Nuclear weapon yield

The explosive yield of a nuclear weapon is the amount of energy released such as blast, thermal, and nuclear radiation, when that particular nuclear weapon is detonated. It is usually expressed as a TNT equivalent, the standardized equivalent mass of trinitrotoluene (TNT) which would produce the same energy discharge if detonated, either in kilotonnes (symbol kt, thousands of tonnes of TNT), in megatonnes (Mt, millions of tonnes of TNT). It is also sometimes expressed in terajoules (TJ); an explosive yield of one terajoule is equal to 0.239 kilotonnes of TNT. Because the accuracy of any measurement of the energy released by TNT has always been problematic, the conventional definition is that one kilotonne of TNT is held simply to be equivalent to 10¹² calories.

The yield-to-weight ratio is the amount of weapon yield compared to the mass of the weapon. The practical maximum yield-to-weight ratio for fusion weapons (thermonuclear weapons) has been estimated to six megatonnes of TNT per tonne of bomb mass (25 TJ/kg). Yields of 5.2 megatonnes/tonne and higher have been reported for large weapons constructed for single-warhead use in the early 1960s. Since then, the smaller warheads needed to achieve the increased net damage efficiency (bomb damage/bomb mass) of multiple warhead systems have resulted in increases in the yield/mass ratio for single modern warheads.

In order of increasing yield (most yield figures are approximate):

In comparison, the blast yield of the GBU-43 Massive Ordnance Air Blast bomb is 0.011 kt, and that of the Oklahoma City bombing, using a truck-based fertilizer bomb, was 0.002 kt. The estimated strength of the explosion at the Port of Beirut is 0.3-0.5 kt. Most artificial non-nuclear explosions are considerably smaller than even what are considered to be very small nuclear weapons.

The yield-to-mass ratio is the amount of weapon yield compared to the mass of the weapon. The highest achieved values are somewhat lower, and the value tends to be lower for smaller, lighter weapons, of the sort that are emphasized in today's arsenals, designed for efficient MIRV use or delivery by cruise missile systems.

Large single warheads are seldom a part of today's arsenals, since smaller MIRV warheads, spread out over a pancake-shaped destructive area, are far more destructive for a given total yield, or unit of payload mass. This effect results from the fact that destructive power of a single warhead on land scales approximately only as the cube root of its yield, due to blast "wasted" over a roughly hemispherical blast volume, while the strategic target is distributed over a circular land area with limited height and depth. This effect more than makes up for the lessened yield/mass efficiency encountered if ballistic missile warheads are individually scaled down from the maximal size that could be carried by a single-warhead missile.

The efficiency of an atomic bomb is the ratio of the actual yield to the theoretical maximum yield of the atomic bomb. Not all atomic bombs possess the same yield efficiency as each individual bombs design plays a large role in how efficient it can be. In order to maximize yield efficiency one must make sure to assemble the critical mass correctly, as well as implementing instruments such as tampers or initiators in the design. A tamper is typically made of uranium and it holds the core together using its inertia. It is used to prevent the core from separating too soon to generate maximum fission, so as not to cause a "fizzle". The initiator is a source of neutrons either inside of the core, or on the outside of the bomb, and in this case it shoots neutrons at the core at the moment of detonation. It is essentially kick starting the reaction so the maximum fission reactions can occur to maximize yield.

The following list is of milestone nuclear explosions. In addition to the atomic bombings of Hiroshima and Nagasaki, the first nuclear test of a given weapon type for a country is included, as well as tests that were otherwise notable (such as the largest test ever). All yields (explosive power) are given in their estimated energy equivalents in kilotons of TNT (see TNT equivalent). Putative tests (like Vela incident) have not been included.