nuclear bunker buster

An underground explosion releases a larger fraction of its energy into the ground, compared to a surface burst or air burst explosion at or above the surface, and so can destroy an underground target using a lower explosive yield. This in turn could lead to a reduced amount of radioactive fallout. However, it is unlikely that the explosion would be completely contained underground.

While conventional bunker busters use several methods to penetrate concrete structures, these are for the purpose of destroying the structure directly, and are generally limited in how much of a bunker (or system of bunkers) they can destroy by depth and their relatively low explosive force (compared to nuclear weapons). The primary difference between conventional and nuclear bunker busters is that, while the conventional version is meant for one target, the nuclear version can destroy an entire underground bunker system. The main principles in modern bunker design are largely centered around survivability in nuclear war. As a result of this both American and Soviet sites reached a state of "super hardening", involving defenses against the effects of a nuclear weapon such as spring- or counterweight-mounted (in the case of the R-36) control capsules and thick concrete walls (3 to 4 feet (0.9 to 1.2 m) for the Minuteman ICBM launch control capsule) heavily reinforced with rebar. These systems were designed to survive a near miss of 20 megatons. Liquid-fueled missiles such as those historically used by Russia are more fragile and easily damaged than solid-fueled missiles such as those used by the United States. The complex fuel storage facilities and equipment needed to fuel missiles for launch and de-fuel them for frequent maintenance add additional weaknesses and vulnerabilities. Therefore, a similar degree of silo "hardening" does not automatically equate to a similar level of missile "survivability". Major advancements in the accuracy and precision of nuclear and conventional weapons subsequent to the invention of the missile silo itself have also rendered many "hardening" technologies useless. With modern weapons capable of striking within several feet of their intended targets, a modern "near miss" can be much more effective than a "hit" decades ago. A weapon need only cover the silo door with sufficient debris to prevent its immediate opening to render the missile inside useless for its intended mission of rapid strike or counter-strike deployment. A nuclear bunker buster negates most of the countermeasures involved in the protection of underground bunkers by penetrating the defenses prior to detonating. A relatively low yield may be able to produce seismic forces beyond those of an air burst or even ground burst of a weapon with twice its yield. Additionally, the weapon has the ability to impart more severe horizontal shock waves than many bunker systems are designed to combat by detonating at or near the bunker's depth, rather than above it. Geologic factors also play a major role in weapon effectiveness and facility survivability. Locating facilities in hard rock may appear to reduce the effectiveness of bunker-buster type weapons by decreasing penetration, but the hard rock also transmits shock forces to a far higher degree than softer soil types. The difficulties of drilling into and constructing facilities within hard rock also increase construction time and expense, as well as making it more likely construction will be discovered and new sites targeted by foreign militaries.

Concrete structure design has not changed significantly in the last 70 years. The majority of protected concrete structures in the US military are derived from standards set forth in Fundamentals of Protective Design, published in 1946 (US Army Corps of Engineers). Various augmentations, such as glass, fibers, and rebar, have made concrete less vulnerable, but far from impenetrable. When explosive force is applied to concrete, three major fracture regions are usually formed: the initial crater, a crushed aggregate surrounding the crater, and "scabbing" on the surface opposite the crater. Scabbing, also known as spalling, is the violent separation of a mass of material from the opposite face of a plate or slab subjected to an impact or impulsive loading, without necessarily requiring that the barrier itself be penetrated. While soil is a less dense material, it also does not transmit shock waves as well as concrete. So while a penetrator may actually travel further through soil, its effect may be lessened due to its inability to transmit shock to the target.

Further thinking on the subject envisions a hardened penetrator using kinetic energy to defeat the target's defenses and subsequently deliver a nuclear explosive to the buried target. The primary difficulty facing the designers of such a penetrator is the tremendous heat applied to the penetrator when striking the shielding at high speed. This has partially been solved by using metals such as tungsten (the metal with the highest melting point), and altering the shape of the projectile (such as an ogive). Altering the shape of the projectile to incorporate an ogive shape has yielded substantial improvement in penetration ability. Rocket sled testing at Eglin Air Force Base has demonstrated penetrations of 100 to 150 feet (30 to 46 m) in concrete when traveling at 4,000 ft/s (1,200 m/s). The reason for this is liquefaction of the concrete in the target, which tends to flow over the projectile. Variation in the speed of the penetrator can either cause it to be vaporized on impact (in the case of traveling too fast), or to not penetrate far enough (in the case of traveling too slowly). An approximation for the penetration depth is obtained with an impact depth formula derived by Sir Isaac Newton.

Another school of thought on nuclear bunker busters is using a light penetrator to travel 50 to 100 feet (15 to 30 m) through shielding, and detonate a nuclear charge there. Such an explosion would generate powerful shock waves, which would be transmitted very effectively through the solid material comprising the shielding (see "scabbing" above).

