Key Points You Need to Know When Talking About Energy: Radiation

Nuclear power plants are part of the mix of our current energy solution.  Some people want nuclear power to go away; some people want it to increase.  The critical factor in nuclear power is safety, so it is important to understand the danger of nuclear power.  So I'd first like to go over some fundamental facts that are sometimes poorly understood.

Radiation vs. Radioactivity

I think the most fundamental misunderstanding a lot of people have is this distinction, so let's clarify it first.

Radiation

Radiation is any sort of energy that comes out from a source and travels through space.  "Radiation" is a very generic term and covers several different types of physical phenomenon.  You can sub-divide radiation based on two primary things: *what* is radiating out from the source carrying this energy and *how the radiation interacts with materials with which it comes into contact*.  

What

The things that can be radiated out are:

1. Photons.  Photon radiation is just light, really, except that we reserve the term "light" for photon radiation that happens to be of a frequency that is visible to our eyes.  There's no fundamental difference, though, between photon radiation of all sorts, except for its frequency.  Photon radiation is called, in ascending order of frequency: radio, microwave, infrared, visible light, ultraviolet, x-ray, gamma.  These are all fundamentally the same thing, though the interaction of photon radiation with various materials can vary greatly based on the frequency.
   
   Another thing to be aware of is that, per photon, the energy of photon radiation goes up as the frequency goes up.  That is, gamma rays are very much more energetic than visible light rays, per photon.
   
   
2. Subatomic particles.  These types of radiation are collectively called "particulate" radiation.  Different types of subatomic particles have different behaviors as radiation and are called different things:
1. Alpha radiation: positively charged particles.  This is normally in the form of two protons and two neutrons.
2. Beta radiation: negatively charged particles--electrons or anti-protons.
3. Neutron radiation: neutral particles.  These are usually neutrons, though technically neutrinos would count here also.

Interaction

There are two main considerations here:

1. Ionization.  The primary way radiation is distinguished here is to separate it into "ionizing" and "non-ionizing" radiation.  An "ion" is an atom with a net positive charge: an atom missing one or more electrons.  "Ionizing" therefore means "able to knock electrons off of molecules".  Whether or not radiation can do this is a key distinction because all chemical interactions involve relationships of atoms to each other at the layer of the electron shell.  Radiation that can create ions can therefore cause chemical reactions at a cellular level and therefore damage tissue chemically.
   
   In contrast, getting too much infrared, microwave or visible light can damage you by conveying too much heat.  It cooks your tissue, rather than chemically changing it.  This can be just as damaging--depending on the amount of heat we're talking about--but it is a different *type* of damage.
   
   In the spectrum of photon radiation, x-rays and gamma rays are ionizing, but lower frequencies are usually not.  Alpha and Beta radiation are definitely ionizing (being charged particle radiation, they definitely interact with the electron layer of atoms).  Neutron radiation is also considered "ionizing radiation", but it actually is only so in an indirect way, which I will discuss a bit later.
   
   
2. Penetration.  The different types of radiation are able to penetrate into a body very differently.  Because Alpha and Beta radiation are both charged particle radiation, they both react very quickly with physical objects and can not penetrate very far into anything.  Beta radiation penetrates a bit more deeply than Alpha radiation (it can penetrate a little into the skin), but not very far and both are stopped by even normal clothing.
   
   On the other hand, Neutron radiation passes right through most normal material.  Neutrons do not react to electrons at all and pass right through the electron layers of atoms as if they weren't there--and the electron cloud is by far the bulk of the volume of an atom, the nucleus being only a tiny mass in the center.  Neutrons passing through your body, then, only interact with you at all if they happen to collide with an atom nucleus as they are travelling.
   
   This can cause damage, however, and if there is enough neutron radiation, it can definitely be fatal.  This type of radiation is the type that must be stopped by thick layers of material or particularly dense materials.  Lead is used for this because lead has a very high atomic number and therefore has a very large nucleus: a bigger target for the neutrons to hit.
   
   For photon radiation, the higher the frequency of the light (in general) the more it is able to penetrate into and through normal objects--this is why we use x-rays to look inside of things.  Gamma rays are more penetrating still.

Radioactive decay and Radioactivity

Now here let's deal with the primary point on which people are confused about radiation.  Radiation is energy radiating out from a source.  "Radioactivity" means some property of a material that causes it to release radiation, and this usually results from radioactive decay.  Usually--but not always!  By these definitions, an x-ray machine is technically "radioactive" when it is turned on but not when it is turned off.  The same, technically, for a light-bulb, which would be "radioactive" when on but not (much) when off.  Be aware that there is some fuzziness to the usage of this term, because it's *normally* used just for things that are radioactive because of radioactive decay.

Ok, so now: what's radioactive decay?

Radioactive Decay

Atoms are composed of a nucleus surrounded by an electron cloud.  Nuclei are composed of various subatomic particles that naturally want to fly apart that are nevertheless bound together as a clump of protons and neutrons by the incredibly strong (and appropriately named) "strong force".  Nuclei are all inherently instable and they will all eventually break apart on their own.  *How* unstable a particular nucleus is depends on its size and structure.  Generally speaking, the larger the nucleus is, the less stable it is.  This is not a simple relationship, however, and there are lots of exceptions and peaks and valleys of instability as you look over the table of elements.  However, in general it is true that larger elements will tend to want to break apart into smaller elements.

