Fallout

Since nuclear radiation appears to be the preparedness theme of the week, let’s take a look at what it means to survive a nuclear event. After the initial blast and fires, the biggest risk is radioactive fallout. All fallout is, is ash and other products of combustion that have had pieces of the bomb itself attached to them. These pieces are going to be radioactive isotopes, and this radiation is produced as unstable isotopes decay into more stable ones. This decay process gives off energy in the form of radiation. There are over 300 different fission products that may result from a fission reaction. Many of these are radioactive with widely differing half-lives. The half lives of some of these are measured in fractions of a second, while a few are long enough that the materials can be a hazard for months or years. Their principal mode of decay is by the emission of beta and gamma radiation.

We measure radiation in the form of RADs (Radiation Absorbed Dose). This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One rad is defined as the absorption of 100 ergs per gram of material. The unit rad can be used for any type of radiation, but it does not describe the biological effects of the different radiations. For that, we use REM (Roentgen Equivalent for Man). The metric version of the REM is a sievert (1 sievert = 100 REM).

A dose of approximately 100 REM will cause mild radiation sickness, and will increase your chances of premature death from cancer by about 6%. Severe illness occurs at 200 REM, and half of those who are exposed to 300 REM will die within days. A dose of 800 REM is fatal to everyone within hours, even with prompt medical care.

About 5% of the energy released in a nuclear detonation is transmitted in the form of initial neutron and gamma radiation. The neutrons result almost exclusively from the energy producing fission and fusion reactions, while the initial gamma radiation includes that arising from these reactions as well as that resulting from the decay of short-lived fission products.

The good news about fallout is that the isotopes that produce the most radiation also tend to decay the most quickly. The rule of thumb that is generally applied here is called the 7:10 rule. Let’s look at that:

If fallout of 1,000 REM per hour arrives at your location 1 hour after the blast, you have to be sheltered or you will receive a fatal dose in less than an hour. The 7:10 rule states that after seven hours, the rate will drop to one tenth, or 100 REM per hour. In another 7*7 hours (forty-nine hours) it will have decayed down to 10 REM per hour. Then 7*49 hours (~ 2 weeks), it will be down to 1 REM per hour. Once the rate drops to 0.5 REM per hour, you can leave your shelter, which would take about 25 days.

Shelter is where we are screwed here in Florida. At least in other areas of the country, there are basements. Shelter means being shielded from radiation, and that means a couple of things: mass, and not ingesting radioactive material by either breathing it or consuming it in food and water. More on shelter later.