Japanese nuclear reactors

The reactor core consists of a series of tubes or zirconium metal rods containing uranium fuel pellets stored in which engineers call fuel equipment.

Water is pumped from the rods to keep them fresh and to create steam that drives a turbine generating electricity.

The cooling of support had problems several times during the past three days in reactors 1, 2 and 3 at the Fukushima plant.

In the normal operation of a reactor, high energy neutrons of uranium fuel and break hit atoms in a chain reaction that generates heat, new radioactive elements such as strontium and cesium, and new neutrons which continue the process.

The chain reaction stopped a few seconds of the earthquake in all nuclear reactors in Japan, including those most affected, and that turn off automatically, control rods made of boron were inserted into the fuel, which absorbed neutrons.

However, the natural breakdown of radioactive materials in the reactor core continues to produce heat, so-called waste heat, falling to a quarter of its original level during the first hour and then slowly disappears.

Normally this heat is removed by cooling pumps at the plant in Fukushima lost emergency power supply because of the earthquake, tsunami, or both.

Emergency workers try to cool the interior core of the reactor and residual heat removal by pumping sea water inside them. Boric acid added to seawater to try to stop further nuclear reactions, as an additional precaution.

The cooling of the reactors is important because although they have stopped the chain reaction, there is still enough heat to melt the metal rods that surround the uranium fuel. If these get hot enough, they react chemically with the surrounding water, producing explosive hydrogen gas.

It was that hydrogen gas which caused the two explosions at the plant in Fukushima, in Unit 1 on Saturday and in the reactor 3 on Monday, according to experts and officials.

Engineers attempted to vent the hydrogen into the atmosphere, which also contributed to some degree of local radiation because the gas contained small amounts of radioactive particles.

The reactor core is inside a thick steel container surrounded by a concrete containment structure. Around the whole building is more open to a fairly thin coverage that is not given an important structural function.

The hydrogen explosion only damaged the outer building that collapsed, not internal structures, officials said.

There remains the risk that underlying the nucleus, which is what happened at Three Mile Island in Pennsylvania in 1979. In that case, the site would be sealed permanently.

Chernobyl in 1986 was a different situation where the control rods failed to control the fission chain reaction, and this led to an explosion that destroyed the reactor, which shed radiation that poisoned to Ukraine and Europe in the worst civilian disaster in history world.