Nuclear Power

Nuclear fission and radioactivity are dependent on the relative stability of various atomic nuclei. In addition to the overall size of the nucleus, each chemical is characterised by the number of protons in its nucleus, because that governs how many electrons it will attract and will, in turn, govern all its chemical properties.

However, the number of neutral neutrons in a nucleus of a chemical can vary. Most stable nuclei have about the same number of neutrons as protons, though the ratio of neutrons to protons goes up as the nuclei get bigger. The most stable nucleus is that of iron which has 26 protons and 30 neutrons.

Sometimes there will be a few more or a few less neutrons than normal in a nucleus – we call these different isotopes of an element. You will have heard of carbon-12 and carbon-14. All carbon isotopes have 6 protons. Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. Carbon-13, with 7 neutrons also exists and is quite stable. Very large heavy nuclei tend to have a bigger imbalance of neutrons to protons, and have a tendency to lose very small fragments to get closer to the size of a more stable nucleus, and, as a result, the atom changes to be a different chemical. Sometimes, to make a more stable balance between neutrons and protons, a neutron will turn into a proton and lose a negative part – which appears as a very high-energy electron, and again, the atom will turn into a different chemical.

When nuclei are big or unstable, the spontaneous loss of little chunks from their nuclei is what we call radioactivity or radioactive decay. The particles that can come out of an unstable nucleus can be Alpha particles, Beta particles or neutrons.

Alpha particles are the same as a helium nucleus – two protons and two neutrons. These are heavy and quite big and don’t move very quickly. They will typically be stopped by a piece of paper. If a neutron loses a negative particle to become a proton, that particle is effectively a high energy electron. It will be slowed down and stopped by a thin sheet of any metal – like thick aluminium foil. Neither of those particles are likely to get as far as another atom’s nucleus. The alpha particle will probably attract a couple of electrons from other atoms, causing some ionisation and just turn into helium. The electron will disappear into the outer shell of any transition metal and take part in normal chemistry fairly soon afterwards. Alpha and beta particles are not too much of a problem to humans.

The particles that can interact with other atom’s nuclei are neutrons, as they have no electrical charge, so are not slowed down by the outer cloud of electrons of an atom. Energy often comes off as Gamma rays, which are electromagnetic waves, in the same family as radio, light, UV and X-rays, but even more energetic than X-rays. Gamma rays can cause random damage and burns, as X-rays can, but more severely. To stop the Gamma rays you need several inches of lead or a foot or two of concrete. Gamma rays also “fry”/destroy electronics.

If neutrons are lost from a nucleus, they usually eventually embed themselves in another atom’s nucleus, once they are travelling slowly enough for another nucleus to capture them. Some nuclei are very happy to absorb additional neutrons, others become unstable if they do, and then emit an alpha or beta particle, or even a neutron again, at a later point.

Sometimes, when a large nucleus is hit by a neutron, it becomes so unstable that, instead of randomly dropping off an odd fragment here or there to make itself more stable, it splits completely into two much smaller chunks. This creates a lot of energy and heat and often other even smaller fragments, alpha and beta particles and neutrons, shatter off at the same time. That process is called Fission. If the fission event produces neutrons as well, and there are enough atoms of the right sort around, then one fission event might cause another.

Neutrons generated by a fission event, rather than from radioactive decay, are usually travelling too fast to be captured by another nucleus. A free thermal (slow) neutron has fairly low energy (in the order of 0.025 eV) and travels at around 2.2 km/s. Fast neutrons have an energy of about 1 million eV and travel nearly 10,000 times faster – typically at something over 14,000 km/s (energy is proportional to velocity squared). Fast neutrons are difficult to stop; this is why uranium-based nuclear plants need such thick concrete and lead shielding, and why the whole nuclear plant ends up becoming radioactive, because of all the neutrons absorbed in the shielding.

Fast neutrons are typically produced from a uranium fission event, but they move too fast to be captured by most nuclear fuels unless they are slowed down, by a material called a moderator. So, in a normal uranium fission reactor the neutrons which fly off when an atom fissions are usually slowed down by moderation to increase the probability of them being captured by another uranium atom, for the process to continue. Some forms of uranium will catch fast neutrons – fast-breeder reactors make use of this.

Some elements are very good at slowing down neutrons – graphite and heavy water are two such materials. Some atoms are very good at absorbing neutrons, staying relatively stable and not emitting them again. The reason that solid-fuelled nuclear reactors only use about 3% of their fuel before the fuel is useless and no longer produces much heat, is that there is a build up of these neutron-absorbing chemicals in the fuel rod as the fission progresses, and because the fuel is solid, the neutron absorbers stay there. One of the most common of these chemicals is xenon, which, if the fuel were liquid, would simply bubble out, as it is a gas.