It can’t all be good news. Yes, using a properly designed nuclear reactor, we can capture vast amounts of useful energy by helping atoms get closer to the ideal, iron. Now to the first big wrinkle, radiation.

Unhappy atoms break up in a few different ways, all releasing energy. We can break it down into three categories:

1. Throw out a nucleon The atom flings off some combination of neutrons and/or protons. Most commonly, this is a helium nucleus comprised of two protons and two neutrons called an alpha decay. Nuclear fission is also in this category and what we care about the most for our chain reaction.

Since we’ve already talked about fission, let’s go through an example of an alpha decay. Uranium-238 goes through alpha decay, loses two protons and two neutrons to become Thorium-234. (Get it? 238 to 234, because four nuclear particles are lost. Uranium has 92 protons, thorium 90.)

2. beta decay: Throw out a nearly mass-less charged particle, an electron or positron So far, I’ve ignored the electrons. Electrons are for chemists. Those losers can’t change atoms; they can only rearrange atoms. We’re nuclear physicists right now.

The most common beta decay, beta negative, involves an atom splitting a neutron into a proton and an electron. The proton sticks around in the nucleus, the electron goes flying outward. Because the proton sticks around, the atomic number of the atom undergoing beta negative decay goes up. Neat.

The Thorium-234 made by an alpha decay of U-238? It’s also unstable, but tends to go through a beta negative decay, gaining one atomic number to 91 and thus becomes protactinium-234. The overall weight doesn’t change, as neutrons and protons remain about the same, but the atomic number does.

3. Spit out energy as a high energy electromagnetic gamma ray. This is similar to glow in the dark stickers, just with far more powerful waves. The atom doesn’t change at all, in a chemical sense, staying with the same weight and atomic number. Therefore, gamma radiation often accompanies another decay, an alpha or beta. The energy released by the atom during the other decay gets stored temporarily in the atom. Its release is what causes the gamma ray.

The huge differences in mass between alpha, beta and gamma radiation really matter. If we think of beta particles as being the size of a bullet, alpha particles are more like cannon balls. At the same energy, the beta particle will be going about forty times faster than an alpha.

Imagine we’re shooting at someone through a wall. The cannon ball, flying 40x slower than the bullet but much heavier, will smash the into the wall, causing huge amounts of damage to the wall (the first thing it hit). But, the person behind the wall will probably be ok, so long as they’re far enough behind the wall. The bullet, much lighter and faster, is far more likely to go straight through the wall–losing some speed, but retaining more on the far side of the wall than the cannon ball. The bullet has a far better chance of hitting the person.

Alpha particles, the cannon balls, can be stopped by a single sheet of paper. Smash! Likewise, the dead outer layer of skin does a damn good job of protecting your living cells from alpha particles. Beta particles, the bullets, go right through paper. A thin sheet of aluminum, or something of similar density and substance, will gobble these up.

Gamma radiation is trickier. Gamma radiation is just a freakishly high energy version of light, with almost no substance. Just like light can pass right through your hand, gamma radiation can pass through all but the heaviest and densest of metals, wreaking havoc deep into the body.

Gamma radiation is the most likely to cause your body misery. Eating an alpha emitter? Not so smart, as your gut takes the big hit rather than the dead layer of skin cells. Beta emitters can cause quite a bit of damage. But it’s gamma rays, passing right into your depths with ease, that really cause misery.

What does radiation do to the body? Mostly, the problem comes from gamma rays shredding DNA and RNA, destroying the cookbooks and scrap paper of the cells, rendering them unable to divide or function properly. At the lowest fatal doses of gamma irradiation, the DNA in blood-forming cells is scrambled to the point these cells stop dividing. Within a few weeks one starts to become anemic from the loss of replacement red blood cells. After a few more weeks, the numbers of white blood cells also drop, as they are also not being replaced anymore. One perishes from opportunistic infections, like one does from AIDS.

At yet higher doses of radiation, the cells in the gut stop dividing–again, thanks to scrambled DNA. The walls of the gut thin, eventually allowing the massive collection of bacteria in each of our guts to enter the body and devour us alive. Even higher doses? So many brain cells are destroyed that the whole brain starts to swell, eventually stopping breathing. Even higher? The cells of the body are literally shot to pieces by all the gamma radiation and fast moving alpha and beta particles. Too many of the cells in the body simply burst and die.

It actually takes quite a bit of radiation to get anywhere close to fatal. Our bodies are adapted to handle quite a bit of radiation, as the world we live in is quite radioactive. Gamma rays are constantly raining down on us from the sun. Water is radioactive. Most metals are radioactive. At least some variants of the atoms that make up most of us–carbon, oxygen, nitrogen and so on–are radioactive. Our cells handle persistent, low-level irradiation with some skill. It’s sudden, massive, bursts that cause big problems.

Lest you believe radiation is only a problem of nuclear power, it’s also worth noting that almost all fossil fuel is contaminated with radioactive metals and gases; carbon isn’t the only thing streaming out of smokestacks.