World Eyes Japan's Nuclear Crisis
[UPDATE 3/21] Heroic efforts are being made to contain the damage, but radiation levels at the reactors have reached the highest levels, putting workers at risk. Leaked water samples had radiactivity levels 100,000 times normal levels. It may take several months to bring the reactors back to normal
Like a recurring nightmare, Japan is once again at the center of a possible nuclear disaster. The country is all too familiar with the effects of nuclear radiation after the bombing of Hiroshima and Nagasaki that ended World War II. Now, the twin earthquake and tsunami that struck the country last week have unleashed another threat -- the possibility of a nuclear meltdown.
Fires and explosions rocked four of the six reactors, leading Japan to evacuate people within a 12 mile (20 kilometer) area. Japan has been scrambling to restore electricity to cool the radioactive material in the nuclear reactors.
What caused the nuclear reactors to fail and what does a meltdown mean? Lets take a step back at how a normal reactor works.
Fission, a chain reaction
Over the past few decades, nuclear energy has emerged as a replacement for non-renewable sources such as coal and oil. At the heart of the reactor is a large vessel that contains radioactive rods made of Uranium 235, an isotope of Uranium -- isotopes are different flavors of the element, each with a different number of neutrons.
This Uranium isotope has a special property and can undergo a chain reaction called nuclear fission. That is, its atom can be made to split, and the free neutrons that are released from the nucleus bombard other Uranium atoms causing them to split and so on. Splitting of the atom releases a tremendous amount of energy in the form of heat and radiation. This heat energy converts water to steam, causing a turbine to rotate and generate electricity.
The amount of energy generated by fission is millions of times greater than that from an equivalent amount of fuel such as gasoline! Hence, fission has to be done in a controlled way -- the Uranium rods are kept immersed in water to keep them cool, special control rods are inserted to absorb the free neutrons and slow down the fission, and the entire unit has to be enclosed in a steel containment vessel to prevent radiation from leaking.
What went wrong in Fukushima?
When the earthquake struck, three of the reactors that were working were immediately shut off. However the intense heat and radiation that is generated inside requires water for cooling. Unfortunately, the pumps to circulate water and generators for backup electricity stopped working.
Then came the explosions -- one theory is that increased radiation caused water molecules (H2O) to split into Hydrogen and Oxygen. The build-up of hydrogen gas is believed to have caused the explosion which ripped the outer concrete walls. The extent of damage to the inner containment vessel is not clear.
Japanese authorities have been spraying water using helicopters and meanwhile, working hard to restore electricity. Without cooling, the Uranium rods could melt down, releasing high levels of radioactive materials into the atmosphere. At the time of writing, two of the six reactors are under control.
How much is too much?
Many ordinary heroes have emerged in Japan -- people who have put their lives at risk to protect their countrymen. Normally, people are exposed to 3-6 millisieverts (unit of measurement) of radiation a year, but workers at the plant have been exposed to 100-150 mS. Too much radiation can cause hair loss, cancer and in the worst case, death. You may have seen people on television wearing suits and using special Geiger counters to check radiation levels on people leaving the area.