Atomic cloud from a nuclear explosion, damaging factors

Atomic cloud from a nuclear explosion, damaging factors

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In a nuclear explosion, as a result of nuclear reactions, a huge amount of energy is released in a very short time in a small volume, as a result of which the temperature here reaches several million degrees. All the materials included in the bomb and part of the captured soil evaporate. A so-called fireball is formed, which in the first moments is a uniformly heated sphere of small volume. Then there is an expansion of the volume while simultaneously lifting the fireball up. So, 0.7 ms after the explosion of a bomb with a TNT equivalent of 1 megaton, the fireball has a radius of about 70 m, and after 10 seconds its radius reaches a maximum value of about 1km.

In a ground explosion, a fireball touches the surface of the earth. As a result of exposure to high temperature and shock wave pressure, a large amount of soil located in the territory where the explosion occurred is involved in the fireball. A large funnel is formed. So, for example, for dry soil at the surface of a bomb explosion with a TNT equivalent of 1 kiloton, the diameter of the funnel will be approximately 38 m, and the depth 7.5 m.

Cloud formation.

Soil, bomb fragments and objects that have fallen into the central zone quickly rise up, carried away by a fireball and ascending air currents. Thus, a fireball from a bomb explosion with a TNT equivalent of 1 megaton has a lifting speed of about 400 km/h in the first minute, and after 4 minutes — about 100 km/ h, and after 4 minutes — about 100 km/h [Wilken, 1050]. During the ascent, the fireball with trapped substances cools, ceases to glow and forms an atomic cloud. The cloud continues to rise for some time. When the rise of the cloud slows down, it expands, taking a mushroom shape, where the “leg” of the mushroom is a column of dust formed during the ascent, and the “hat” is the expanded cloud.

In a ground explosion, the “leg” will be represented by a column consisting of falling earth that has been lifted into a cloud. The height of the rise, the vertical size and diameter of the cloud depend on the power of the explosion and on meteorological conditions (Fig.4).

The effect of the tropopause on cloud formation.

The height of the location of the tropopause has some influence on the formation of the cloud. The tropopause is the boundary between the troposphere and the stratosphere. Its height depends on the time of year and the latitude of the area. So, in tropical latitudes, the height of the tropopause is on average about 15-18 km, and in middle latitudes 8-9km. In summer, the tropopause is 2-4 km higher than in winter. It is believed (2) that only a cloud penetrates into the stratosphere from ground explosions of bombs whose power is higher than 1 megaton and from high air explosions.

At different altitudes in the atmosphere, the wind directions and speeds are different, as a result of this, the cloud may be stratified and torn into several separate parts.

From the diagram shown in Fig. 4, it can be seen that the cloud from the explosion of a bomb with a capacity of 20 kilotons reaches a height of 2-4 km, with a capacity of 1 megaton – a height of about 17 km, etc. To set the height of the rise of an atomic cloud during an air explosion, the height at which the explosion was made is added to the height of the rise determined according to the schedule.

Particles of the atomic cloud.

The soil particles captured during a nuclear explosion undergo the following changes. Particles trapped in the central part of the fireball at a temperature of several million degrees evaporate: particles trapped in an area with a lower temperature are melted. A large amount of crushed soil is picked up by ascending air currents, introduced into the cloud and mixed in it. Upon cooling, the particles of evaporated and molten soil turn into teardrop-shaped or spherical particles, often sticking to the non-molten soil particles.

Depending on the size, the raised particles are distributed differently in different parts of the atomic cloud. Soil particles with a size of 80 µ are located mainly in the “leg” of the atomic mushroom, particles with a size of 80-150 µ make up the majority in the lower part of the cloud, and particles less than 80 µ make up about 70% of the total mass of particles in the “cap” of the mushroom.

The composition of the radioactive cloud.

The composition of radioactive cloud particles depends mainly on the composition of the soil located at the explosion site. During the nuclear weapons tests, the method of formation, composition and characteristic features of radioactive particles were thoroughly studied.

Part of the materials captured by the fireball evaporates, the other part is intensively mixed in the “leg” and the atomic cloud. Evaporated materials, as they cool, condense either directly or on the surrounding non-evaporated particles. Basically, two types of particles are described in the literature: those formed during ground explosions carried out on calcareous soils and those carried out on sandy soils [Adams, Falow, Schell (Adams, Farlow, Schell, 1960)].

Particles of the first kind.

