Answer to Question #1290 Submitted to "Ask the Experts"

Q

If a nuclear burst occurs in the atmosphere, not involving the surface, can we expect radioactive contamination? If not, why not?

A

Nuclear explosions in the atmosphere release radioactive by-products regardless of whether the explosion takes place on the ground surface or whether the fireball touches the surface of the ground or not. The radioactive material released includes fission products, activation products from the casing of the bomb, unfissioned nuclear fuel, and activation products from the nuclear fuel. If test is on or near the surface of the ground, such that the neutron flux reaches the ground surface, components of the soil matrix can also become activated by neutron absorption. The origin of this question is probably the frequently quoted idea that detonations where the fireball does not reach the ground do not produce radioactive fallout. This is an over-simplification. The real difference is that for detonations near the surface of the ground, large amounts of soil are vaporized and sucked into the cloud, which is followed by cooling and condensation of the debris into particles. The particles are produced as a result of fusing of radioactive material with entrained soil. These relatively large particles will fall to earth quickly as a consequence of gravitational settling. The particles can have significant amounts of radioactive by-products inside them or plated on their surface. This phenomenon is the source of "close-in" and "intermediate-range" fallout. If the nuclear detonation is not close enough to the ground's surface to suck material into the fireball, the debris cloud will primarily consist of material from the bomb casing that condenses into small spherical particles. Not only is the amount of material in the debris cloud much less, the particles formed are smaller and will stay aloft longer. By staying aloft longer, the radioactivity encased in the particles will decay more before the particles finally settle to the earth. Moreover, the debris clouds of high-altitude detonations will also disperse more by the time the fallout settles to the ground. Hence the amount of radioactivity deposited close to the site of a detonation will be significantly less if the burst does not involve the ground surface.

Steve Simon, PhD
National Cancer Institute