All cooling water is gone from the spent-fuel pool at one of the crippled nuclear reactors in Japan, causing the release of high levels of radiation, Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission, told members of Congress.
 
Jaczko’s testimony followed warnings by the International Atomic Energy Agency, physicians and nuclear engineers that the greatest threat to public health from the damaged Fukushima nuclear plant is spent fuel in pools of water atop of the plant’s six reactors.
 
Radioactivity has been heating the pools at three of the plant’s reactors since the plant’s cooling systems were disabled by a March 11 earthquake and tsunami, officials said. Exposed, the spent fuel rods can catch fire and melt, spewing radiation into the atmosphere, said Robert Kelley, an engineer in Vienna who used to lead the Nuclear Emergency Response at Los Alamos National Laboratory in New Mexico. Unlike the plant’s reactors, the pools aren’t encased by steel and concrete.
 
“We believe that the secondary containment has been destroyed and there is no water in the spent-fuel pool,” Jaczko said at a hearing of a House Energy and Commerce Committee in Washington. “We believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures.”
Fatal Dose
 
The pool water also shields workers. With the top of the fuel rods exposed, a worker at the pool lip would receive a fatal radiation dose in 16 seconds, David Lochbaum, a nuclear physicist for the Union of Concerned Scientists and a former NRC safety instructor, said in a conference call.
 
As water evaporates and exposes the fuel, the uranium in the rods can burn through a protective sheath emitting heat and radioactive cesium. After that, the uranium may mix with any remaining water to start an uncontrolled nuclear reaction that sends radiation into the atmosphere, scientists said.
 
“Dissolving uranium in water is the way to make a certain kind of nuclear reactor,” Kelley said in an e-mailed response to questions. “In this uncontrolled situation, the ‘reactor’ will have no human control and begin fissioning.”[/b]
 
Water in the spent fuel pool at the No. 4 reactor may be boiling, Japan’s Nuclear and Industrial Safety Agency said on March 15. Temperatures in the rod-cooling pools of the shuttered No. 5 and No. 6 reactors rose as high as 63 degrees Celsius (145 degrees Fahrenheit) at 2 p.m. yesterday from 60 degrees Celsius at 7 a.m., a Tokyo Electric official said.
Burning Fuel
 
Reactor pool No. 4 contains 142 tons of fuel that could burn on exposure to the atmosphere, Marvin Resnikoff, a nuclear physicist for Washington-based Physicians for Social Responsibility, said in a telephone press conference arranged by the group. He cited Tokyo Electric statements.
 
Japanese authorities are concerned about the condition of the pools of units 3 and 4, the International Atomic Energy Agency said yesterday. Military helicopters may be used to drop water on reactor No. 3 and workers are planning to spray water into unit 4, according to the IAEA.
 
“This is going to be a problem for months,” Resnikoff said. “Water serves as both cooling and radiation shielding and workers can’t get close to it when the water is gone.”
 
The pools contain enough radioactive cesium to make areas around the plant uninhabitable should it spread by fire and wind, Ira Helfand, a past president of Physicians for Social Responsibility. Whether contamination occurs remains to be seen, he said.
Falling Pressure
 
The crisis at Fukushima worsened yesterday when Tokyo Electric said the containment chamber at the No. 2 reactor may have been breached because pressure dropped suddenly. Clouds of steam were seen rising from the reactor building after a fire at the No. 4 Reactor. The 50 workers remaining at the plant were pulled out yesterday after radiation temporarily rose to unsafe levels. They later returned.
 
“Japan seems to be worried about the balance between the health of the workers and the safety of the site,” said Tony Roulstone, an atomic engineer who directs University of Cambridge’s master’s program in nuclear energy. “Higher radiation on the plant makes it harder for the workers to stay there and do what they need to cool it.”
 
The way to stop the spent rods from reaching this critical heat and mass is to flood their pools with water and large quantities of boric acid, similar to the mixture being used to cool the reactors, scientists said. Getting water into the fuel ponds on the roofs of the building is complicated, said Kelley.
‘Level of Desperation’
 
“If you just drop water on there you don’t know where it’s going to land, if it will disturb ponds or the reactor,” Cambridge’s Roulstone said. “It’s much better to observe what’s going on and add water; shows a level of desperation and brings in uncertainty.”
 
The risk of the fuel ponds getting hot enough to start a nuclear reaction remains remote, said Geoff Parks, a nuclear engineer, also at the University of Cambridge. The pond would need to reach about 2,200 degrees Celsius, he said.
 
Tokyo Electric is building a cable to supply power to the plant’s cooling systems, a spokesman said. The systems were knocked out by the March 11 earthquake and tsunami and the company has been pumping seawater into the reactors to keep them from melting down.
 
The highest measurement of radiation so far taken at the plant was 400 millisieverts, 20 times the annual limit for nuclear industry employees and uranium miners, according to the World Nuclear Association.

[Dernière minute] Une secousse sismique vient d’ébranler des bâtiments à Santiago, la capitale du Chili. La magnitude de 5,3 avec épicentre près de Valparaiso a été mesurée par l’institut de géophysique américain (USGS).

Confusion sur les chiffres, veuillez nous excuser, voici le correctif : Des précisions sur la baisse de radioactivité à Fukushima : un niveau de 752 microsieverts par heure a été enregistré mercredi à 17h00 (heure japonaise) à l'entrée principale de la centrale. A 05h00 jeudi, les appareils affichaient 338 microsieverts par heure. Ce niveau reste supérieur à ce qu'il devrait être sans être dangereux, selon l'agence japonaise de sûreté nucléaire.