What is Nuclear Monitoring?

A nuclear explosion occurs as a result of the rapid release of energy from an intentionally high-speed nuclear reaction. The driving reaction may be nuclear fission, nuclear fusion or a combination of the two.

Atmospheric nuclear explosions are commonly associated with mushroom clouds, although mushroom clouds can occur with large chemical explosions, and it is possible to have an 'air-burst' nuclear explosion without these clouds.

All nuclear explosions produce nuclear radiation and radioactive debris that can produce devastating and long-lived effects in the local environment.

Nuclear weapons have only been used twice during wartime – both times by the United States against Japan, during the closing days of World War II. The first was on the morning of 6 August 1945, when a Uranium 'gun-type' device code-named 'Little Boy' was dropped on the city of Hiroshima. The second occurred three days later when a Plutonium 'implosion-type' device code-named 'Fat Man' was dropped on the city of Nagasaki. These two bombings resulted in the immediate deaths of around 220 000 people, with over 200 000 more dying subsequently from lethal radiation overdoses.

Effects of nuclear explosions

The dominant effects of a nuclear weapon (the blast and thermal radiation) are the same physical mechanisms as produced by conventional explosives, however, the energy produced by a nuclear explosive is millions of times more and the temperatures reached are in the order of tens of millions of degrees Celsius (Co). Nuclear weapons are quite different from regular weapons because of the huge amount of explosive and thermal energy they can produce. Also, the devastating impact of the explosion does not stop after the initial blast, as with regular explosives. A cloud of nuclear radiation travels from the epicentre of the explosion, causing widespread impacts on both flora and fauna even after the pressure and heat waves have passed. The radiation can cause genetic mutation, radiation poisoning, and death.

Types of nuclear weapons tests

Nuclear weapons tests have historically been classified into categories reflecting the medium or location of the test: atmospheric, underwater, and underground.

  • Atmospheric test explosions take place in or above the atmosphere. Generally these have occurred as devices detonated on towers, balloons, barges, islands, or dropped from airplanes. A limited number of high altitude nuclear explosions have also been conducted, generally fired from rockets. Nuclear explosions which are close enough to the ground to draw dirt and debris into their mushroom cloud can generate large amounts of nuclear fallout due to irradiation of the debris. High altitude nuclear explosions can generate an electromagnetic pulse (EMP), and charged particles resulting from the blast can cross hemispheres to create an auroral display.
  • Underwater test explosions have usually been conducted to evaluate the effects of nuclear weapons against naval vessels (such as in Operation Crossroads – near Bikini Atoll in the South Pacific), or to evaluate potential sea-based nuclear weapons (such as nuclear torpedoes or depth-charges). Underwater tests close to the surface can disperse large amounts of radioactive water and steam, contaminating nearby ships or structures.
  • Underground test explosions are nuclear tests which are conducted at varying depths under the surface of the earth. Underground nuclear testing made up the majority of nuclear tests by the United States and the Soviet Union during the Cold War. When the explosion is fully contained, underground nuclear testing emits a negligible amount of radioactive fallout. However, underground nuclear tests can 'vent' to the surface, producing considerable amounts of radioactive debris as a consequence.

Separately from these designations, nuclear tests are also often categorized by the purpose of the test itself. Tests which are designed to garner information about how (and if) the weapons themselves work are weapons related tests, while tests designed to gain information about the effects of the weapons themselves on structures or organisms are known as weapons effects tests.

Nuclear-weapons-related testing which purposely results in no yield is known as subcritical testing, referring to the lack of a creation of a critical mass of fissile material. Additionally, there have been simulations of nuclear tests using conventional explosives.

Topic contact: hazards@ga.gov.au Last updated: June 17, 2013