What is the worst scenario for the Japan's nuclear incident?

It can't be worse than Chernobyl's disaster. It can't even be as bad as Chernobyl. I wish the media would get some scientific input before they say that. Some of them do actually, if you scroll past the first page of sensationalism and get to the part at the end where they have physicists comment.

Chernobyl had a very poorly designed reactor. It used graphite, instead of water, to slow down neutrons. The use of graphite makes a nuclear reactor prone to sudden increases in heat. At Chernobyl, one of these sudden increases occurred. The operators attempted to shut down the reactor when they realized, but their control rods were also poorly designed, causing the reactor to heat up first while they were inserted.

The reactor at Chernobyl heated out of control in a matter of seconds. This caused the coolant water in the reactor to immediately flash into steam, which caused a massive build up of pressure, which caused the reactor itself to explode.

At the Japanese plant, the reactor is not going to to become superheated in a matter of seconds. As long as the workers are paying attention, they will be able to vent any steam from the reactor before the pressure increases to a dangerous level. So the Japanese reactors aren't going to explode, unless the workers all give up and go home.

Graphite is also flammable. After Chernobyl exploded, the graphite from the reactor caught on fire and burned for hours, releasing massive amounts of radiation into the air. Obviously, the water the Japanese are using will not burn.

Then the Soviets attempted to hide the accident. No information was given to nearby residents until Sweden detected a radiation increase 36 hours later. That's when the Soviets began evacuating people. So the tens of thousands of people who were living near the plant were exposed to harmful amounts of radiation for over a day before the government bothered to say anything.

Most of the people living near the Japanese plant have already been evacuated, so they won't be exposed to significant amounts of radiation. (Anyone who is staying has refused to leave. Obviously, that's their choice, and the nuclear industry and Japanese government aren't responsible for their failure to evacuate.)

The worst-worst case scenario is that there is a complete meltdown with loss of containment. The resulting steam explosion (that will occur when the melted fuel rods impact the water table) will spread large quantities of radiation in the area around the plant. (By large quantities I don't mean the type that will kill you right away; that will likely only occur at the plant itself. I mean quantities that are likely to cause a significant increase in cancer.) Exactly how big that area is depends on how strong the explosion is, how the winds blowing, what type of soil is underneath, and so on. It's unlikely that it would be a larger area than Chernobyl.

In a worst-worst case scenario, the wind blows the radiation south to Toyko, and the city has a minor increase in cancer over the next couple of decades. There would be food and water shortages, since anything that would be sourced from the contaminated area will be unsafe to consume.

Any effect on other nations would be minimal, at the worst.

I have no idea where the 48 hour figure came from. I suspect that someone made it up.

That depends on if it reaches a "core-on-the-floor" type accident. At that point what will happen is no one knows as that has never happened before. The location of the plant is right next to the ocean. Half of the world's oxygen is produced via phytoplankton photosynthesis in the ocean.

The worse scenario is that the the meltdowns are not contained and the wind is blowing exactly toward Tokyo affecting millions of people and trillions in commerce.

the same what the first person said

well leaks in ocean kills all fish washes on shore and beaches we play in it we die so it will not be good

Nuclear Meltdown and fallout in each of the 6 reactors, which would be 6x worse than Chernobyl. Cernoby's disaster affected only 1 reactor. I repeat...1 reactor.