As early as 1944, the Barnes Wallis Tallboy bomb and subsequent Grand Slam weapons were designed to penetrate deeply fortified structures through sheer explosive power. These were not designed to directly penetrate defences, though they could do this (for example, the Valentin submarine pens had ferrous concrete roofs 15 feet (4.5 m) thick which were penetrated by two Grand Slams on 27 March 1945), but rather to penetrate under the target and explode leaving a camouflet (cavern) which would undermine foundations of structures above, causing it to collapse, thus negating any possible hardening. The destruction of targets such as the V3 battery at Mimoyecques was the first operational use of the Tallboy. One bored through a hillside and exploded in the Saumur rail tunnel about 60 feet (18 m) below, completely blocking it, and showing that these weapons could destroy any hardened or deeply excavated installation. Modern targeting techniques allied with multiple strikes could perform a similar task. Development continued, with weapons such as the nuclear B61, and conventional thermobaric weapons and GBU-28. One of the more effective housings, the GBU-28 used its large mass (4,700 lb or 2,130 kg) and casing (constructed from barrels of surplus 203 mm howitzers) to penetrate 20 feet (6 metres) of concrete, and more than 100 feet (30 m) of earth. The B61 Mod 11, which first entered military service after the Cold war had ended, in January 1997, was specifically developed to allow for bunker penetration, and is speculated to have the ability to destroy hardened targets a few hundred feet beneath the earth. While penetrations of 20 to 100 feet (6 to 30 m) were sufficient for some shallow targets, both the Soviet Union and the United States were creating bunkers buried under huge volumes of soil or reinforced concrete in order to withstand the multi-megaton thermonuclear weapons developed in the 1950s and 1960s. Bunker penetration weapons were initially designed within this Cold War context. One likely Soviet Union/Russian target, Mount Yamantau, was regarded in the 1990s by Maryland Republican congressman, Roscoe Bartlett, as capable of surviving "half a dozen" repeated nuclear strikes of an unspecified yield, one after the other in a "direct hole". The Russian continuity of government facility at Kosvinsky Mountain, finished in early 1996, was designed to resist US earth-penetrating warheads and serves a similar role as the American Cheyenne Mountain Complex. The timing of the Kosvinsky completion date is regarded as one explanation for US interest in a new nuclear bunker buster and the declaration of the deployment of the B-61 Mod 11 in 1997. Kosvinsky is protected by about 1,000 feet (300 m) of granite. The weapon was revisited after the Cold War during the 2001 US invasion of Afghanistan, and again during the 2003 invasion of Iraq. During the campaign in Tora Bora in particular, the United States believed that "vast underground complexes," deeply buried, were protecting opposing forces. Such complexes were not found. While a nuclear penetrator (the "Robust Nuclear Earth Penetrator", or "RNEP") was never built, the US DOE was allotted budget to develop it, and tests were conducted by the US Air Force Research Laboratory. The RNEP was to use the 1.2 megaton B83 physics package. The Bush administration removed its request for funding of the weapon in October 2005. Additionally, then US Senator Pete Domenici announced funding for the nuclear bunker-buster has been dropped from the US Department of Energy's 2006 budget at the department's request. While the project for the RNEP seems to be in fact canceled, Jane's Information Group speculated in 2005 that work might continue under another name. A more recent development (c. 2012) is the GBU-57 Massive Ordnance Penetrator, a 30,000-pound (14,000 kg) conventional gravity bomb. The USAF's B-2 Spirit bombers can each carry two such weapons.

Note that with the exception of strictly earth penetrating weapons, others were designed with air burst capability and some were depth charges as well. Mark 8 nuclear bomb (1952–1957): earth penetrating W8 for SSM-N-8 Regulus (cancelled): earth penetrating Mark 11 nuclear bomb (1956–1960): earth penetrating Mk 105 Hotpoint (1958–1965): laydown delivery B28 nuclear bomb (1958–1991): laydown delivery and ground burst Mark 39 nuclear bomb (1958–1962) laydown delivery and ground burst B43 nuclear bomb (1961–1990): laydown delivery and ground burst B53 nuclear bomb (1962–1997): laydown delivery B57 nuclear bomb (1963–1993): laydown delivery B61 nuclear bomb (1968–present): laydown delivery and ground burst Mod 11 (1997–present): earth penetrating, laydown delivery, and ground burst W61 for MGM-134 Midgetman (cancelled): earth penetrating B77 nuclear bomb (cancelled): laydown delivery B83 nuclear bomb (1983–present): laydown delivery and ground burst W86 for Pershing II (cancelled): earth penetrating Robust Nuclear Earth Penetrator (cancelled): earth penetrating Nuclear Deterrent System-Air-delivered (NDS-A) (under study or in early development)