The biggest danger of radiation is that it is so hard to grasp. I’ve learned some new facts about the radiaction in the last weeks, but am still quite helpless and lost. This post is an attempt to organize the knowledge.
For starters, the radiation by itself (the alpha, beta and gamma rays) is not much different from the sunlight or X rays. There is a source, sending the rays. To protect from them, you want to place an barrier between the source of the radiation and you, a barrier that would be opaque for these rays. The more density this barrier has, the better it would stop the radiation. Even the normal air is a (weak) barrier by itself, but several centimeters of it will stop alpha rays, several meters of it will stop beta rays, and several hundred meters of it will stop gamma rays. Generally, radiation rays would travel along a straight line, but there are materials that could reflect them like a mirror reflects the visible light. So I would consider myself absolutely safe from direct radiation if I’m farther than 50 km away from its source (due to the Earth curvature), and reasonably safe on the distance of 1 km.
Sometimes, when rays hit a material, the material itself can produce new radiation as a result of this impact, but this effect seems to be not a huge one, because it is rarely considered in my information sources.
Alpha rays is what we have inside of our CRT monitors, TV sets and the amplifier lamps (hello to all fans of the warm Hi-End sound). Beta rays is what we use in medicine. Gamma rays are similar to X rays and are used for material defects detection etc.
When you’re exposed to radiation, you get a dose. This behaves similarly to a sunburn: the stronger the radiation and the longer you are exposed to it, the worse is the dose. Like with a sunburn, you can expose either some parts of your body, or the whole body to it. Unlike the sunburn, the dose and the negative effects of it accumulate over the whole life span. The body is trying to work against the radiation, repairing the damage, so that being slowly irradiated over a year is much better for the health than getting the same dose in just one day. But body cannot compensate for the radiation damages completely, therefore one should avoid gathering unnesessary doses. Note that it has nothing to do with heavy metals accumulating in the body – this is a separate effect (I’ll touch the metals later on).
There are many units of the dose: sievert, gray and roentgen. They are all different in size and also have different meaning, but, considering that during an uncontrolled accident such as Fukushima there is no possibility to measure your dose exactly, one may reasonably assume that 1 sievert = 1 gray = 100 roentgen. It is not so easy to remember and to convert, particularly due to the “milli” and “micro” prefixes that are often used. I’m using a rule of thumb that a sievert (like $) is like a dollar to the roentgen, which is like a cent.
To get some feeling about the values, one should note these milestones:
an acute (over one day) dose of 3 sieverts and above is deadly;
cancer, infertility and offspring mutations very probably in the next 20 years after accumulating the dose of 0.25 sieverts;
average natural radiation dose per year is 0.006 sieverts.
To measure the strength of radiation, one can use the dose one gets per hour. The natural radiation on Earth has strength of 0,00000023 sievert per hour (0.2 mkSv/h). Something around this level your geiger counter would measure, if you bought one before the Fukushima-related panic sales exploded the prices.
So far I was speaking about a quite simple situation: visible and well identified sources of radiation, i.e. something that humans can detect and protect themselves against. Bad news are that the radiation is produced by individual atoms of the radioactive material. In case of an accident or a controlled nuclear explosion, the structure of the material will be damaged, either mechanically by an explosion, or due to burning because of high temperatures, so that the radioactive atoms (radioisotopes) will contaminate air, water and soil in form of gases and small particles, both invisible for human eye but still radiating.
For some unknown reason, the amount of radioisotopes is measured not in grams or moles, but in becquerel or curie. Curie is very large: 37 GBq, one would normally use it if something went seriously wrong. 1 Bq is very small, it is just one atom decay per second. Because the radiation strength produced by one becquerel depends on the kind of radioactive material, and because radioactive materials have different half-life and therefore different danger level, one typically measure amounts in Bq for each radioisotope that has been released to environment.
After release of the radioactive isotopes, they would fallout or otherwise contaminate some area, volume or things. When soils are contaminated, one typically uses Bq per square km or per square meter to define the contamination level. When air or water are contaminated, one typically uses Bq per cubic cm, per liter or per cubic meter. (Note that one liter is 1000 cubic cm, and one cubic meter is 1000 litres). When other things are contaminated, for example food, they can use Bq per kilogramm or gramm to indicate the contamination levels.
Contaminated environment can either irradiate you externally (you stay on a radioactive dust and it sends gamma rays through you) or internally (you breath isotopes with air, drink them with beverages, or eat with food). The latter exposure is much more dangerous, because the radiation have less barriers (in form of clothes and skin), and because the isotopes tend to stay in tissues of your body and continue irradiating you until they decay completely. Because, during your lifetime, you inevitably breath, drink or eat radioisotopes, the annual dose contributed to internal radiation should rise, so that your overall lifetime dose should rise quicker and quicker with age.
The meanest internal contamination can happen with the food. Fishes and mushrooms would get radioisotopes with contaminated water and accumulate them in their tissues, being in a sense biological radiation accumulators.
For contamination levels, there are limits defined by law, which are quite specific and detailed. For example, the Russian standard from 1999 allows for Caesium 137 (half-life 30 years), for children below 2 years, the mean contamination levels of 27 Bq / m3 in the air, and 11 Bq / kg in drink water. For Iodine-131, the numbers are 7,3 Bq/m3 and 6,3 Bq/kg respectively. At the same time, 1000 Bq / kg in food is defined to be the warning level for general population (the food is allowed, but population must be warned and educated) and 10000 Bq / kg is defined to be the action level (such food is not allowed and must be destroyed).
The Japanese limits seems to be 2000 Bq / kg in food and 40 Bq / kg in water. According to TEPCO and MEXT measurements on April 5th, they have levels of Iodine-131 from 2200 to 41000 Bq / kg directly on the Fukushima coast line, up to 420 Bq / kg in the sea distance of 15 km from NPS, and max. 66 Bq / kg at 30 km.
The outer fallout zone remained after controlled nuclear explosions seems to be defined by the value of 1 curie / km2 (37 Bq / m2) — supposedly, anything below that level is either hard to measure or doesn’t have much environment impact. Around Chernobyl, the prohibited zone is defined to be 40 curie per km2 or more, and the “periodic control zone” is 5 to 15 curie per km2.
There are tables allowing to calculate the dose in sieverts knowing the dose in Bq, as well as calculate the dose in Bq knowing, say, the air contamination in Bq / m3. If you don’t have them, just remember you breath 16 liters of air per minute, and you can get annual Sv by dividing Bq by 200 000 000 (for Cs-137).
Calculation example: you have breathed air with 1000 Bq / liter in it for 15 minutes. You have breathed 240 liters, or 240000 Bq. If it was Cs-137, it will remain in your body and reduce itself by 50% in the next 30 years, so that your annual dose will rise by around 1.2 mSv.
Another example: you’ve drunk one cup (250 ml) of 40000 Bq / kg water coming directly from the reactor. You’ve got 10000 Bq and your annual dose increased by 50 mkSv.
Any comments and additions / corrections are welcome.