Radioactive particles of the first type were formed during test explosions on the atolls of Eniwetok and Bikini. According to their shape, these particles were of two types: angular and spherical with a size from a few microns to 1 mm or more. Their density depended on the density of calcium compounds and inclusions of air bubbles or gases and ranged from 2 to 2.8 g/cm3.

The angular particles consisted mainly of fine-grained calcium oxide, often interspersed with unchanged coral. Radioactive isotopes have always more or less uniformly covered the surface of the particles, diffusing slightly inward. With the exception of a few cases when radioactivity was caused by black balls sticking to angular particles (size from 10 p or less), the sources of radioactivity were not visible even at high magnification.

The spherical particles consisted mainly of calcium oxide coated with a thin layer of calcium carbonate. Radioactivity was distributed unevenly over the entire volume of these particles.

Angular particles were formed at a temperature of 800-900 °, and spherical — 2570 ° (melting point of the calcium compounds that make up the particles).

Particles of the second kind.

The particles of the second type were collected and studied during test explosions at a landfill in Nevada (USA) and in Australia. These particles consist of glass, which was formed from the fusion of silicate soil materials. The radioactive particles were of two types: transparent balls of yellow-green color and dark yellow-brown irregular shapes that did not differ in appearance from the original soil. Both types of particles had a diameter of 2-3 mm or less. The transparent balls contained radioactivity homogeneously distributed throughout the object.

Obviously, they were formed from surface soil minerals heated to a liquid state.

Irregular particles, as found in thin sections, consist of transparent light brown and colorless glass with random zones of dark brown and black. They contain inclusions of molten particles and air bubbles and look as if they were formed from a foamy viscous mixture fused with non-molten soil minerals.

Distribution of radioactivity in the atomic cloud.

Radioactivity is distributed unevenly across all particles, the greatest radioactivity is noted in the areas with the most intense staining.

During radiochemical and spectrometric analyses of the isotopic composition of radioactive particles, it was found that there is some difference between particles of different types. Radioactive isotopes condense on angular particles in the colder part of the fireball and later than on spherical ones.

It is obvious that angular particles will receive less radioactivity and that the effect of isotope separation will affect the condensation and capture of later radioactive decay products from short-lived isotopes.

Fallout from the atomic cloud.

All the particles raised in a nuclear explosion fall back over time. The rate at which particles land from the point of explosion depends on the size of the particles and the wind speed in the path of these particles.

All radioactive fallout can be divided into three main types [Wilkens, 1959; Libby (1956)].

Local.

Local — with a large particle size. These precipitation falls within 10-20 hours after the explosion in an area up to 400-500 km from the center of the explosion.

Semi-global.

Semi-global, or continental, precipitation consists of small particles trapped in the upper layers of the troposphere. The precipitation of these particles is slower due to their small diameter (5 µ or less).They continue to fall to the ground for 2-3 weeks from the moment of the explosion.

During this time, the cloud manages to circumnavigate the globe. In semi-global precipitation, most of the short-lived radioactive isotopes disintegrate by the time of precipitation.

Global.

Global precipitation consists of small particles (less than Iµ) that have been introduced into the stratosphere.These precipitation falls very slowly. The period of their precipitation on the earth’s surface, according to various data, ranges from six months to 5-7 years.

In the future, the hazard factors that arise from the local precipitation of a ground-based nuclear explosion will be considered. However, all provisions with some amendment can be applied to all types of radioactive fallout.

According to some researchers (Wilkens, 1959), during a ground explosion, about 90% of the entire raised mass of soil settles near the explosion site in the form of local precipitation. Special cases occur when radioactive and rain or snow clouds are mixed.

In this case, raindrops or snowflakes capture radioactive particles and accelerate their transfer to earth. It is believed that the efficiency (1) of capturing radioactive particles by raindrops and snowflakes is the same.

When rain acts on the explosion cloud, local pollution spots may occur. For example, as calculations and observations carried out in the USA in 1955 showed, about 2996 of all particles with a diameter of 10 µ are washed out by rain with an intensity of 1 mm/hour in 1/4 hour.

1 See: Meteorology and Nuclear energy. Edited by E. K. Fedorov. Publishing house of foreign literature.

 

From the book:

PROTECTION FROM RADIOACTIVE FALLOUT edited by A.I.Burnazyan

R.V., Petrov, V.N. Pravetsky, Yu.S. Stepanov, M.I. Shalnov

State Publishing House of Medical Literature Moscow -1963

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