The plant's best hope may be restoration of outside power, which could allow workers to restart cooling operations for the reactors and the cooling pools, provided they have not been too severely damaged. NHK TV in Japan said TEPCO hoped to run power lines into the site from another power plant Thursday. TEPCO wants to finish the installation "as soon as possible after reviewing the procedures in order to keep the workers' radiation exposure to a minimum," NHK reported.
The problem of spent fuel storage
Nuclear reactor operators must store spent fuel removed from reactor cores for several years at least, in large pools at reactor sites until the remaining heat from the uranium fuel cools sufficiently. In the United States, much of the fuel units remained stored underwater in pools but some are removed for storage in large casks.
A report to Congress in 2006 by a National Research Council panel investigating terrorist threats to spent fuel storage concluded that "under some conditions," if a pool were partially or completely drained, that "could lead to a propagating zirconium cladding fire and the release of large quantities of radioactive materials to the environment."
The fuel rods in most cases consist of uranium dioxide pellets encased in zirconium alloy tubes or cladding. Heat from uncovered fuel could ignite the zirconium cladding, and the super-heated metal could then oxidize steam, releasing hydrogen and oxygen. Leaks of hydrogen from damaged reactors at units Nos. 1 and 3 is blamed for explosions at the tops of the outer, secondary containment structures, and an explosion within the No. 2 primary containment structure. Officials said hydrogen released from the spent fuel pool at No. 4 may have caused a fire there.
The National Research Council report said that as pool water levels drops, through a leak or other causes, temperatures of the fuel rods increase, accelerating oxidation of the cladding and the production of hydrogen gas. The reactions can become self-sustaining at high temperatures, if there is sufficient water or oxygen present, causing the cladding tubes to rupture. "The result could be a runaway oxidation reaction" and the release of radioactive fission gases" and some of the radioactive fuel material.
The mix of radioactive particles released into the atmosphere would vary with the length of time expired since the fuel units were moved from reactors to the pools. A particular concern is the spread of Cesium-137, which can enter the body or contaminate agricultural products, Lyman said.
Neff said the consequences of exposed spent fuel at the Fukushima Daiichi plant are not clear. If a pool is completely dry, the oxidized fuel units may be lying on the pool's bottom. "I think the zirconium fire [at unit No. 4] is probably over."
But the resulting radiation could be so high, "it would be almost impossible to get anyone in there," to continue supplying water to the damaged reactors, he said. That could put the cores in a meltdown scenario that could lead to an explosion within the core or a leak of puddle fuel from the bottom of the reactor -- "doomsday" scenarios that create the ultimate test of a reactor's designed defenses, he said.
"There is a possibility if the fuel is in the right configuration, has been out of the reactor long enough, and is sufficiently air cooled that a fuel fire would not start," said one U.S. expert. "If there were a fuel fire, the radiation levels off site would go off the charts -- which they have not," he said.
Tom Clements, southeastern nuclear campaign coordinator of Friends of the Earth, said the fuel in pool No. 4 was hotter than in the plant's other pools because it had more recently been transferred into the pool. "So, it may well have boiled faster, and it had more fuel in it."