The Basics
Fundamentally, a nuclear weapon is an explosive device that uses the energy released from the fission or fusion of two or more nuclei. To develop a nuclear weapon, one must acquire Uranium 235 (U-235) or Plutonium 239 (P-239), both of which are fissile materials. Without one of either of these elements, a nuclear explosion is impossible to create. The property that deems these materials fissile is their ability to split into two equal masses when hit by a single neutron. When an atom is split it releases these byproducts: two lighter nuclei, a couple free neutrons, and heat and energy. The newly freed neutrons are then sent out to collide with and divide other nuclei. The rapid succession of this process, in a matter of microseconds, develops a nuclear explosion. This method of nuclear power development was used to design the Atom Bombs that fell on both Hiroshima and Nagasaki during World War II. http://www.fas.org/nuke/intro/nuke/design.htm
A much more potent nuclear method that produces a greater, deadlier yield of explosion, would be the fusion method. Fusion weapons use isotopes of hydrogen, deuterium, and tritium to release greater quantities of neutrons and energy. The detonation of a fusion explosion is reliant on a “primary” chemical and fission explosion. When created, a small portion of U-235 or P-239 is placed around the fusile material. When, let’s say U-235, ignites, it releases neutrons and energy that will compress the fusile material. The fusile material then fuses together nuclei releasing far more energy and neutrons than a regular fission explosion. This fusion component would be referred to as the “secondary” explosion. A weapon that uses the fusion method is considered a thermonuclear weapon and is often referred to at the Hydrogen Bomb. http://www.ucsusa.org/nuclear_weapons_and_global_security/solutions/us-nuclear-weapons/how-nuclear-weapons-work.html#.VM_L-LvhfZt
The Impacts
The release of energy of a nuclear weapon mainly impacts the physicality of the bombarded area and the inhabitants. After the electromagnetic radiation is released from the bomb, it quickly heats. This creates an air burst in which the gas explodes, taking the form of a mushroom-shaped cloud, rising and expanding. A diameter of a one megaton air burst can expand up to 2,200 meters per 10 seconds and rise up to 150 meters every second. The intensity of such explosion produces a blast wave which can wipe out an entire city. The most damage comes form blast waves due to the change in air pressure causing high winds to travel across the terrain, demolishing buildings, trees, power lines, etc., evidently resulting in casualties.(http://www.fas.org/nuke/intro/nuke/effects.htm)
The effects of thermal radiation differ from that of blast waves. Thermal radiation is the flash of light seen after the blast wave disperses. Although it should only momentarily blind those looking directly at the explosion, it can cause serious damages to the eyes, i.e., retinal burn. In addition, first-degree, second-degree, and third-degree skin burns may occur to those relatively close to the hypocenter (within a 5-7 mile radius). Likewise, flammable objects/materials in the surrounding area may ignite. Winds will carry the flames across the ruins resulting in an uncontrolled fire, affecting the geographical plain and population. In addition, radiation is emitted from the explosions for as long as fission is occurring. Initial radiation can lethally affect those closer to ground zero, whereas residual radiation may contribute to long term effects. Radiation creates environmental contamination that lasts for a long time, depending on half-life of the fissile products released. Impacts of radiation on humans range from hair loss, vomiting, diarrhea, and fatigue to brain, thyroid, heart, gastrointestinal and reproductive tract damage. The severity of the damages depends on the dose of rems released. http://www.atomicarchive.com/Effects/effects2.shtml
The effects of thermal radiation differ from that of blast waves. Thermal radiation is the flash of light seen after the blast wave disperses. Although it should only momentarily blind those looking directly at the explosion, it can cause serious damages to the eyes, i.e., retinal burn. In addition, first-degree, second-degree, and third-degree skin burns may occur to those relatively close to the hypocenter (within a 5-7 mile radius). Likewise, flammable objects/materials in the surrounding area may ignite. Winds will carry the flames across the ruins resulting in an uncontrolled fire, affecting the geographical plain and population. In addition, radiation is emitted from the explosions for as long as fission is occurring. Initial radiation can lethally affect those closer to ground zero, whereas residual radiation may contribute to long term effects. Radiation creates environmental contamination that lasts for a long time, depending on half-life of the fissile products released. Impacts of radiation on humans range from hair loss, vomiting, diarrhea, and fatigue to brain, thyroid, heart, gastrointestinal and reproductive tract damage. The severity of the damages depends on the dose of rems released. http://www.atomicarchive.com/Effects/effects2.shtml