first millisecond the ball’s diameter is 150m wide, while its widest diameter (after 10 seconds) can reach 2,200m.  The sudden widening of the fireball compresses the cold air on its outside and that way forms an acoustic wave. Within a minute the fireball cools down and the rise stops. The typical mushroom shape is formed like this, which can be condensed vapour or dust, in the case of a ground level explosion.
Depending on the height of the burst, there are aerial, aboveground, underground and atmospheric detonations.
The height of the aerial detonation is less than 30kms, but high enough for the fireball not to reach the ground. With the changing of the altitude we can maximize the effects of the blast, heat- or radioactive radiation. This is the most effective method against infantry because it causes burns on wide areas (more square miles) and eyesore on even wider territories. In this case the nuclear fallout doesn’t reach the ground on the place of the explosion.
When an aboveground detonation takes place, the fireball reaches the ground, so one part of its energy is absorbed by the earth. Its effect is not as great as that of an aerial detonation. The nuclear fallout, however, is significant.
An atmospheric detonation happens above 30 km. Because the atmosphere is thin, the range of X-rays is bigger (more hundred kilometres), therefore the fireball is greater. The considerable ionization of the atmosphere leads to the collapse of telecommunication (satellites, airplanes) systems. The electromagnetic impulse can ruin sophisticated electronic instruments. Their use is unlikely because they can paralyse a whole continent’s communication system.

*Blast

When a bomb explodes, one part of the released energy transforms into the heat energy of the surrounding atoms. The fast atoms start to move away from the detonation’s centre, pushing the still cold air. This way a very powerful shockwave is formed (a classic acoustic wave). This shockwave is behind in time compared to the fireball (but is still faster than the speed of sound). At the moment the shockwave catches up with the fireball, the air smoulders because of the great pressure, and a bright flash can be seen. The velocity of the shockwave decreases and after a while reaches the speed of sound.
The shockwave causes considerable damages: buildings start to get damaged at even 0.35 atm overpressure. The wind formed after the wave can reach a speed of more hundred kilometres. Whole cities can be demolished this way.
The size of a shockwave (and its range) depends a great deal on the size of the bomb (these data refer to a typical atmospheric detonation).

• 0.7 km 1 kiloton bomb
• 3.2 km 100 kiloton bomb
• 15 km 10 megaton bomb

*Electromagnetic impulse

The electromagnetic radiation developed during the detonation manifests its destructive power in the form of heat radiation. This heat can cause burns and blindness. Its range is way bigger than that of the blast and increases considerably with the bomb’s power. Therefore bombs above 1 megaton are mainly fire-bombs.

*Radioactive radiation

The radioactive radiation emitted during an explosion can be felt even after more decades. The prompt radiation appears in the first one minute and it is the result of the nuclear reactions taking place inside the bomb. The residual radiation, on the other hand, is the result of the fissure of radioactive isotopes formed during the explosion.
5% of a bomb’s energy is neutron- or gamma radiation. So beginning from 50 kiloton weapons, the prompt radiation is irrelevant compared to the heat radiation and the blast.
The nuclear fallout is a form of residual radiation. When a fission bomb explodes, moderately heavy (with 100 atomic mass) decay products are formed (even 300 different elements) which are mainly radioactive. Some of them have half-lives of more months or years, so represent danger for a long time. On the other hand, fission bombs don’t use up all their fissure material. They are dispersed together with other decay products. These elements have long half-lives (U-235 and Pu-239) and are alpha emitters and are therefore not so dangerous.
The powerful neutron radiation can activate these elements scattered in the appropriation of the bomb, which this way become radioactive. In the case of an aboveground explosion these decay into gamma and beta radiations with the contribution of sodium, magnesium, aluminium and silicon found in the soil. This doesn’t represent a great danger because we are speaking about small arias. However, with time, one part of the surface evaporates and condenses into small particles. Generally these particles return to the earth within a day, but thanks to the wind, are scattered on larger territories. Rain or snow accelerate the process and narrow the affected area.
In the case of an atmospheric detonation the radioactive elements turn into tiny particles (0.1-20 micrometers). Reaching the stratosphere, they represent a danger for months or even years.
Nuclear disasters
The Chernobyl disaster
The Chernobyl disaster happened on 26th April 1986 in the nuclear power station near the Ukrainian (then member state of the Soviet Union) cities of Pripyat and Chernobyl. At 1:23:58 am local time, the unit 4. reactor suffered a steam explosion that resulted in a fire, a series of additional explosions, and a nuclear meltdown. This was the gravest disaster in the history of nuclear power. The lack of protective buildings resulted in radioactive waste falling on the western part of the Soviet Union and other regions of Europe and the eastern part of the United States of America. Vast territories of Ukraine, Belarus and Russia were contaminated and around 200,000 people had to be evacuated. About 60% of the radioactive fallout landed in Belarus.
It is hard to judge the number of deaths caused by the catastrophe because many people died afterwards because of the complications (e.g. cancer). There are people who are sill alive but the cause of their illnesses is not evidently the result of the accident. The International Atomic Energy Agency attributed 56 direct deaths; 47 accident workers and 9 children with thyroid cancer, and estimated that as many as 4,000 people will ultimately die of illnesses caused by the explosion.
The plant had four nuclear reactors type RBMK-1000, each with the performance of 1,000 MW. and it covered 10% of Ukraine’s energy production. The construction of the plant began in the 1970s, with reactor No. 1 commissioned in 1977, followed by No. 2 (1978), No. 3 (1981), and No. 4 (1983). Two more reactors were under construction at the time of the accident.
On April 25, 1985 the Unit 4 reactor was scheduled to be shut down for routine maintenance, using this opportunity to test the rundown phase of one of the reactor's turbine generators. They wanted to know how much electricity can be obtained from the rotation impulse of the turbine’s rotors and whether it was enough to power the most significant consumers of the unit until the diesel generators started functioning (in the case of a tension stoppage).
The experiment wasn’t properly prepared; its safety was handled formally. The local administrators were unprepared, they didn’t see the possible dangers. During the execution they violated the strongly objectionable plan procedures.