I recently posted a copy of my old radiation safety notes. Those were written for young physicists, so they aren’t exactly models of clarity. Since people asked, here’s a completely nontechnical summary of the key things to know about radiation.
Radiation consists of high-energy particles of all sorts. They can injure you by crashing into cells. If they hit DNA they can damage it, and if enough DNA gets damaged then some damage will make it past your body’s bad-DNA-removal systems and turn into cancer. If you get really a lot of radiation, it can kill cells directly, especially in internal organs; this causes radiation sickness and is quickly (and unpleasantly) fatal.
Elements are the different types of atoms which exist in the world. Each atom has a nucleus which is made out of a mixture of protons and neutrons. The number of protons, a.k.a. the atomic number or simply “Z,” determines all of the chemical properties of the element, and so which element it is. (For example, Carbon has Z=6, Uranium Z=92) The total number of protons and neutrons is called the atomic weight, or sometimes “A.” Isotopes are variants of the same element with different atomic weights. (“Uranium-238″ is the isotope of Uranium with a total of 238 protons and neutrons) Most combinations of protons and neutrons aren’t perfectly stable; over time, they tend to fall apart (“decay”) and emit small parts of themselves, i.e. radiation. Substances like these which naturally emit radiation are radioactive materials. The more unstable a nucleus is, the faster it will decay; the amount of time it takes half of the nuclei in a sample to go away is called the half-life of the nucleus. This can range from milliseconds to millions of years. Radioactive materials with short half-lives will be very radioactive for a short while, but then stop quickly; radioactive materials with longer half-lives will stay moderately radioactive for a very long time.
Because chemistry only depends on Z, different isotopes of the same chemical will mix thoroughly and can’t be separated by chemical means. This means that if you need a lot of one particular isotope, you have to do something complicated and mechanical. This also ties in to one of the main dangers of radioactives, below. “Low-Z” materials are materials with a Z up to about 20; this includes most organic materials, plastics, and so on. These materials are relatively transparent to radiation, which is why airport backscatter X-rays can’t see explosives. “High-Z” materials include most metals (except Aluminum!), and absorb radiation well.
There are four common types of radiation. Alpha particles, which are light atomic nuclei, can cause a lot of damage (because they’re relatively heavy, and being made of nuclear matter interact a lot with other nuclear matter). However, they’re relatively easy to block; even paper or clothing offer considerable protection. They can be very dangerous if ingested, though. Beta particles are electrons and positrons, and penetrate more deeply than alphas, though they do less damage. Gamma rays are simply high-energy light (part of a wider category including UV light and X-rays) and are the most penetrating. Betas and gammas are most effectively blocked with a thick layer of high-Z material such as lead, or several feet of concrete. Neutrons are emitted by the decay of certain very large radioactive elements, including those used in nuclear power. They can cause as much damage as alphas, but because they have no electric charge they are much more penetrating. Also, high-Z nuclei tend to absorb neutrons into themselves. This makes them good at shielding neutrons, but in the process they tend to turn into other, often radioactive, isotopes. This means that metals used in neutron-rich environments like reactors tend to become radioactive over time.
The common units for measuring radiation doses are the Sievert and the rem.* This chart can give you a notion of some effects of radiation; section 5.6.3 of The Nuclear Weapons FAQ goes into more detail and talks about acute doses.
Danger levels: For a normal adult, up to 5 rems per year total (up to 1.5 for a pregnant woman) is safe and should cause no adverse effects. Doses up to 50 rems will cause increasing risks for cancer; generally, each 0.01 rems you get is about one “micromort” (a one-in-a-million increase in your chance of dying, about the equivalent to driving 230 miles) Doses above 50 rems can cause acute radiation sickness. Children are more vulnerable because they’re smaller, so they get more radiation per unit of body mass.
Normal levels: By simply walking around and existing, you pick up about 0.3 rems per year, from natural radiation in rocks, from space, and so on. A chest X-ray is good for about 0.01 rems; an airplane flight, about about 0.04 rems; a mammogram, about 0.3 rems. The most radioactive fruit is the banana, which is rich in Potassium. (The radioactive isotope Potassium-40 is about 1% of all Potassium in nature) Eating a banana is good for about 0.01 mrem, or one Banana Equivalent Dose. You can learn how to protect yourself from them here.
Nuclear fission is a reaction in which a large, unstable nucleus breaks into two smaller nuclei, generally throwing off some spare protons (and a lot of excess energy) in the process. Only a few isotopes do this. If one of these isotopes is hit hard by a neutron (which is heavy but electrically neutral, so it can get in close to the nucleus) it can be enough to break that nucleus, causing it to fission and emit more neutrons. This can cause a chain reaction; if enough neutrons hit other nuclei, rather than being absorbed or escaping, the total number of fissions keeps increasing. An assembly of fissile material which can do this is a critical mass.** Nuclear reactors work by having a large pile of fissile material, generally a 3-5% solution of Uranium-235, which is just at the level of criticality but no more. These are mixed in with control rods, which are rods of some neutron-absorbing substance like Cadmium or graphite, which can be moved in and out to control the speed of the reaction. Together these form the reactor core, which gets very hot from all this energy release; that heat is used to heat water or some other fluid, and ultimately to make steam which turns a turbine, like in any other power plant.
