A commenter requested that I explain the difference between radiation and radioactivity. These two words are often used interchangeably by reporters, but they have different meanings. Confusing them is related to other misunderstandings.
Radioactivity is the phenomenon of energy emission by unstable atoms. Radiation is what is emitted. More generally, Wikipedia defines radiation as:
“a process in which energetic particles or energy or waves travel through a medium or space.”
Radioisotopes are elements with unstable nuclei that are therefore radioactive.
Radiation includes light, heat, and microwaves, as well as nuclear radiation. A good chart of electromagnetic radiation can be found toward the bottom of this page. Nuclear radiation also includes the emission of particles, which are different from the sorts of radiation shown in the chart. This is a historical artifact of the names given when these emissions were discovered and their nature was not yet determined.
Nuclear radiation is emitted by unstable atoms, so there is always matter associated with the radiation. The radiation from a nuclear reactor originates in both the fission process and the materials in core of the reactor. Most hospital irradiation facilities rely on a gamma-ray emitter like cobalt-60 or cesium-137.
When steam is vented from the Fukushima reactor core, it carries some radioactive material with it. Normally, there are small amounts of radioisotopes in the coolant water, but damage to the fuel elements has likely released more. The most volatile radioisotopes are released with the steam. Some are gaseous and rapidly dissipate to negligible levels in the atmosphere. Others, like iodine-131, form solid particles as they cool in the atmosphere and fall out as dust does. If there have been fires in the spent fuel pools, they will release a different balance of isotopes in particulates.
Being exposed to radiation, whether from a reactor or a medical accelerator, does not make a person radioactive, except at the very highest doses that cause death in a short time. Radioactive particles on skin or clothing can be transferred to other people.
As environmental measurements are released from Japan, more are measured in becquerels. Sieverts, which I’ve mentioned in my other posts, are a measure of biological effect. A becquerel is one count per second, or one alpha, beta, or gamma per second. Alphas, betas, and gammas have different biological effects, so becquerels are not easily translated into sieverts.
When you’re measuring in becquerels, the numbers are likely to be large. Each count represents a single atom. For comparison, in a cubic centimeter of water (about a tablespoon), there are about 3 x 1022 molecules, or 1023 atoms. A billion is 109. The superscripts indicate the number of zeroes after the first number (assumed to be 1 in the last two numbers).
Cheryl Rofer holds an A.B. from Ripon College and an M.S. from the University of California at Berkeley, both in chemistry. She is retired from the Los Alamos National Laboratory, where she worked from 1965 through 2001 on tthe nuclear fuel cycle, management of environmental cleanups, and other topics. She has also been involved with cleanups in Estonia and Kazakhstan of former nuclear sites. She is immediate past president of the Los Alamos Committee on Arms Control and International Security and a member of the Board of Trustees of Ripon College (Ripon, Wisconsin). She also blogs at Phronesisaical (http://phronesisaical.blogspot.com/)