Perhaps more important than the overall size of the nucleus, though, is its geometric configuration.  This means that the number of neutrons compared to the number of protons in a particular nucleus greatly influences how stable it is.  The number of neutrons in a particular nucleus determines its isotope: two different atoms with the same number of protons but a different number of neutrons will be different isotopes of the same element: chemically the same (because the electron layer is the same),  but different at a nuclear level, and usually of very different stability.

Radioactive decay is what happens when an atom breaks apart.  An atom of one heavier element will split apart into two or more atoms of lighter elements, at the same time releasing some energy in the form of a mix of Alpha, Beta, Gamma and Neutron radiation.  What elements are formed by the decay and what is the exact mix of radiations produced depends on the element that is decaying and the form of the breakup--normally, even for a given isotope, there are multiple "failure modes" of the nucleus and therefore multiple types of by-products of its breakup.  These happen at a predictable percentage, though, so we can characterize the total radiation byproduct of a given isotope as the weighted average of these different nuclear "failure modes".

All elements are subject to eventual decay; therefore all materials are inherently radioactive.  However, some elements and isotopes are subject to decay much more frequently than others, and therefore we typically reserve the word "radioactive" for elements and isotopes that decay rapidly and therefore emit radiation at a rate that concerns us.

But do keep in mind that this is a distinction of degree and not of kind.  Radioactivity is an inherent property of all matter.  Phosphorous, for example, is high up on the scale of radioactivity among very ordinary elements, and because bananas have high levels of phosphorous you will get a reaction out of a Geiger counter if you hold it up next to a banana.

Nuclear power, including both nuclear power plants and nuclear weapons, make use of materials with very, very high rates of radioactive decay, and therefore necessarily involve materials which are constantly emitting high levels of radiation.

How radioactivity and radiation are harmful in different ways

So now we are equipped to understand the fundamental ways in which radioactive material is harmful in a different way from simple radiation.  Radiation, we saw, can be harmful in that it damages cells on a chemical level by ionization.  If you get a large dose of radiation, this can cause a lot of damage to your body, possibly permanent and possibly lethal.  However, such damage is a one-time event and it does not perpetuate in your body.

The situation is different if, for some reason, you inhale or ingest the dust of some radioactive material.  We said that Alpha and Beta radiation doesn't penetrate far into the body, being mostly stopped even by just skin.  However, if you happen to inhale or ingest particles of some radioactive element, those particles will be continuously decaying and producing continuous levels of Alpha and Beta radiation *inside your body*.  This causes a lot more damage because the internal tissues of the body are not designed to withstand radiation damage in the same way the skin is.

Furthermore, the most radioactive materials are, as I said, the heavy elements--they're called "the actinides" on the periodic table.  A major health problem with these heavy metals is that *the body does not have a good way of eliminating them*.  This is why lead poisoning is a problem, by the way, even though lead is pretty inert chemically: the body doesn't have a good way of eliminating lead from the system and so any lead dust you you ingest or inhale tends to slosh around inside your system forever and clog things up.

So this is why the real long-term horror of a nuclear weapon or a catastrophic nuclear power plant failure is not the explosion, but the fallout.  It is the spreading of radioactive dust around that gets into everything and cannot be easily purged if allowed entry into your body.

How radioactivity "spreads"

This leads to the final confusion between radiation and radioactivity I want to cover, which is how radioactivity spreads.  I think everyone is dimly aware that radioactivity "catches" somehow--that if you bring something into contact with a source of radiation, for some reason the thing "infected" with the radiation will itself become radioactive.

This is true for both radioactive things *and* for a very specific type of radiation, but for very different reasons.

For radioactivity, the way radioactivity spreads is that radioactive particles get into things.  So if you have some plutonium dust, and it gets on something, that item will have plutonium dust on it.  It will therefore emit radiation because of the plutonium dust and will therefore "be" radioactive.  If you are able to wash all of the plutonium dust off of this thing, then it will no longer be radioactive--the radioactivity is strictly contained in the plutonium dust. 

The difficulty is with being able to separate out the radioactive particles from the non-radioactive ones; there are always ways to do this, but many of them are not feasible at a large scale.  If you can't get the radioactive particles out, then you are stuck with the radiation for as long as the half-life of the radioactive material dictates.

Radiation itself does not cause other things to be radioactive, with the sole exception of Neutron radiation.  We said before that neutron radiation interacts with materials by impacting into the nucleus of the atom, because it doesn't interact with the electron cloud.  When it does this, it either bounces off, splits up the atom there and then, *or* merges with the nucleus, causing the atom to become a new isotope of the original element.

This last scenario is why neutron radiation causes things to become radioactive.  When neutron radiation is absorbed by an exposed material, the isotope that is created is usually not stable at all.  The absorption will only "take" for a little while, and then the element will decay in a bang of Alpha, Beta and Gamma radiation.  And *this* is why neutron radiation is considered ionizing radiation, because the primary by-product of the nuclear decay that it causes is other forms of ionizing radiation.

There is an important distinction to understand here, though.  Nuclear material that comprises a nuclear bomb or a nuclear power plant is almost all actinide--those heavy metals that release tons of radiation and may take millennia to decay to the point of safety.  If some ordinary material is made radioactive by neutron radiation, it becomes temporarily radioactive but it does *not* become an actinide.  This is why something that is radioactive because of exposure to neutron radiation is so for a matter of days or maybe weeks only.  Nuclear material of this kind poses a radically different (and lesser) threat than does other types of nuclear waste.

And remember: this type of radioactivity "spread" is specific to neutron radiation *only*.  You can expose an object to x-rays or gamma radiation for as long as you want and you will *never* make that object radioactive, no matter how much damage you cause otherwise.