Nuclear fusion is a reaction in which two small nuclei are rammed together to form a slightly larger nucleus. For certain combinations of nuclei, this will create a net release of energy. Fusion releases more energy than fission, and its result products are nonradioactive.*** This is great for clean energy, but it’s much harder from an engineering perspective.****
At the Fukushima-Daiichi power plant, seismometers detected the earthquake and immediately shut down the reactor by driving the control rods fully in. However, even with control rods fully in and the chain reaction stopped, the baseline radioactivity of the core requires constant cooling. The earthquake knocked out the power to the cooling system, and the tsunami destroyed the backup systems. Plant operators moved on to the next level of backup cooling methods, which involve pumping seawater directly into the hot bits. While the cores of several reactors have melted, the core containment units have held, and so none of the core materials were released. Another problem at the plant was that pressure was building up in the cooling system, because what water was there initially was boiling (and also being decomposed into Hydrogen and Oxygen – water does that at high enough temperatures) and the automatic venting system had lost power as well. This was dealt with by manually venting the steam. Some of this steam was radioactive, and this is where the radiation releases happened. A third problem are the spent fuel rods, which were in temporary (cooled) storage in the reactor as well; their cooling water was not being replenished, and humans have had to go in and assist. All of this has been complicated by problems such as “the building is on fire” and “if hydrogen builds up in an area and isn’t vented quickly enough, that part of the building will explode.” Heroic work by the reactor crew has so far minimized the effects.
At Chernobyl, on the other hand, thanks to a combination of poor design, poor maintenance, and poor operations, the core temperature went out of control and this caused the control rods themselves to burst into flame and explode. This destroyed the core’s containment vessels, and spewed bits of the core itself into the air and the environment. The worst radiation doses in the Fukushima area have been in the ballpark of 0.3 rems per day, for about two days; the worst radiation doses in the Chernobyl area were several thousand rems.
The difference between the two has to do with the amount of stuff emitted, and the chemistry of the stuff emitted. The radioactive steam from F-D got its radioactivity by being ordinary water bombarded by neutrons from the reactor itself. Low-Z materials tend to transform into relatively safe isotopes with short half-lives. (For example, Oxygen in nature is 99.7% O-16, which gets turned by a neutron into O-17 (stable); 0.04% O-17, which turns into O-18 (also stable); and 0.2% O-18, which turns into O-19 (beta source, half-life of 27 seconds; turns into Flourine-19, which isn’t healthy but isn’t radioactive). [You can look this stuff up here] So if you irradiate the oxygen in water, you get a very small amount of a source which decays quickly) The contents of the core, on the other hand, include such exciting isotopes as U-238, Cs-137, and Sr-90. Anything which spews that into the air is likely to be Very Bad.
Strontium-90 is a good example of a dangerous isotope. It’s a beta source with a half-life of 28.9 years. More importantly, Strontium is chemically very similar to Calcium; if you get it inside your body, your body will try to build bones out of it. Radioactive bones, which come with little radiation sources shining straight in to your bone marrow. Cesium-137 is similar to Sodium, although not as much as Strontium is to Calcium; Cs-137 is commonly used for radiotherapy for cancer. Plutonium-239, used in nuclear weapons, is one of the worst; it’s a powerful alpha and beta emitter; it’s chemically a more reactive version of Iron, and so reacts with Hemoglobin to form Plutoglobin, which your body then transports into your bone marrow; and when exposed to air, it forms a pyrophoric oxide which not only burns, it oxidizes itself (think Magnesium) and so can’t easily be extinguished.
So to summarize: Radiation is made of energetic particles which harm you by ramming into cells and damaging them; radioactives are materials which naturally emit radiation. You can naturally heal radiation damage, and block many kinds of it, but ingesting radioactives is particularly dangerous. Some radioactives look like elements used by your body, and so your body will ingest them particularly thoroughly. Others (like those in the steam) are fairly neutral and nonreactive, and so once they stop being radioactive themselves are safe. If you hear about a radiation dose, remember that anything up to a rem per year (10 mSv per year) is not cause for panic.
* The roentgen is a unit of radiation emission. The rad measures how much radiation you absorb; one rad is 0.01 Joules of energy per kilogram of your body mass. (More body = more ability to absorb things) The rem (short for Radiation Equivalent in Man) multiplies rads by a factor which takes the different damage levels from different kinds of radiation into account; betas and gammas are 1, alphas and neutrons (depending on their energy) anything from 1 to 20. The Sievert is roughly 100 rems, but Sieverts use a different table of medical effects. You’ll commonly see milliSieverts (1 mSv = 0.1 rem) and microSieverts (1μSv = 0.0001 rem).
** There are actually two different kinds of critical reaction, prompt critical and delayed critical. The former is used in nuclear bombs, the latter in nuclear power. Achieving prompt criticality isn’t something you can do by accident; nuclear reactors can never undergo the sort of reaction that triggers a nuclear bomb.
*** The stability of a nucleus depends on the difference between the energy of the nucleus as a whole, and the energy it would have if broken up into pieces. If this difference is negative, then the nucleus is stable; if it’s positive, then it’s unstable, and the bigger it is, the more unstable it gets. (Which means a shorter half-life) Energy-producing reactions work by making something more stable and using the energy difference you get. Fission works by taking a big, unstable nucleus, and kicking it so that it breaks into two smaller pieces; that energy difference is released as kinetic energy. But the smaller pieces aren’t guaranteed to be stable, just “more stable than what you started with.” Fusion works by taking two nuclei and combining them into something which is more stable than the originals were; once again there’s a difference, but this time by definition what you’re left with is stable. There’s no “chain reaction” for fusion; you need some way to keep the particles fusing, even though they electrically repel one another, and keeping that pressure up is the hard part. Some techniques are crushing the matter with a nuclear bomb (useful for H-bombs), compressing it with lasers, compressing it with giant magnetic fields, or crushing it with gravity. (Which is how the Sun works)
**** Practical fusion energy is about 20 years away, and it has been for the past 50 years.