Kaboomly / Fallout explained

What Is Nuclear Fallout?

Nuclear fallout is radioactive material that falls back to the ground after a nuclear explosion. It is carried by dust and debris lifted into the mushroom cloud, drifts with the wind, and can land far from the explosion itself.

A short definition

A nuclear explosion splits heavy atoms and leaves behind radioactive fission products. In the fireball these are vaporized together with the rest of the weapon and anything drawn in from below. As the cloud rises and cools, the radioactive material condenses onto particles of many sizes. When the particles fall back to Earth, the radioactivity they carry is called fallout.

Glasstone and Dolan's The Effects of Nuclear Weapons (1977), the standard public reference, covers the subject in chapter IX. It separates fallout from the initial radiation of the explosion, which is released within the first minute (§9.01). Fallout is part of the residual radiation that comes later and can arrive far from ground zero (§9.02).

Local and global fallout

The book splits fallout by timing (§9.03):

• Early, or local, fallout is whatever reaches the ground within the first 24 hours. It is made of larger, heavier particles and lands mostly downwind of the explosion. This is the part that can give high radiation doses.
• Delayed, or global, fallout reaches the ground after the first day. It is made of very fine particles that can stay in the upper atmosphere for months or years and settle in low concentrations over much of the world. Its radioactivity has decayed a great deal by the time it lands.

For a surface burst on land, Glasstone and Dolan estimate that 40 to 70 percent of the radioactivity ends up in the early fallout, and they use 60 percent as a working value (§9.59). The rest becomes delayed fallout.

Why surface bursts make more local fallout

The amount of local fallout depends mostly on whether the fireball touches the ground. In a surface burst, large amounts of soil and debris are sucked up into the fireball and the rising cloud. Radioactive material condenses onto these particles, and the larger ones start to fall back within minutes to hours. Larger particles fall faster and carry more activity, so they contaminate the area near and downwind of ground zero most heavily (§9.62).

In an airburst where the fireball stays well above the ground, there is little soil to carry the radioactivity down. Glasstone and Dolan explain that by the end of about a minute, essentially all of the residues have risen so high that their radiation no longer reaches the ground in significant amounts; the fine particles then spread widely and come down very slowly (§9.04). The main exception is rain or snow falling through the cloud, which can wash some of it out (§9.67).

How wind shapes the pattern

Where fallout lands is decided by the wind at every height from the ground to the top of the cloud (§9.66). Heavy particles land close to ground zero. Smaller ones can take many hours to fall and may be carried hundreds of miles, and the finest can circle the Earth before they come down (§9.64).

When the wind blows steadily in one direction at all heights, the result is a long, narrow area the book calls "cigar-like" (§9.85). A faster wind stretches it farther downwind; a slower wind makes it shorter and more concentrated near ground zero. When the wind turns with height by about 20 degrees or more, the pattern becomes wider and shorter, and with extreme turning it can spread in several directions at once (§9.99). Rain, uneven terrain, and gusts create "hot spots" of much higher contamination than the surrounding area (§9.66, §9.67).

How fallout radiation fades: the 7:10 rule

Fallout is most radioactive soon after it forms and then fades, quickly at first and more slowly later. Glasstone and Dolan give an approximate rule, often called the 7:10 rule, in §9.15: for every sevenfold increase in time after the explosion, the dose rate decreases by a factor of ten.

• At 7 hours, the dose rate is about one tenth of its value at 1 hour.
• At 49 hours (about 2 days), it is about one hundredth.
• At 343 hours (about 2 weeks), it is about one thousandth.

The book says the rule is accurate to within about 25 percent up to about 2 weeks, and within a factor of two up to roughly 6 months. It is only an illustration. Real fallout is a changing mixture, and the book stresses that in any real situation, actual measurements would be needed (§9.15).

What the Kaboomly simulator shows

The fallout map in the Kaboomly nuke simulator draws fallout for a surface burst only. It uses Glasstone and Dolan's idealized contours from Table 9.93 (§9.93), which describe the dose rate one hour after the explosion for a steady effective wind of 15 miles per hour. The wind is fixed and always points east. It never changes with the city, the season, or real weather.

To give a sense of scale, for a Little Boy surface burst (15 kt) the model's 100 rad per hour contour reaches about 48 km (30 mi) downwind, and the outermost shaded contour, 1 rad per hour, reaches about 218 km (135 mi). For a B83 surface burst (1.2 Mt) the outermost shaded area stretches about 1,564 km (972 mi). In both cases that is far beyond the widest blast ring, which matches the book's point that fallout can reach well past the other effects (§9.65).

Those figures describe a textbook shape, not a prediction. Glasstone and Dolan warn that the idealized patterns tend to make the downwind extent as long as possible, because they assume the wind barely changes direction with height (§9.99), and that real patterns are much less regular (§9.100). The contours are for a smooth, open surface; real terrain would lower the dose rates (§9.95). The simulator also never shows death or injury numbers, only distances. The How It Works page explains the method in full.

Historic context: Castle Bravo, 1954

The best documented example of heavy local fallout is the BRAVO test at Bikini Atoll on March 1, 1954, which Glasstone and Dolan describe in §9.104 to §9.110. The device, with a total yield of approximately 15 megatons, was detonated about 7 feet above a coral reef. Its fallout contaminated an elongated area extending over 330 miles downwind and up to more than 60 miles wide (§9.104).

Even for this well studied test, the book notes that because there were no measurements over large areas of ocean, the exact shape of the pattern is "largely a matter of guesswork" (§9.105). What is certain is that contamination reached 300 miles or more downwind. Inhabitants of Rongelap Atoll were exposed to fallout before they were evacuated (§9.109). The event is one reason fallout became a major subject of study, and the book stresses that it came from a particular combination of yield, a very low burst height, the surface below, and the weather (§9.111).

Official guidance

This page explains the science and is not safety advice. For real-world preparedness information in the United States, see Ready.gov's page on radiation emergencies, which covers nuclear explosions and fallout. Outside the United States, use your national or local emergency management agency.