San Francisco Example: Radiation & Fallout

Radioactive Fallout

The initial radiation in the form of gamma rays and neutrons can be a major source of fatalities in the first few miles from a nuclear blast of 1.8 megatons. Several factors in this scenario reduce the impact that initial radiation has on the overall number of fatalities. With the blast centered more than a mile off shore much of the lethal radiation zone is located over water where there are few people. Additionally, the heavy fog will attenuate the radiation faster than dry air, and the hills will shield many people. Since most people are inside, they will receive further shielding by buildings - typically between 10% and 40%.

Because of the dirty nature of this blast, fallout will be an especially important effect to consider. This blast will likely deposit measurable, if not harmful, amounts of radioactive debris across much of the U.S., in the form of delayed fallout over the next several months. Even more important are the localized effects that the early fallout, defined as all the fallout occurring in the first 24 hours, will have on the survivors, rescue workers and those not directly affected by the blast but living in the vicinity of San Francisco.

In a surface blast, fallout consists of neutron activated weapon debris, fission products, unused fission material, activated sea water and sea floor material. Surface blasts generally produce more early fallout since the average size of the fallout particles are larger and thus fall back to earth faster. Over 300 species of radioactive materials are produced in a blast. Each has its own half life and some interact in particular ways with the human body.

If we assume that there is little wind, the fallout will settle back to earth in a circular pattern centered around ground zero. For a 1.8 megaton blast where roughly one megaton of yield comes from fission, we can expect the following total dose levels.

After the first 24 hours, when most of the early fallout has settled, the radiation level decreases roughly in proportion to elapsed time. Reducing exposure by leaving the area or remaining in a fallout shelter will greatly reduce the radiation effects on people

Significant levels of radiation (greater than 200 rads will cause radiation sickness) will extend out much farther than other effects such as blast overpressure. Radioactive contamination will hamper rescue, cleanup and recovery efforts as most of the city will have dangerously high levels of radiation for many weeks to come.

The radiation given off by fallout is dangerous because of the way it interacts with the body's cells. There are three basic types of radiation given off by radioactive fallout: gamma, beta and alpha. The gamma rays are high energy photons that penetrate deeply into tissue and cause damage by scattering off of, and knocking loose, electrons that are a part of the body's cells. The beta rays (electrons) only penetrate shallowly but can cause severe burns. The alpha rays (helium nuclei) are the most massive and do the most damage when they interact with the body's cells, ionizing elements they contact. Neutrons interact with the nuclei to lock out protons that then ionize other elements.

In addition to the direct effect that radiation has, some elements can be absorbed into the body causing greater internal damage to specific organs. Strontium 90 (created in the fission process) is similar in chemical composition to calcium and lodges in the bones, especially in children. Iodine 131 is absorbed by the thyroid gland and can result in cancer of the thyroid or hypothyroidism. Cesium 137 is taken into the body much like potassium and is distributed fairly evenly.

Fallout Effects

Dose-rem Effects
5-20 Possible late effects; possible chromosomal damage.
20-100 Temporary reduction in white blood cells.
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