Shots Across the Bow

A Reality Based Blog


A Nuclear Power Primer: Part 2b: What is Radiation? Radioactive Decay

So last time, we talked about the structure of the atom, and where nuclear energy comes from. We finished by describing nuclear energy as the binding energy of the nucleus that is released when the nucleus splits. Today, we're going to talk about how that split happens.

First of all, we need to define our terms. Instead of a list definitions, which will bore you into deep slumber, I'm going to use the words in context so you'll retain them better. When we're talking about splitting a nucleus, we're talking about fission. Fission can occur spontaneously, or we can make it happen. When it happens spontaneously, we call it decay, and elements that fission spontaneously are called radioactive since they naturally emit radiation.

See? Simple!

When we're talking about radioactivity, we're talking about the rate of decay for that element. Each element, and each isotope (remember, an isotope is an element with the normal number of protons, but a different number of neutrons) has it's own decay rate or activity. To make things a little less straight forward, the decay rate is not constant over time, but is affected by the quantity of the isotope as well. The more there is of it, the faster it decays In order to take this rather strange property of radioactivity into account, the decay of an isotope is measured by the time it takes for half of it to decay. For example, Cobalt 60, a radioactive isotope produced in nuclear reactors has a half life of 5.27 years, so if you have 1 pound of it, in 5 years and 14 weeks, you will have half a pound of Cobalt 60, and half a pound of it's decay products, in this case, nickel 60, which is a stable element. In another 5 years 14 weeks, you'll have a quarter pound, and so on.

Now what should be obvious when you think about it, as the amount goes down, and the rate of decay slows down, the energy it gives off goes down as well, which is a good thing, because eventually, the radiation given off decreases to a point where it is low enough not to matter.

So, how does this decay occur?

Well, we talked about how the electrostatic forces that tend to cause the protons to try and scatter are countered by the nuclear force that holds the nucleus together. As we add more protons and neutrons to the nucleus, it gets larger. The electrostatic force gets larger, but the nuclear force gets weaker. Just like gravity, the further you get from the center of the object, the weaker the force becomes. Eventually, you get to a point where the nuclear force is overcome by the electrostatic forces, and the nucleus becomes unstable. The degree of instability determines the activity of the isotope and it's decay rate. (This is a huge simplification of all the forces in play, but it is accurate enough for our purpose here. Naturally occurring radioactive elements have nuclei that are unstable due to their size, and they decay spontaneously.

As you can imagine, isotopes that are highly unstable decay pretty quickly, which means we don't find them in nature because they've all decayed away. On the other hand, isotopes that are barely unstable are going to stick around for awhile, giving up their energy very slowly.

Now when you think about a nuclear fuel, you want one that releases energy fairly quickly so we have a problem; the isotopes that are most abundant are the ones least suited to generate power. We have to come up with a way to make these normally low activity isotopes give off enough activity to be useful.

If you've been paying attention, you already know the answer, but we'll talk more about that later. Next time, we need to talk about the different ways the nucleus can fission and the different types of energy that is released.
Posted by Rich
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