B61 (nuclear weapon)
The U.S. B61 nuclear weapon is a gravity bomb of low to medium yield, from as low as 300 tons and as high as 340 kt.  Its physical weight has been described as in the 700-800 pound range; the bomb is 142 inches long and 13.4 inches and diameter It can be delivered by every U.S. aircraft certified to drop nuclear weapons, from the F-16 Fighting Falcon to the B-2 Spirit, and, under "dual key" arrangements, by aircraft of Belgium, Germany, Greece, the Netherlands, and Turkey.
The yield can be changed in flight. The bomb has a wide variety of high and low altitude fuzing modes, from high to low airburst, surface burst, and in one variant, subsurface burst. It is the only U.S. weapon that is considered "tactical" although it can be delivered on by "strategic" aircraft.
The B61 has an enriched uranium, boosted fission Primary. "W-80, W-81 (now retired and dismantled), W-84 (now retired and in the inactive stockpile), and the W-85 (which was retired, and then readapted to yield another B61 variant)."
- Primary consists of beryllium reflected plutonium (the derivative W80-0 contains supergrade plutonium)
- Lithium-6 (95% enrichment) deuteride fusion fuel
Safeguards and surety
Mods 7 and 11 have Category D and tactical models have Category F Permissive Action Links. While most of its implosion system uses the insensitive high explosive LX-17, the B61-4/7/10/11 weapons contain some LX-07 explosive."
|Mod 0||Withdrawn and re-manufactured|
|Mod 1||Withdrawn and converted to Mod 7|
|Mod 2||Withdrawn and re-manufactured|
|Mod 3||0.3 kt, 1.5 kt, 60 kt, and 170 kt||This is the highest yield tactical bomb mod. Along with the mod 4, this was the first mod developed using PBX-9502 IHE, and the first to be equipped with a microprocessor based arming and firing system (in 1980). Design used for W85 GLCM warhead by Lawrence Livermore National Laboratory.|
|Mod 4||0.3 kt, 1.5 kt, 10 kt, and 45 kt||Tactical bomb. Design used for W85 Pershing II warhead by Los Alamos Laboratory|
|Mod 5||Withdrawn and re-manufactured|
|Mod 7||10 kt - 300 kt.||Strategic mission; highest yield in series (including derivatives such as the W80; re-manufactured with PBX-9402 replacing PBX-9404; some converted to Mod 11|
|Mod 8||~1 kt / 10 kt / 100 kt / 345 kt|
|Mod 9||10 kt / 100 kt / 345 kt / 500 kt|
|Mod 10||0.3 kt, 5 kt, 10 kt, and 80 t.||Converted W85 Pershing II missile warhead, which was derived from B61 Mod 3 and 4|
|Mod 11||10 kt - 340 kt||Field modification of B61 Mod 7|
One version, the B61-11, has a limited earth-penetrating capability, for use against hardened underground structures. It will penetrate about 20 feet into earth, which causes more efficient transfer of blast energy to ground shock.  one-piece case hardened steel center case, and a new nose piece and rear subassembly to provide ground penetration capability for defeating buried targets ("bunker busting"). The parachute assembly has also been removed, and new aerodynamic fins added for high-velocity, accurate delivery. The B61-11 buries itself 3-6 meters underground before detonation, transfering a much higher proportion of the explosion energy to ground shock, compared to surface bursts. The actual warhead itself is identical to the Mod 7. This is the first new model of a U.S. warhead to go into service since warhead production was suspended in 1989. It is being produced by field modification of existing Mod 7s.
The -11 will generate fallout, but by being even slightly underground when it detonates, a lower yield might be adequate than a higher-yield surface burst. The U.S. assumes, however, that the greater efficiency of the B61-11 will allow it to replace the 9 Mt B53 bomb. In no way, however, should it be assumed the B61-11 penetrates deeply enough to contain its fallout. Were it used in an urban area, there would be very substantial radiation casualties; it is unlikely that any air-launched weapon could withstand the impact of penetrating deeply enough to contain even a small explosion. 
- Nuclear Weapons Archive, U.S. Nuclear Weapon Enduring Stockpile
- Cochran, Thomas B.; William M. Arkin & Milton M. Hoenig (1984), Nuclear Weapons Databook, Volume I: U.S. Nuclear Forces and Capabilities, Nationsl Resources Defense Council
- The B61 (Mk-61) Bomb: Intermediate yield strategic and tactical thermonuclear bomb, Nuclear Weapons Archives, 9 January 2007
- Safe Handling of Insensitive High Explosive Weapon Subassemblies at the Pantex Plant, Defense Nuclear Facilities Safety Board, September 1999, DNFSB/TECH-24, p. 4-1
- Nelson, Robert W., Low-Yield Earth-Penetrating Nuclear Weapons, Federation of American Scientists