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:

Joseph Cirincione, who was the Director for Non-Proliferation at the Carnegie Endowment for International Peace and author of Deadly Arsenals: Nuclear, Biological and Chemical Threats.

Dr. Michiko Kaku, a theoretical physicist at the City University of New York.

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.

Japan's Nuclear Reactors: Essential Questions

CIRINCIONE: A lot depends on weather and wind conditions. If the wind is blowing out to the ocean, as it usually does, then most of the contamination goes out there. If the wind shifts south, well then, some of the most heavily populated areas of Japan are at risk including Tokyo itself…

It is possible the fires could be so hot that it could send radioactive particles into the upper atmosphere, carrying it across the Pacific. It would hit the southern parts of Alaska and the west coast of Canada and the United States in about ten days.

How dangerous would the levels be?

KAKU: The radiation would not kill immediately, but it would eventually cause cancer because it is breathed in and ingested. So the 12-mile evacuation zone would be completely useless.

CIRINCIONE None of this radioactivity would kill people in the continental United States, but it would expose people to the increased risk of cancer. Think of it like asbestos. You don't want to breath it, it doesn't kill you right away, it just makes your chance of getting cancer that much higher.

When is the point when the danger is passed?

KAKU: Even if the sea water successfully prevents this scenario, we are talking weeks to months to clean up this accident. The heat from the core will last for weeks, and cleanup operations are hampered by the chaos caused by the earthquake. Also, since these fission byproducts are water-soluble and would get into crops, this could also ruin the agricultural economy.

CRINCIONE: We have been talking about either a meltdown or shutdown but we have a third possibility that is emerging. That these reactors staying in a crisis state, venting radioactive steam for an extended period of time, maybe weeks — that means that the people who have evacuated, 200.000 and growing so far, can't return. You could be looking at hundreds, maybe thousands of [square] kilometers that would be uninhabitable for weeks, maybe months….

Tokyo is far away from these reactors that are in danger now, but we just had a U.S. carrier that is a hundred miles from this site redeployed to get even further away. A hundred miles is about Tokyo suburbs' distance. You've got to be a little worried if you are in Tokyo.

Japan's Nuclear Reactors

CIRINCIONE: We will be out of danger when the temperatures inside these reactors drop. Then we will know we have actually stabilized the reactions… The impact of a meltdown is almost immediate and it depends how strong the winds are, but people would be exposed immediately in the area and then within hours in nearby cities.