A nuclear attack is the deliberate detonation of a nuclear weapon against a target, releasing an enormous burst of energy through atomic reactions. A single warhead can destroy an entire city through a combination of blast waves, extreme heat, radiation, and radioactive fallout. The effects unfold in stages, from a blinding flash lasting fractions of a second to fallout that can contaminate land for weeks or longer.
How Nuclear Weapons Release Energy
Nuclear weapons get their destructive power from reactions inside the atom itself. There are two types of reactions involved: fission and fusion.
Fission weapons, sometimes called atomic bombs, work by splitting heavy atoms like uranium-235. When a neutron strikes a uranium atom, it causes the atom to break apart into lighter fragments, releasing energy and an average of 2.5 additional neutrons. Those neutrons then strike other uranium atoms, triggering a chain reaction that multiplies in microseconds. A single gram of uranium-235 releases roughly 80 million kilojoules of energy through this process, millions of times more than the same weight of conventional explosives.
Fusion weapons, often called hydrogen bombs or thermonuclear weapons, add a second stage. A small fission bomb detonates first, generating temperatures high enough to force light atoms (typically isotopes of hydrogen) to fuse together into helium. This fusion reaction releases energy comparable to fission per reaction, but because the fuel is lighter and the process scales up more easily, fusion weapons can be made far more powerful. The largest nuclear weapons ever tested used this two-stage design.
What Happens in the First Seconds
A nuclear detonation produces a fireball that reaches tens of millions of degrees. For a relatively small 1-kiloton weapon, the fireball stretches nearly 200 feet across. Larger weapons produce fireballs that can be more than a mile wide. Anything inside or near the fireball is vaporized instantly.
The extreme heat radiates outward as thermal radiation, a pulse of light and infrared energy that can ignite fires and cause severe burns miles from the blast. In Hiroshima, thermal radiation caused flash burns on exposed skin more than a mile from the detonation point. The flash is bright enough to cause temporary or permanent blindness at much greater distances.
Immediately following the thermal pulse, the blast wave expands outward. This is a wall of compressed air moving at supersonic speed, and it causes most of the structural destruction. The pressure is measured in pounds per square inch (psi) above normal atmospheric pressure. At just 1 psi of overpressure, houses become uninhabitable. At 2 to 3 psi, concrete block walls shatter. At 5 to 7 psi, houses are nearly completely destroyed. At 10 psi, total building destruction is expected. The blast wave is followed by powerful winds that hurl debris at lethal speeds.
Radiation Exposure and Its Effects
A nuclear detonation releases a burst of ionizing radiation, including gamma rays and neutrons, in the first minute. This initial radiation is lethal at close range and dangerous for miles depending on the weapon’s size. A dose of about 1 gray (a unit measuring absorbed radiation) received in a short time causes radiation sickness: nausea, hair loss, and skin reddening. Between 2 and 10 gray, roughly half of those exposed will die without medical treatment. Exposure above 6 gray causes severe vomiting and diarrhea within an hour and is frequently fatal. Above 10 gray, death occurs within weeks regardless of treatment, often from catastrophic damage to the brain and blood vessels.
These effects depend heavily on distance from the blast and whether any shielding, such as concrete walls, stands between the person and the detonation. The initial radiation burst travels at the speed of light, so there is no time to react to it specifically. Anyone who survives the blast wave has already received whatever initial radiation dose they will get.
Air Bursts vs. Ground Bursts
Where the weapon detonates relative to the ground dramatically changes the aftermath. An airburst, where the weapon detonates above the surface, maximizes the blast wave’s reach and causes the widest area of destruction. It produces relatively less radioactive fallout because the fireball doesn’t touch the ground.
A surface burst, or ground burst, is different. The fireball contacts the earth and scoops up thousands of tons of soil and debris. Only a small fraction of this material becomes radioactive, but it mixes with the intensely radioactive weapon residue inside the fireball. As the mushroom cloud rises, it cannot carry all that weight. The heavier particles begin falling back to earth quickly, creating a visible trail of radioactive dust around the detonation site and stretching downwind for tens or hundreds of miles. This is fallout, and it represents a prolonged radiation hazard that an airburst largely avoids.
Subsurface bursts, detonated underground or underwater, contain much of the blast energy but can contaminate large areas with radioactive material pushed through the ground or water.
How Fallout Decays Over Time
Radioactive fallout is most dangerous in the first hours after detonation, and its intensity drops rapidly. Emergency planners use the 7:10 rule: for every sevenfold increase in time after the blast, the radiation exposure rate drops by a factor of 10. If the radiation level is 400 roentgens per hour at 2 hours after detonation, it will fall to about 40 roentgens per hour by 14 hours, and to roughly 4 roentgens per hour by about 4 days.
This steep decay curve is why sheltering immediately after a blast is so critical. The first 24 hours are by far the most dangerous period. After that, radiation levels have typically dropped enough to allow brief movement outdoors, though contaminated areas can remain hazardous for much longer depending on the weapon’s size and type of burst.
Sheltering From Fallout
The single most effective survival action after a nuclear detonation is getting inside a solid building as quickly as possible. Brick and concrete provide the best protection. Once inside, move to the basement or the center of the building, staying away from outer walls and the roof. The goal is to put as much dense material as possible between your body and the radioactive particles settling outside.
Vehicles, mobile homes, and outdoor areas do not provide adequate shielding. A large, multi-story building or an underground space is far more effective. FEMA recommends staying inside for at least 24 hours unless local authorities give different instructions. During that first day, the radiation intensity outside is dropping by roughly 90%, making the difference between sheltering and not sheltering potentially life-saving.
Large-Scale Environmental Effects
A single nuclear weapon causes devastating local destruction. A full-scale nuclear conflict, involving thousands of warheads, could alter the global climate. Research published in Science modeled exchanges of several thousand megatons and found that soot from burning cities and forests would encircle the earth within one to two weeks, reducing sunlight to just a few percent of normal levels. Land temperatures in these models dropped to between negative 15 and negative 25 degrees Celsius, even during what should be summer months.
This scenario, known as nuclear winter, has a surprisingly low threshold. As little as 100 megatons detonated over major urban centers could generate enough smoke to create subfreezing land temperatures for months. The soot and dust would also accelerate the spread of radioactive particles from the Northern Hemisphere to the Southern Hemisphere, making even distant nations vulnerable. The resulting disruption to agriculture would threaten food supplies worldwide, potentially causing more deaths from famine than from the explosions themselves.