Nuclear power II
Radioactivity
The fission of a nucleus is not a neat process. It can break into two or three big pieces, which in general are not stable isotopes. The fission fragments then start turning into stable nuclei with the same value of A by turning neutrons into protons. Much of this process happens quickly, within minutes or hours, but there are many unstable isotopes that take thousands of years to decay. This has several implications for a nuclear reactor and nuclear power:
*Stopping the fission process does not stop the release of energy. The unstable nuclei continue to turn into stable ones, which releases part of the power of the reactor when it is running. It's similar to stopping a moving freight train: turning off the engine is only the first step.
*Even after it has been removed from the reactor, the nuclear fuel assembly remains hot and extremely radioactive for a long time. A wide variety of unstable nuclei are present; some decay rapidly (which makes them dangerous in the short term), while others decay slowly
(which makes them less dangerous, but for a correspondingly longer time).
*The radiation produced by the radioactive materials is biologically active: it is a kind of poison.
Ensuring that none of this gets out of the power plant in normal operation is already hard; it complicates disposal of the spent fuel, and then we have to worry about accidental release of radioactivity .
Nuclear radiation
The principal hazard associated with nuclear reactions is radioactivity and radiation. These refer to unstable nuclei and the energetic emissions that come from them. There are four kinds of radiation:
* an operating nuclear reactor (or a nuclear explosion) emits neutrons. These are the most dangerous kind of radiation, because they penetrate matter easily and then deliver a lot of energy to one small place. They are also dangerous because normal nuclei absorb them and become radioactive. However, they are not produced any other way than during nuclear fission, so you need not worry about this if you stay away from nuclear reactors and nuclear explosions, as I expect you all will.
*many unstable nuclei emit "gamma rays." These are relatives of Xrays. Similar to Xrays, they can penetrate into matter; they can cause chemical reactions (breaking up or modifying molecules) on their way through. They are more penetrating than Xrays and are hard to stop; a wall of lead bricks is needed.
*when a neutron turns into a proton, it frequently emits a high energy electron (called a "beta ray" in this context). These are pretty easy to stop; a sheet of aluminum foil has a significant effect.
*heavy nuclei, such as uranium, radium, and radon, emit high energy helium nuclei (called
"alpha rays"). These are very easy to stop, but they carry a lot of energy.
Our biological senses do not detect these kinds of radiation at all. The good news is that there are relatively simple electronic devices that can detect them extremely well. This means that it is very easy to check something for the presence of radioactivity. These devices are so good that they lead to a different kind of hazard: the meters will reveal that stone walls, air in your basement, and a banana are slightly radioactive (in each case, not enough to cause harm; the hazard in this situation is the meter itself, which is giving you information that you don't need).
© J. P. Straley Nuclear2.doc
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Hazards of radioactivity
There are several kinds of radioactive hazard, and we need to understand the distinction between them.
To begin with, we need to distinguish between radioactive materials and radioactive emissions.
There are many kinds of unstable nuclei (all the isotopes in the white band in the diagram, except for the stable ones that are indicated). When these decay, they emit energy in some form (the various "rays" discussed above). There are two very different cases to consider: the material is outside of you, or it is inside you.
If the radioactive material is outside of you, the first issue is to keep it there, by not eating, drinking, or breathing it (this advice is only useful if you know the radioactive stuff is there). Of course, you will remove yourself from the area, or remove the radioactive material, if this is possible. It's also relevant to know how easy it is to shield yourself from the "rays."
When the radioactive material is inside you the situation is more difficult. Radioactive isotopes are chemically identical to stable ones, and so they become concentrated (or not) according to the biological uses of the element.
There is radioactivity in the environment, some due to humans and some of natural origin. It is useful to discuss these, because they give a context in which to discuss the radioactivity hazard of nuclear power.
The testing of nuclear weapons in the atmosphere in the 1950's produced an isotope Strontium-
90. Because it is similar to calcium, it becomes part of bones. Strontium-90 decays rather quickly, so most of it is now gone, but because it decays quickly, any that is in your bones will decay during your lifetime, doing a little damage to your bone marrow – an important source for certain specialized cells.
There is an isotope of potassium (Potassium-40) that lasts a very long time. There is potassium-
40 in the environment that is left over from the creation of the universe. The decay of potassium-40 in your body is a significant part of the radiation dose that you receive from natural causes. Since you need potassium to live, you should eat your bananas anyway.
There is a little bit of the heavy elements Uranium, Radium, and Radon in the soil everywhere, though Kentucky has slightly more than its share. Radon is a gas, and the air in a basement is likely to contain more than normal. Radon emits alpha particles, which don't travel very far, so the radon outside of you isn't very relevant. But when the radon in the air in your lungs decays, it zaps the lung lining directly, and then turns into a different radioactive element that isn't a gas and stays in your lung.
© J. P. Straley Nuclear2.doc
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