It takes around 10 seconds for the fireball from a nuclear explosion to reach its maximum size, but the effects last for decades.

A nuclear explosion releases vast amounts of energy in the form of blast, heat and radiation. An enormous shockwave reaches speeds of many hundreds of kilometres an hour. The blast kills people close to ground zero, and causes lung injuries, ear damage and internal bleeding further away. People sustain injuries from collapsing buildings and flying objects. Thermal radiation is so intense that almost everything close to ground zero is vaporized. The extreme heat causes severe burns and ignites fires over a large area, which coalesce into a giant firestorm. Even people in underground shelters face likely death due to a lack of oxygen and carbon monoxide poisoning.

Radiation

Unlike conventional weapons, nuclear weapons release ionizing radiation: particles and rays given off by radioactive materials. At high doses, radiation kills cells, damages organs and causes rapid death. At low doses, it can damage cells and lead to cancer, genetic damage and mutations. In human beings, it causes most types of leukaemia, or blood cancer, as well as solid cancers such as thyroid, lung and breast cancers.

Increased rates of leukaemia and thyroid cancer among exposed children begin to appear after five years, while the incidence of most solid cancers rises after about 10 years, with the increased risk persisting throughout one’s life. Radiation exposure can also heighten the risk of hereditary effects in future generations. Radiation exposure can occur externally (from particles in the air, water and soil) or internally (from breathing, eating and drinking). Many radioisotopes are concentrated in plants and animals, and thus the food chain.

Effects of a 100-kt nuclear bomb

• 3 km radius A radioactive fireball hotter than the Sun and with the force of 100,000 tonnes of TNT kills everyone.
• 5 km radius The vast majority of people die quickly from blast injuries, asphyxiation or (over weeks) radiation sickness.
• 10 km radius About half die from trauma and burns. Many succumb soon after to fires and radiation sickness.
• 80 km radius Radioactive fallout spreads. Over time, many thousands will die from radiation sickness and cancers.