Nuclear Weapon Basics

Not everyone is a specialist in everything. Nuclear weapons have their own lingo and the way a physicist, an engineer, a military specialist, an arms control specialist, and a defense planner talk about and view nuclear weapons is sometimes widely divergent. The following is a basic primer on nuclear weapons that can help get you up to speed.

Nuclear weapons are either strategic or non-strategic (also called tactical). These definitions can be confusing, as they do not relate to the size of the nuclear bomb itself, rather its intended use. Nuclear weapons include Intercontinental Ballistic Missiles (ICBMs), Submarine Launched Ballistic Missiles (SLBMs), and Cruise Missiles, which can be Air Launched (ALCM), ground launched (GLCM), or submarine/surface ship launched. The United States has eliminated its ground and ship based nuclear cruise missiles, converting them to conventional roles. The Russians maintain a significant, though indeterminate arsenal of all types of nuclear capable cruise missiles, some of which are likely still nuclear. New START does not limit the stockpile of cruise missiles, rather it limits the number of nuclear capable “strategic bombers” that can carry nuclear ALCMs, air dropped bombs, or short-range attack missiles (SRAMs). Non-strategic weapons are those not covered by treaty, which may include nuclear air dropped bombs delivered by fighter/bombers in a tactical role, artillery shells, submarine launched cruise missiles, anti-ship weapons, surface to air missiles, short range missiles, depth charges, and missile defense systems.

 

Strategic nuclear missiles consist of three primary components, the launch vehicle that can be single or multi-stage, a warhead bus, and the nuclear weapon(s) in a “reentry vehicle.” For instance, the American Minuteman III may carry a Mark 12A Reentry Vehicle Bus, which can carry three nuclear weapons (called Multiple Independent Re-entry Vehicles or MIRVs), either the 335-kiloton W78 warhead or the 300-kiloton W87 warhead (SIPRI 2013). To comply with the New START limitation on deployed warheads, the United States has mounted only a single warhead on each missile, placing the remainder in storage (SIPRI 2013). Strategic warheads are usually thermonuclear in design, in other words they incorporate a fusion device.

  Figure 1 - Mark 12A Reentry Vehicle Bus with three weapons mounted  (Source: Wikipedia Commons)

     

The first nuclear weapons were fission devices, also called “atomic” bombs or A-bombs. These used either an implosion-based design that compressed a plutonium core to critical mass in order to achieve a chain reaction, or a uranium “gun” device that used an explosive charge to launch a subcritical mass of high-enriched uranium (HEU) into another, larger subcritical mass in order to achieve critical mass and a fission chain reaction. “Fatman,” the bomb dropped on Nagasaki, was an implosion device (as was the Trinity test device), while Hiroshima’s “Little Boy” was of the gun design (Rhodes 1986).

Nuclear weapons measure their explosive power, known as yield, in kilotons (kt). A kiloton is the equivalent explosive power of 1,000 tons of TNT explosive. The Hiroshima and Nagasaki bombs had yields of approximately 10-20 kilotons. Atomic based weapons are limited in their yield to weight ratios. The larger the explosive power, the greater the mass of the weapon. Sophisticated design can miniaturize a warhead, but it comes with technical tradeoffs that may affect reliability. Warhead size and weight determines the ability of a missile to “throw it,” from which the term “throw weight” comes. For much of the Cold War Soviet missiles typically had greater throw weights and larger warheads/yields than American systems. In part, this was to counter the United States, which developed MIRVs prior to the Soviets. This disparity between the two stockpiles was a key negotiating point in arms reduction throughout the Cold War and after (Segal 2002).

The United States and the Soviet Union developed fusion weapons originally known as the Hydrogen bomb or H-bomb in the 1950s. These bombs use a fission chain reaction to set off a larger fusion reaction - a thermonuclear reaction. Fusion weapons have significantly better yield to weight ratios and can achieve explosive power measured in megatons (mt). A megaton is the explosive power equivalent to one million tons of TNT. The largest weapon ever detonated was the Soviet Tsar Bomba, a 100-mt device deliberately detonated at a reduced 50-mt.

Yields from modern nuclear weapons vary, but Russian weapons continue to have larger throw weights and yields than American systems. The United States also developed a system nicknamed “dial-a-yield” that allows the bomb to have its yield set prior to launch. In the case of the American B61-3, the yield ranges from 0.3-kt to 170-kt. This is where the tactical and strategic designation becomes confused. The non-strategic B61-3 and B61-4 deploy on specially modified F-16 fighter planes and have a yield larger than some MIRV warheads delivered on a Trident II-D5 SLBM. Further, the B61-7 and B61-11 are strategic weapons dropped from B-2A bombers. The Soviet era SSC-1B Sepal ABM/Air Defense missile, designed to break up large bomber concentrations and intercept incoming nuclear missiles, yields 350-kt, which is greater than most of the strategic weapons in the present American stockpile.

The designation will become more complicated in the coming years as the United States B61 modernization program comes on line. The United States is combining most of its stockpile of B61 tactical and strategic weapons into the new B61-12. While the overall yield will be smaller, this change effectively eliminates the American stockpile of non-strategic nuclear weapons. With the current American policy of pursuing additional cuts with the Russians, especially in the area of non-strategic weapons, the Americans will no longer have anything equivalent to trade in kind. This leads to an examination of the present stockpiles, which are increasingly divergent, and the subject of future posts.

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