Some Nuclear Datum Made Simple
In the wake of the Japan nuclear discussions some background information might be useful.
Nuclear radiation, that is energy and particles orginating in the nuculei of atoms, come in four varieties. Alpha, beta, and gamma radiation + (hot or energetic) neutrons. Well, to be honest there are also neutrinos being tossed about … but they only interact weakly … really really really weakly and can be ignored.
Alpha particles are, basically, helium nuclei. That is a bound group consisting of two neutrons and two protons. The penetration of alpha radiation, because it is heavy and highly charged it has a very short mean path in ordinary materials. Their mean free path (average distance traveled before capture) is only a few meters for example in air. Your outer (dead) skin will stop all of the output of any but an incredibly intense alpha source. When a nuclei emits an alpha particle, its number drops by two (it loses 2 units of charge) and mass by 4.
Beta particles are electrons. When an electron is emitted by a nuclei, its atomic number goes up by one. Basically a neutron in the nuclei “turns into” a proton, and since the neutral neutron wants to become a positive charge to balance it has to spit out a negative charged “thing”. That “thing” is an electron. They won’t be stopped by your dead skin, but a few mm of Aluminium will suffice. Their penetration too is limited.
Beta and alpha radiation are easily controllable and avoidable … there is only one real large danger that their limited penetration poses … if the source is taken internally as food … then there is no skin or distance which you can put between you and the source.
Gamma radiation is are high energy photons (light). Their energy is much much higher than visible light. As you go up in energy (and down in wavelength) our classification of electromagnetic spectra goes from radio, microwaves, visible, uv, x-rays and finally gamma radiation … with radio waves being the lowest energy and longest wavelength to gamma radiation with short wavelengths and high energies.
Neutrons are one of the two primary constituents of the nuclear matter. They are neutral and pass a fair distance through matter without stopping. Neutron radiation can be seen in some test nuclear pool reactors as looking like a blue glow. Neutrons in the wild (outside of a nuclei) unless captured decay into a proton and beta particle (+ a neutrino) in about 900 seconds (half life). Neutrons captured yeild, quite often, stimulated radioactivity. That is the nuclei that captures said neutron is probably unstable, and itself will give off a beta or alpha particle shortly.
One of the basic notions one has in matter interactions is that things only interact with other things of comparable size. Light wavelengths we just mentioned, but matter too has a wavelength, the momentum of a particle yeilds (by the Plank) constant its wavelength. Why is this important?
Well, the neutrons, gamma radiation, and beta particles aren’t going to interact with other atoms or nuclei very well unless they are of comparable size. The “hot” neutrons coming out of a nuclear decay is going very fast. That means its wavelength is very very short. Nuclear reactions are stimulated when a U235 “captures” a neutron. The chance (cross section in Physics parlance) of capturing a neutron is much better when the wavelenght of the neutron is about the same as the size of the nuclei … both of which depend on their momentum (or in other words … temperature).
Nuclear reactors consist of four basic elements. A heat exchange material, to take the heat generated away to be used. A nuclear fuel, either enriched Uranium (Uranium in which the U235 isotope has been increased from the mostly naturally occuring U238 or a Plutonium isotope). A moderator, this is a material containing light nuclei which those hot neutrons can bounce off of and lose energy … cooling them off so that their wavelength lengthen to the point where they have a better chance of capture by another U235 nuclei. And finally a control material, this is a material that is a good (or at least much better than the U235) abosrber of neutrons. It’s a simple matter of design an engineering to put these four things together in a way that gives you a safe controllable reactor. It might also be noted that to get an uncontrolled explosive chain reaction, like in a bomb, takes both careful design and a fuel that is far more highly enriched than that found in reactors. Enrichment, is really really hard to do, so there is basically zero chance a reactor will “become” an atom bomb … that is to say it’s about as likely as all the air molecules in your room suddenly, by accident, going to the other side of the room and leaving you gasping in a vacuum.
Most US/Western reactors are water cooled and moderated. That means the water (or more specifically the hydrogen, protons, in H2O) act both as the coolant and the moderator. Chernobyl was water cooled but used graphite as the moderator. There was 1 (!) graphite moderated reactor in the West (in the UK) but that has been shut down decades ago.
A radioactive material is one which contains an unstable isotope of something in a significant concentration. How much danger that isotope poses depends on what type of radiation (of those four above) it yields on decay and its half-life. The half life is the measure in time at which half of the atoms present will have decayed. A material with a short half life will be more intense for a shorter time, basically.
Filed under: Mark O. • Science
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