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(Nuclear Energy)
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Japan’s nuclear crisis provoked doubts, fears, and hysterical outbursts. Headlines like ‘nuclear explosion’ and ‘apocalypse’ didn’t help. Neither did the industry’s history of secrecy and cover-ups. 
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So here’s the science behind some of the jargon you may have encountered since the March 11, 2011 tsunami  pushed the Fukushima Daiichi nuclear power plant to the edge of catastrophe.
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<h3>How does a nuclear reaction happen?</h3>

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Power plant nuclear reactions are ‘fission’ reactions. Fission is the splitting up of the nucleus of an atom of a heavy element—usually uranium or plutonium. It is done by bombarding atoms with neutrons.
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Fission generates huge amounts of energy in the form of heat. It also generates radioactive gamma rays and neutrons that zoom around the reactor core causing more fission as they collide with other atoms, thereby creating a ‘chain reaction’.
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In a nuclear power station the chain reaction is controlled by absorbing some of the neutrons produced. In a nuclear bomb, the chain reaction causes an explosion.
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<h3>How does a nuclear reactor generate power?</h3>
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Nuclear fission in the reactor core generates heat. A coolant that circulates around the core takes in that heat which is then used to boil water. The resulting steam turns turbines, which drive electric generators.
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There are several different types of reactors in use. Most use water as a coolant, like the one in Fukushima. Other reactor types use liquid metal, gas or molten salt.
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<h3>What does the reactor core do?</h3>
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This is the central part of the nuclear reactor. Reactors consist of several structures that sit inside one another. In the middle is the core, it is enclosed by a reactor vessel which is, in turn, enclosed by a containment vessel, which sits inside the building that is visible from the outside.
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The core contains the fuel of the nuclear reaction and also any ‘moderator’ such as water or graphite that helps control nuclear fission by slowing down the neutrons produced.
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The Boiling Water Reactor (BWR) at the Fukushima power plant uses water as both a moderator and a coolant. The water boils inside the reactor core itself to create steam.
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<h3>How does the cooling system work?</h3>
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It depends on the design of the reactor. Reactors like the ones at Fukushima were designed in the 1960s and rely on pumps, valves and motors to get cooling water around the system. These pumps failed after the March 11 earthquake and tsunami knocked out power to the plant.
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More modern designs have fewer pumps, valves and motors and tend to rely on passive safety features that use natural forces such as gravity, circulation or evaporation rather than relying on active systems that need electrical power to work.
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<h3>What are fuel rods?</h3>

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These are long metallic tubes containing the uranium or plutonium fuel pellets that sustain the nuclear reaction. These rods are loaded into the reactor core. In Fukushima the reactors use uranium oxide fuel pellets contained in zirconium alloy tubes.
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Because they become so hot during nuclear fission, and because zirconium reacts with steam producing more heat, fuel rods must be covered by a coolant such as water or they may overheat.
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<h3>How is the reactor shut down?</h3>

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Control rods (often graphite) are lowered into the reactor core beside the fuel rods. Control rods are designed to absorb the neutrons generated by nuclear fission so that the chain reactions within the core can be slowed down or stopped.
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When the March 11 earthquake struck, control rods were automatically lowered into the Fukushima reactors, stopping the chain reactions.
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<h3>What is a ‘meltdown’?</h3>

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If they are not covered by water the fuel rods will heat up because of the fission process. This heat can melt the fuel rods.
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In Fukushima the fuel rods were exposed to air because the water in the core boiled off but more water couldn’t be pumped in because of the pressure of the steam.
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The rods heated up, and the zirconium casings were damaged because of the steam, leaking radioactive gases and hydrogen. Hydrogen build-up almost certainly caused the explosions at the reactors.
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This ‘partial meltdown’ may not have damaged the nuclear fuel itself. In the worst-case scenario of a full meltdown, the fuel pellets melt and fall through the bottom of the reactor vessel onto the floor of the containment vessel.
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<h3>What is a containment vessel?</h3>

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A sealed unit made of steel and reinforced concrete that contains the reactor vessel. This is the last barrier to radiation leakage into the outer building.
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In Fukushima, some of the outer buildings have been wrecked by explosions so the containment vessels are exposed to the environment. At Chernobyl, there was no containment vessel.
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What’s the difference between radiation and radioactivity?
Radioactivity is the decay of the nucleus of an unstable atom, turning that atom into a new form, or isotope, of the same element or a different element altogether. Unstable atoms exist naturally (as in granite rocks) and decay in their own time, but nuclear fission is an accelerated form of radioactive decay.
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For example, decaying uranium can turn into cesium or iodine: radioactive particles of both these elements have been detected around the Fukushima nuclear power plant.
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Radioactive decay emits radiation, electromagnetic waves or sub-atomic particles like alpha particles, beta particles and gamma rays.
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In Fukushima, the ultimate fear is that the radioactive products of fission escape and repeatedly give off radiation as they decay again and again over many years.
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<h3>When is radioactivity dangerous to human health?</h3>

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When it is large doses of ‘ionising’ alpha particles, beta particles and gamma rays: this radiation can create DNA-disrupting ions within the body possibly leading to damage and mutation of cells.
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Radiation is a fact of life, arising from the earth's crust and from cosmic radiation. A typical human receives 2 millisieverts of this ‘background radiation’ a year but it can be as high as 50 millisieverts a year in some parts of the world. A spinal X-ray delivers about 1 millisievert of radiation.
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Only when radiation levels get to several hundred times the background radiation level is there a threat to human health.
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In Fukushima levels as high as 400 millisieverts per hour have been recorded, 20 times the yearly recommended limit for nuclear power plant workers, forcing the temporary evacuation of the plant.
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But radiation can dissipate quickly, which is why the levels of radiation measured at the plant have fluctuated so wildly, sometimes falling back to 20 microsieverts per hour - only a few times more intense than the radiation experienced on a passenger jet.
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How severe is the Fukushima incident compared to other nuclear accidents?
The International Nuclear and Radiological Event Scale (INES) developed by the International Atomic Energy Agency (IAEA) identifies the severity of nuclear incidents. It goes from Level 1, which indicates very little danger to the general population, to Level 7, a "major accident" with a large release of radioactive material and widespread health and environmental effects.
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According to the Institute for Science and International Security (ISIS) in Washington D.C. the Fukushima incident has gone from an initial Level 4 accident to a Level 6 incident, between the Level 5 of the Three Mile Island reactor meltdown in 1979 and the Level 7 of the Chernobyl explosion in 1986.
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Sources: World Nuclear Association, Institution of Engineering and Technology, International Atomic Energy Agency, BBC, Kyodo News, New York Times via Allianze SE
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A Beginner’s Guide to Nuclear Power