Explainer: What’s Happening Inside Japan’s Damaged Nuclear Reactors

As emergency workers struggle to pump sea water into the damaged reactors at the Fukushima power plant in Japan, the water inside the overheated core is boiling off faster than they can replace it. That steam from the boiled water has increased pressure inside the reactor, and as the water level dropped, they left fuel rods exposed to air.

The lower the water level drops, the higher the danger of rods melting in the rising heat.

Water levels dropped precipitously Monday inside one of the reactors at Fukushima, leaving the uranium fuel rods completely exposed twice and raising the threat of a meltdown. It happened just hours after a hydrogen explosion tore through the building housing a different reactor.

Water levels were restored after the first drop, but the rods remained partially exposed late Monday.

If attempts to cool the fuel rods fail, and the heat causes their casings to melt, radioactive materials could leak into the reactor. Scientists worry that those materials could melt into one large mass and burn through the floor of the reactor.

The reactor is housed behind two layers of concrete walls that are meant to contain radiation. A similar design protected U.S. citizens from major exposure near the Three Mile Island nuclear plant when it malfunctioned in 1979.

A representative of the Tokyo Electric Power Company told Japan’s NHK television that some radiation has been detected near the affected reactor. U.S. officials say that 17 personnel from the aircraft carrier Ronald Reagan tested positive for low levels of radiation after a helicopter rescue mission near Fukushima.