Four of the troubled Fukushima Dai-ichi plant’s six reactor units have seen fires, explosions or partial meltdowns in the week since the tsunami. While the reactor cores where energy is generated are a concern, water in the pools used to store used nuclear fuel are also major worries. Water in at least one fuel pool — in the complex’s Unit 3 — is believed to be dangerously low, exposing the stored fuel rods. Without enough water, the rods may heat further and spew out radiation.
"Dealing with Unit 3 is our utmost priority," Chief Cabinet Secretary Yukio Edano told reporters.
Frantic efforts were made Thursday to douse a number of units with water, and authorities were preparing to repeat many of those efforts Friday. However, they said they would not continue helicopter drops of water. Televised footage of the air drops Thursday appeared to show much of that water blowing away.