Alpha, Beta, Gamma and you

Washington Post

Atomic science 101 and why the US State Department was wrong

Japan’s current advice is that people stay more than 20 km away from the plants, and if they’re 20-30 km away, that they stay inside.

First, the inverse square rule.     As you move away from a radiation source, every time you double the distance away from the source, the radiation level drops to 1/4 of what it was.

Atomic reactions generate three types of radiation – alpha, beta and gamma

Alpha radiation is harmless – it is non-ionizing beyond a few inches – meaning it causes no changes to atoms.  A sheet of paper stops alpha radiation.   If you have a couple feet of air between you and the source, it won’t reach you.    Your smoke detector uses an alpha ray emitter to detect smoke binding with the ions it is creating and that change in the ion flow is what sets off the alarm.

Beta radiation is harmful – if you’re close to it.   Get more than maybe 10 feet away, it doesn’t reach you.   A thin coat of lead will stop it.   (quick, get the lead out of the paint in your homes! – I digress)   Even if Beta reaches you, it actually does less ionization than alpha.

Gamma radiation is the bad stuff – alpha and beta are particles, gamma rays are pure energy.     Pure energy means they travel at the speed of light – gamma rays and x-rays are basically the same thing, just different because of how they are created.    A gamma ray travels in a straight line like the light from a light bulb.   It takes about 3 inches of lead to stop a gamma ray.    Gamma rays are absorbed by the air, and the further away you are, the less “bright” the gamma ray light is.    If you’re 20 km away from the plant, the gamma rays won’t get to you.

Spent fuel storage tanks use the 26 feet of water to block the gamma radiation going upwards – the sides of the tank are 6 feet of concrete and steel – enough to stop the gamma “light” from coming out the side.    If you remove the water from a spent fuel tank and the walls are intact, the gamma radiation will only emit upwards.   If you’re in a helicopter dropping water over a gamma ray emitting storage tank with no water covering the fuel, you get a serious dose of gamma radiation.   If you are on the ground below the tank and use a water cannon, you are not getting gamma radiation from the pool (as long as the bottom of the tank is still intact.   

So if you’re beyond the 20 km limit, the only way you’re going to get exposed to radiation is if the radioactive fuel escapes from the plant site – so that the alpha, beta and gamma ray emitters are sitting inside your house, not inside the reactor.   Cesium 137 is a potent beta and gamma ray emitter created only by nuclear fission.   It has a half life of 30 years.    You are exposed to Ce137 every day because of the atmospheric tests that were done in the 1950s that released large of amounts into the environment.   Cesium is why the area around Chernobyl is uninhabitable and food can’t be grown near it.

Radioactive iodine 131 is another radioactive byproduct of nuclear fission of uranium.   This is why *some* people *might* be advised to take a potassium iodide pill,   The reason is that Iodine 131 has a very short half-life compared to Cesium.   Ioidine 131 loses half of its potency every 8 days.   Unless the nuclear reaction resumes, no new Iodine 131 is being created and the existing I131 in the fuel rods is decaying.   By the time I131 could arrive at the United States by blowing in the wind, much of it would have turned into inert hamless Xenon gas and it would be widely dispursed.

Iodine 129 is also created – but in smaller quantities (about 1/5 the amount of I131).   Because it has a half life of 15.7 million years, it radiates almost no radiation, but once created will be around forever.       I129 is a marker that fission has occurred, but no health consequences.   The body excretes Iodine over time and the I129 will be flushed out long before any atoms decay and release radiation inside your thyroid gland.

Radioactive isotopes of an element can either generate a lot of radiation for a short time, or a small amount for a long time, but not both.    These are the common isotopes of Plutonium, for example.