The blast injured 11 people, one seriously.
The string of earthquake- and tsunami-triggered troubles at the Fukushima Daiichi plant began with the failure of the primary and back-up cooling systems, necessary to keep reactors from overheating.
On Saturday, a similar explosion occurred at unit 1. Trace amounts of radioactive elements cesium-137 and iodine-131 have been detected outside the plant.
The U.S. Seventh Fleet said on Monday that some of its personnel, who are stationed 100 miles offshore from the Fukushima Daiichi plant, had come into contact with radioactive contamination. The airborne radioactivity prompted the fleet to reposition its ships and aircraft.
Using sensitive instruments, precautionary measurements were conducted on three helicopter aircrews returning to USS Ronald Reagan after conducting disaster relief missions near Sendai. Those measurements identified low levels of radioactivity on 17 air crew members.
The low level radioactivity was easily removed from affected personnel by washing with soap and water, and later tests detected no further contamination.
Like the Saturday explosion at unit 1, the blast at unit 3 took place after a buildup of hydrogen was vented by the reactor. The hydrogen was produced by the exposure of the reactor’s fuel rods and their zirconium alloy casing to hot steam.
In normal conditions, the fuel rods would be covered and cooled by water.
The explosion occurred as Tokyo Electric entered day four of its battle against a cascade of failures at its two Fukushima nuclear complexes, using fire pumps to inject tens of thousands of gallons of seawater into two reactors to contain partial meltdowns of ultra-hot fuel rods.
The tactic produced high pressures and vapors that the company vented into its containment structures and then into the air, raising concerns about radioactivity levels in the surrounding area where people have already been evacuated. The utility said that at one of the huge, complicated reactors, a safety relief valve was opened manually to lower the pressure levels in a containment vessel.
But the limited vapor emissions were seen as far less dire than the consequences of failure in the fight against a more far-reaching partial or complete meltdown that would occur if the rods blazed their way through the reactor’s layers of steel and concrete walls.