NEW YORK/VIENNA (Reuters) - Pumping seawater into troubled nuclear reactors in Japan should keep them from a catastrophic full-scale meltdown, but conditions are still so volatile that it is far too early to declare the emergency over, nuclear experts said.
It is probably the first time in the industry's 57-year history that seawater has been used in this way, a sign of how close Japan is to facing a major nuclear disaster following the massive earthquake and tsunami on Friday, according to the scientists.
Tokyo Electric Power (TEPCO) workers on Sunday were pouring seawater into two reactor cores at the coastal Fukushima Daiichi power plant and were considering using seawater on a third. Authorities have been forced to vent radioactive steam into the air to relieve pressure in the plant and reactors at the company's nearby Daini plant are also troubled.
"I am not aware of anyone using seawater to cool a reactor core before. They must be desperate to find water and the seawater was the only thing nearby," said Richard Meserve, former chairman of the U.S. Nuclear Regulatory Commission and president of the Carnegie Institution, in an interview on Sunday.
He said that suggests the company has decided it will sacrifice the reactors altogether, in what has become the worst nuclear accident since the Chernobyl disaster in Ukraine in 1986.
The method being used to regain a semblance of control of the reactors smacks of last-resort desperation, said Robert Alvarez of the Institute for Policy Studies and formerly a senior policy adviser at the U.S. Department of Energy.
"I would describe this measure as a Hail Mary Pass but if they succeed, there is plenty of water in the ocean and if they have the capability to pump this water in the necessary volume and at the necessary rates...then they can stabilize the reactor," said Alvarez in a press conference on Saturday.
In a sign of how volatile the situation is, the Union of Concerned Scientists said in a statement on Sunday in Washington that it fears the situation "took a turn for the worse as serious problems developed" at the Unit 3 reactor at Daiichi.
It said that statements from Tokyo Electric officials indicate that water levels have dropped so far that approximately 90 percent of the fuel rods in the core of the reactor were uncovered and that despite pumping in seawater the water level is still well below where it should be.
The Daiichi plant was shut immediately after the March 11 earthquake when outside power was lost. Diesel generators kept the cooling water running over the superheated uranium fuel rods in the reactor core for about an hour until water from the tsunami caused them to stop.
Without circulating cooling water, the water inside the core was heated by the rods and enough evaporated causing some to partially melt. Adding the seawater should keep the rods from melting further, the scientists said.
The fear is that if the uranium fuel rods did not cool, then they could melt the container that houses the core of the reactor, or even explode, releasing a radioactive material cloud.
The experts interviewed by Reuters cautioned that it is still far too early to definitively say that the day has been saved, especially as the information from the company and the authorities is incomplete.
But they say that with every hour that goes by, the chances of a major catastrophe is diminished -- as long as water from the sea or elsewhere, keeps reactor cores from overheating.
Japanese authorities "appear to be having enough success to have forestalled a definite core melt accident that's difficult to control," said Mark Hibbs of the Carnegie Endowment for International Peace. "After three days that is very good news."
Still, he said it was "still a touch-and-go situation."
A meltdown of nuclear fuel -- which contains most of the radioactivity -- would not lead to a major escape of potentially dangerous clouds into the air as long as thick walls shielding the reactor cores were not breached. The danger is that a full-scale meltdown could cause the pressures that could create that breach through an explosion.
The passage of time may help to reduce the danger, said Professor Richard Wakeford at the Dalton Nuclear Institute of Britain's University of Manchester.
"The reactor cores were still hot when the reactor shut down, as time goes on that radioactive decay heat will get less and the problem will get less," he said in a statement.
TEPCO officials have not publicly addressed the danger facing workers involved in the seawater operation at the three reactors. However, a statement from TEPCO Director Tsuyoshi Otani suggested there is some concern.
"TEPCO staff are currently working near and in the plants and injecting seawater and boric-acid into the reactors to cool down the core under such high radiation atmosphere in order to prevent catastrophe," he said in a press release.
Japan's Chief Cabinet Secretary Yukio Edano said there might have been a partial meltdown of the fuel rods at the No. 1 reactor at the Daiichi plant.
A German industry expert, however, said any partial meltdown "is not a disaster" and that a complete meltdown was unlikely.
Robert Engel, a structural analyst and senior engineer at Switzerland's Leibstadt nuclear power plant, said he believed Japanese authorities would be able to manage the situation at the damaged Fukushima facility north of Tokyo.
Engel was an external member of a team sent by the International Atomic Energy Agency (IAEA) to Japan after a 2007 earthquake that hit Japan's Kashiwazaki-Kariwa plant, at the time the largest such event to affect a nuclear complex.
"I think nobody can say at this time whether there is a small melting of any fuel elements or something like that. You have to inspect it afterwards," he said.
Normally, Engel said, the water level inside a reactor core is 3 to 4 meters above the fuel. If the rods are not covered by water for a short period of time then they will be damaged and a core melting is possible.
"I think they will be able to manage it ... when the (reactor) containment is intact only a small amount of radioactivity can go out, like in Three Mile Island," he said referring to the 1979 nuclear accident at a plant in Pennsylvania in the United States that badly setback the nuclear industry in the United states.
At Three Mile Island, a cooling fault led to a build-up of pressure in the radioactive core and resulted in a relatively small radiation leak.
But Wakeford, the University of Manchester professor, said the Japanese authorities were doing the right thing by evacuating people in the case the worst happens.
"If the fuel is uncovered by cooling water it could become so hot it begins to melt -- if all the fuel is uncovered you could get a large scale meltdown," he warned.
By Scott DiSavino and Fredrik Dahl