To restrict the flow of potentially polluting water out of the plant, Tepco has put up screens around the ocean inlets at the plant and on Friday began use of a mineral agent known as zeolite that can absorb radiation. It has applied the mineral in the ocean and said it would also use it to treat other water at the plant.
While China and South Korea expressed strong concerns about the release of the water, Russia has taken a more reserved view.
"We understand that Japan did not have any other choice but to release the radioactive water," Vladimir Uiba, head of Russia's Federal Medical-Biological Agency, said at a Friday press conference in the Russian Embassy in Tokyo. "We would have done the same had we been in the same situation."