Nuclear Disaster: 3 Essential Questions About Japanese Reactors

March 14, 2011 -- Officials and energy workers in Japan struggled today to control the reactors at the Fukushima Daiichi plant, badly damaged in Friday's earthquake. Officially, the Japanese government seemed focused on keeping people calm even as they evacuated residents from near the nuclear reactors.

For the latest news on the reactors, click HERE.

ABC News asked three of the most essential questions of ABC's nuclear experts:

What is worst-case scenario in Japan?

CIRINCIONE: The worst case is multiple reactor meltdowns. That means that the reactor core, the fuel, gets so hot that it fuses together into molten lava and that bursts right through the reactor container vessel, and right through the concrete containment box that is built around the reactor. Then radioactivity goes in the air, in the ground, in the water. It is a mess.

The danger is that it happens, not just at one reactor but two or three reactors at once. That means you have an arch of radioactivity over hundreds of squared kilometers contaminating, perhaps, possible thousands of square kilometers.

KAKU: The worst-case scenario is a steam/hydrogen gas explosion which blows the reactor vessels apart, sending uranium dioxide fuel rods and radioactive debris into the air. This might happen if the core is fully exposed for a few hours, which is a distinct possibility. This is what happened at Chernobyl, when such an explosion blew about 25 percet of the core's radioactive byproducts into the air.

How far can the radiation travel in the worst case?

KAKU: [In the event of fuel rods and reactive debris being sent into the air,] a plume containing iodine, cesium, and strontium would rise above the reactor. The winds would blow it 20-50 miles downwind from the site. The plume, like a lighthouse beam, would be pencil-thin, about 15 degrees, and would sweep back and forth depending on the wind.