The anniversary of the Chernobyl disaster is exactly 30 years ago this week. There have been 3 major nuclear disasters (and countless near misses arguably). The first was Three Mile Island in 1979. This put some people off but the alternatives weren’t great. The second was Chernobyl which changed my mind about nuclear power. The third was of course at Fukishima which has confirmed many of our fears. Now of course there are many and an increasing number of alternatives (which are also cheaper). In all three disasters a total core meltdown was narrowly avoided. Chernobyl was the worst since of the three since there was no proper containment. The fear many of have is that the nuclear industry have little idea once a core meltdown is underway how to stop it and so far doing so has relied on a degree of luck.
Each disaster has cranked up Nuclear costs as more safety features are built in by regulators. In the Western world this has led to a mere handful of reactors being started since Chernobyl and now an increasing number of pro-nuclear academics are questioning the existing safety regime in a bid to lower these costs. Part of their argument is that radiation is not as dangerous as all that, so if there was another disaster things would not be that bad. (I’ve actually heard this said on the radio over the last few months.) Just what is radiation and just how dangerous is it?
First a bit of physics. An atom is composed of a nucleus with elections orbiting around it (in simple terms). The nucleus consists of protons (positively charged) and neutrons (as the name implies no charge). These are added together and called the atomic mass (electrons being smaller are ignored). The atomic mass is shown as a superscript in front of the element symbol (see below). The atomic number (the number of protons is shown as a subscript). The atomic number tells us which element it is. The neutrons have no effect on the chemistry of the atom (slight exaggeration) and their number can vary for the same element. Its the ratio of neutrons to protons that decides whether the element is stable. A high number of extra neutrons (generally in heavier elements) makes it less likely. Atoms that are not stable (and most are stable) undergo radioactive decay and produce sub-atomic bits we call radiation. There are four main types of radiation all of whose sources are a bit weird on the face of it and a full understanding probably depends on a deep knowledge of sub-atomic physics. In releasing radiation the element changes into another. As a final point these elements can be present in chemical compounds and that makes no difference to their decay.
The first type of radiation is when a neutron converts to proton and in doing so emits an electron (told you its weird). This form of radiation is not too dangerous unless you ingest the source. Electrons travel only very short distances through any matter including air. In this example radioactive carbon 14 decays to nitrogen 14 (the normal form not radioactive).
The next form produces anti-matter (yes it does exist) when a proton converts to a neutron. This produces a positive electron. When is this meets a normal electron its annihilated producing gamma rays (see below). Little danger here unless the source is inside you are very close contact. In this example sodium decays to neon, beta is positively charged.
The third form is alpha particle radiation. This is a helium nuclei and is made up of protons and neutrons. Again air and materials stop it, but if swallowed this is much more of a danger since the particles are bigger. Here in this example the element radon decays to the radioactive gas radon.
The last form is gamma rays these are formed of electromagnetic radiation like light (except a much shorter wavelength). These will travel large distances in air and through matter. They are formed from the decay of alpha and beta emitters. These can do great damage.
When a particular individual atom decays is a random unpredictable event. How fast a group of atoms decays is predictable and the rate depends on the starting radioactive element. These rates we call half lives. This is simply the time that its takes mass x to decay to half its radioactivity. The half life can vary from microseconds to 1019 years with everything in between. Atoms that decay will end up as a stable non-radioactive elements. Many atoms decay through a series of radioactive steps, the decay of radium shown above is part of the decay of uranium which ends up as lead. In the next post I will look at the biological effects and the controversy.