Nuclear reactions as energy sources

1. A chemical reaction only rearranges the electrons on the outside of atoms, so every nucleus is exactly the same before and after it.
2. A nuclear reaction changes the nucleus itself, so the number of protons or neutrons is different afterwards and the atom can end up as a different element.
3. A nucleus is written as ${}_Z^{A}X$, where $A$ is the mass number, the total number of protons and neutrons, and $Z$ is the atomic number, the number of protons.
4. Nuclear reactions release enormously more energy per atom than chemical reactions, which is why a few kilograms of nuclear fuel can do the work of thousands of tonnes of coal.
5. Three kinds of nuclear reaction can be used as a source of energy, and you need to be able to recall all three.
   1. Nuclear fission, in which a large nucleus splits into two smaller ones.
   2. Nuclear fusion, in which small nuclei join to make a larger one.
   3. Radioactive decay, in which an unstable nucleus emits an alpha particle, a beta particle or a gamma ray.
6. Of these three, the one you must be able to explain in detail is the fission of uranium-235.

The uranium-235 nucleus

1. Uranium-235 is written as ${}_{92}^{235}U$, so it holds $92$ protons and $235-92=143$ neutrons.
2. That is one of the largest nuclei found in nature, and the more protons a nucleus contains the harder it is to hold together, because every proton repels every other proton.
3. A uranium-235 nucleus is therefore only just stable, and absorbing one extra neutron is enough to tip it over the edge.
4. A nucleus that will split after absorbing a neutron in this way is described as fissile.
5. Most natural uranium is uranium-238, which is not fissile in the same way, so reactor fuel is processed to raise the proportion of uranium-235 it contains.

The fission of uranium-235

1. A slow-moving neutron reaches a uranium-235 nucleus and is absorbed by it rather than bouncing off or passing through.
2. The nucleus now has one more neutron, making it ${}_{92}^{236}U$, and this new nucleus is highly unstable.
3. Within a tiny fraction of a second the unstable nucleus splits into two smaller daughter nuclei.
4. At the same instant two or more neutrons are thrown out, because a large nucleus needs proportionally more neutrons to hold it together than the two smaller ones do.
5. A large amount of energy is released, mostly as the kinetic energy of the two daughter nuclei flying apart, with some carried away as gamma radiation.
6. The fast-moving daughter nuclei collide with surrounding atoms and are slowed down, so that kinetic energy ends up as an increase in the thermal energy of the fuel.
7. In words, the whole process is $U-235\; nucleus+neutron\to two\; daughter\; nuclei+two\; or\; more\; neutrons+energy$.
8. One possible split is ${}_{92}^{235}U{+}_0^{1}n\to {}_{56}^{141}Ba{+}_{36}^{92}Kr+3{}_0^{1}n$, and both the mass numbers and the atomic numbers balance across the arrow.
9. The two daughter nuclei are not equal halves, and the same nucleus can split in many different ways, so barium and krypton are only one of the possible pairs.
10. This kind of fission is described as induced, because a neutron has to arrive and trigger it, unlike radioactive decay, which happens on its own.

Where the energy comes from

1. Add up the masses of everything produced by one fission and the total is very slightly less than the mass of the uranium nucleus and the neutron that went in.
2. That missing mass is the source of the energy released, so the reaction does not create energy from nothing.
3. The fractional loss of mass is tiny, but because so many nuclei are involved the energy adds up to an enormous amount from a very small mass of fuel.
4. One fission of uranium-235 releases roughly a million times as much energy as burning one carbon atom, which is the reason a nuclear station needs so little fuel.