Nuclear Energy

Nuclear power

Nuclear power plants operate on the same principle as the thermal power station, they use the steam to generate electricity. This electricity is sent directly into the grid, because it can not be stored. It is on high voltage lines, 400 000 volts, that our electricity is walking at the speed of light. Nuclear power plants use 2, 4 or 6 nuclear reactors to heat water.
The steam generated spins the turbines which in turn lead to an alternator to ultimately produce electricity. In these power plants, the fuel is coal, oil or gas but uranium. Uranium is found throughout the Earth's crust, especially in granitic and sedimentary rocks, at levels of about 3 g/ton. It is widespread in the world and maintains its natural radioactivity due to the heat of the mantle, avoiding a sudden cooling of the planet.
In these nuclear power, uranium is placed in the pipe assemblies, inside nuclear reactors to heat water.
To extract nuclear energy, engineers used a special kernel of uranium, uranium-235 in particular, to break easily in its nucleus. Indeed its nucleus is unstable, it decays naturally, i.e. the cohesive force is not sufficient to hold the protons and neutrons.

It ejects particles and produces radioactive radiation (radioactivity) and energy.
When a neutron is sent on a core of uranium 235, it makes it even more unstable it becomes fissile and breaks into two parts.
This fission releases particles, radiation and high energy.
It is this energy that will be used to heat water. Fission releases energy giant, for comparison, 1 gram of uranium-235 releases as much energy as burning 1000 kg of coal, or 69 kg of oil.
In 2007, worldwide there were 439 operating nuclear reactors. The nuclear power generation, is 2600 TWh, supplying about 15% of worldwide electricity.

Image: The pellets of uranium dioxide (UO2) are stacked in zirconium tubes, 4 feet long. The pencils (pellets and ducts) are blocked at both ends and pressurized with helium to keep water in liquid form.
The rods are then assembled into bundles of 289 elements (17x17).

Chain reactions

In nuclear power plants are not used only for uranium-235 but also of uranium 238.
These two elements contain 92 protons but different numbers of neutrons, 143 for uranium 235 and 146 for uranium 238. Both elements are unstable but uranium 238 is fertile, it does not break.
In a block of uranium, there are 0.7% uranium 235 and 99.3% and uranium 238. To sustain nuclear fission, which will occur in assemblages of pencils, the block must be enriched to contain 3-5% uranium 235.
Inside the reactor, fuel assemblies, containing uranium pellets are immersed in water.
The fission process starts when the emission source sends out neutrons.
The neutrons are absorbed by the nuclei of uranium-235 and instantly crack and release their energy. Cracked kernels, also eject 2 or 3 neutrons which will in turn reach out to other nuclei of uranium 235, which will also release energy and neutrons, and so on...
Pencils to pencils, the neutrons will trigger nuclear chain reactions and produce more and more heat.
This heat is used to heat water. This is not the water content in the reactors, which will turn into steam, because this water is radioactive.
The water was heated by fission, reached inside the reactor, a temperature of 320 ° C.
It is in the liquid state because it is kept under pressure in a totally closed circuit, called the primary circuit.

This source of heat from the pipes, heating the steam generator which contains water and this hot water will flow into the secondary circuit.
Steam is sent through the turbines so that they carry a generator to provide electricity.
The steam is then cooled to a liquid state it is returned to the steam generator. Uranium 238 which is present in greater quantities, do not fission, but also absorbs neutrons, it will thus be transformed into plutonium 239. This element, like uranium 235 is unstable and fissionable material. Plutonium-239 will produce 1 / 3 of the energy of the nuclear reactor.
One part of plutonium 239, which does not fission, is recovered to be restated. This mixture (plutonium, uranium), used to make another fuel, MOX (mixed oxide), used in many engines. MOX is isolated during the processing of spent fuel. Uranium and plutonium are not consumed will be part of highly radioactive waste that will be vitrified.

NB: MOX (mixed oxide) is a nuclear fuel made from plutonium and depleted uranium. MOX fuel contains uranium dioxide (UO2) and plutonium dioxide (PuO2).

Image: Nuclear power plant in Fukushima, Japan.
In 2011, Japan has 54 nuclear reactors in 17 power plants in operation, which ranks the 3rd largest producer of nuclear electricity.
Nuclear power provided about 35% of the electricity needs of the country.

Image: Nuclear power plants in the world

Percentage of nuclear electricity generated

The world's fleet of nuclear plants is increasing.
France comes in first place with 78% of its electricity from nuclear.
The share of nuclear electricity in total electricity production is 14% in 2009. The first national park of nuclear power plants is that the United States (104 nuclear reactors for power of 99 GW), followed by France (59 reactors with a power of 63 GW).
In 2008, nuclear energy provided over 21% of the total electricity production in the OECD. In total, 17 of the 30 OECD member countries exploit this energy today.
The OECD countries account for around 83% of the global nuclear energy. Fourteen non-member economies of the OECD for the remainder of production.

Image: percentage of electricity produced in the country, from nuclear power.
Credit : Panorama des statistiques de l'OCDE 2010.