Physics

Nuclear energy


Among the main forms of electricity production in the world, nuclear power accounts for about 16% of this electricity. However, there are some countries that are more reliant on nuclear power: while in Brazil, for example, only 3% of the electricity used is produced by nuclear plants, in France 78% of the electricity generated by them (2008 data).

In the United States there are more than 100 nuclear power plants, although some states use this type of energy more than others; while in Brazil we have only two in operation: Angra 1 and Angra 2, with a third (Angra 3) being installed, all constituents of Almirante Álvaro Alberto Nuclear Power Plant.

The main question is: how nuclear plants work?

For starters, it is important to define what nuclear power is. It is the energy released in the transformation of atomic nuclei. Basically, what happens is the transformation of an atomic nucleus into several other lighter nuclei, or isotopes of the same element.

At nuclear fissions, reactions that consist of breaking one heavier nucleus into smaller and lighter ones after a neutron collision in the initial nucleus, are the basis for energy production in nuclear power plants.

As uranium is a widely available element on Earth, it is the main resource used in the nuclear reactions of these plants. Uranium 238 (U-238), for example, which has a half-life of 4.5 billion years, makes up 99% of the planet's uranium; uranium 235 (U-235) makes up only 0.7% of the remaining uranium and uranium 234 (U-234), even rarer, is formed by the decay of U-238.

Although less abundant, U-235 has an interesting property that makes it useful for both energy production and nuclear bomb production: it naturally decays, like U-238, by alpha radiation and also spontaneously fissures in a small time interval. However, the U-235 is an element that can suffer induced fissionwhich means that if a free neutron crosses its nucleus, it will be instantly absorbed, becoming unstable and dividing.

Consider, then, a neutron approaching a nucleus of U-235. When capturing the neutron, the nucleus splits into two lighter atoms and throws two to three neutrons - this number depends on how uranium has split. The two newly formed atoms emit gamma radiation according to how they fit into their new states.

The probability of induced fission occurring on a U-235 atom is very high: in a properly functioning reactor, each ejected neutron causes a new fission. In addition, neutron capture and subsequent nucleus division occur very rapidly at intervals of 10-12s. Not to mention that a single nucleus, when divided, releases an enormous amount of energy, both in the form of heat and gamma radiation. This energy production is governed by the well-known equation. E = mc2, due to the mass difference between the fission products and the original atom.

For a uranium sample to have the above properties, it must be enriched to contain 2% to 3% more than U-235. The 3% enrichment is sufficient for use in a nuclear reactor that works in energy production.