State of spent fuel rods in storage pool remains mystery
The condition of the spent fuel rods in the storage pool at the No. 4 reactor of the crippled Fukushima No. 1 nuclear power plant has experts and government officials scratching their heads.
Fire erupted twice at the No. 4 reactor building after an explosion near the No. 4 reactor's storage pool on March 15. The outer walls of the building were considerably damaged, and the pool was left exposed. The damage was as severe as that at the No. 1 and No. 3 reactors, whose buildings were destroyed by hydrogen gas explosions.
When the Great East Japan Earthquake rocked northeast Japan on March 11, the No. 4 reactor was under regular inspection--all the fuel rods in the reactor had been moved to the storage pool. The pool contained about 1,300 partially and completely spent fuel rods, which were releasing a larger amount of heat than fuel rods stored in pools at five other reactors at the plant.
Experts believe the explosion occurred because the heat from the fuel rods warmed and evaporated water, exposing the fuel rods to the air, and the fuel cladding tubes reacted with hot water vapor and generated flammable hydrogen gas, which exploded. The experts say the surfaces of the fuel rods were destroyed in the explosion.
On Tuesday, the Tokyo Electric Power Co., the plant operator, collected water samples from the pool using a concrete pumping truck and analyzed the concentration of radioactive materials. The samples contained 220 becquerels of iodine-131, 88 becquerels of cesium-134 and 93 becquerels of cesium-137 per cubic centimeter. These concentration levels were lower than those of the contaminated water found in the turbine building of the No. 2 reactor, which is thought to have leaked radioactive materials after the magnitude 9.0 earthquake.
The industry ministry's Nuclear and Industrial Safety Agency said Thursday that the figures would have been larger if the fuel rods were damaged, indicating, as did TEPCO, that serious damage was unlikely. In contravention of the experts' opinions, the agency does not deny the possibility that the March explosion at the No. 4 reactor might have occurred because the hydrogen gas had been generated elsewhere and had migrated to the No. 4 reactor.
Decomposition of water into hydrogen and oxygen by radiation from the fuel rods is also conceivable as a mechanism of hydrogen generation. In this case, an explosion can take place, at least logically, even when the fuel rods have not been seriously damaged.
But Keiji Miyazaki, professor emeritus of nuclear reactor engineering at Osaka University, is skeptical, saying that the amount of hydrogen generated by this mechanism is too small to explode.
"There is no channel of hydrogen inflow from the No. 1 through No. 3 reactors," Miyazaki said. "It is, after all, most natural to think that the exposed fuel cladding tubes reacted with water. Some may argue that the fuel rods were not damaged seriously, but I cannot quite believe it."
In response to arguments about possible problems in TEPCO's method of water sample collection, a TEPCO official said, "It is unlikely that samples were collected erroneously from somewhere else, because we verified the water surface using a camera."
The water sample analysis results released are preliminary figures, and TEPCO plans to reconfirm the figures.
(This article was written by Tatsuyuki Kobori, Ryoma Komiyama and Keisuke Katori.)
