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Bunker-busting nuclear weapons, also known as earth-penetrating weapons (EPW), are a type of nuclear weapon designed to penetrate into soil, rock, or concrete to deliver a nuclear warhead to a target. These weapons would be used to destroy hardened, underground military bunkers buried deep in the ground. In theory, the amount of radioactive nuclear fallout would be reduced from that of a standard, air-burst nuclear detonation because they would have relatively low explosive yield. However because such weapons necessarily come into contact with large amounts of earth-based debris, they may still generate considerable amounts of fallout, even with their reduced yields. Warhead yield and weapon design have changed periodically throughout the history of the design of such weapons. Penetration by explosive force Concrete design remains little changed since 60 years ago. 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. Raymond T. Moore 1 was able to create a "human sized hole" in 18 inch (45 cm) thick reinforced concrete in less than 48 seconds with a mere 20 lb (9 kg) of explosive and a bolt cutter. 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 opposite side of 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 (this does not necessarily mean that the barrier itself must have been penetrated at this point). The crater volume varies approximately inversely with the square root of the concrete's compressive strength. Therefore, increasing the compressive strength of the concrete by 50% will yield an approximately 25% smaller crater. As the compressive wave propagates to the opposite side of the concrete and is reflected, the concrete fractures, and scabbing occurs on the interior wall. As such, an asymptotic relationship exists between the strength of the concrete and the damage that will be done between the crater, aggregate, and scabbing. 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. Penetration with a hardened penetrator Further thinking on the subject envisions a penetrator, dropped from service height of a bomber aircraft, using kinetic energy to penetrate the shielding, and subsequently deliver a nuclear explosive to the buried target. The problems with such a penetrator is the tremendous heat applied to the penetrator unit when striking the shielding (surface) at hundreds of meters per second. This has partially been solved by using metals such as tungsten (with a much higher melting point than steel), and altering the shape of the projectile (such as an ogive). Additionally, altering the shape of the projectile, to incorporate an ogive shape has yielded substantial results. Rocket sled testing at Eglin Air Force Base has demonstrated penetrations of 100 to 150 feet in concrete when traveling at 4,000 ft/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 slow). An approximation for the penetration depth is obtained with an impact depth formula derived by Sir Issac Newton. Combination penetrator-explosive munitions Another school of thought on nuclear bunker busters is using a light penetrator to travel 15 to 30 meters 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). Problems with proposed weapons The largest problem with nuclear munitions is nuclear fallout. The goal of an earth-penetrating nuclear "bunker buster" is reduce the required yield needed to ensure the target is destroyed by coupling the explosion to the ground, producing a shock wave like an earthquake. For example, the United States retired the B-53 warhead, with a yield of 9 megatons, because the B-61 Mod 11 could attack similar targets with much lower yield (400 kilotons) because the latter can penetrate into the ground. So, the fallout of a B-61 Mod 11 will be less than that of a B-53, all other things being equal. However, the fallout from the weapon is actually increased by making it earth-penetrating. An explosion above ground has enormous blast effects, but does not necessarily radioactively contaminate and throw up tons of debris, as an explosion below ground does. The Union of Concerned Scientists points out that at the Nevada Test Site, the depth required to contain fallout from a nuclear test was between 100 and 500 meters. It is improbable that any type of bomb or missile could be made to penetrate so deeply. With yields between 0.3 and 340 kt of TNT, it is improbable that the blast would be completely contained. Another problem is that bunkers can be built farther into the earth to make them more difficult to reach. If a tunnel can be built 300 m into the side of a mountain, then it can be built 1000 m into the mountain using the same equipment and techniques. The target's vulnerability is then limited to openings like the ventilation system, which conventional bombs can handle. Politically, as well, such nuclear bunker busters are unpopular in some circles. Most targets are near cities, and even minimal fallout will inflict unacceptable levels of collateral damage. Furthermore, the testing of new nuclear weapons would be prohibited by the proposed Comprehensive Test Ban Treaty, though the United States is not a signatory. Many fear that developing such weapons would lead to a new global nuclear arms race. Lastly, the requirement to use nuclear weapons in this role is questionable. Very effective conventional ground penetration weapons were designed by the British aerodynamic engineer Barnes Wallis in the 1940s. These weapons were able to destroy very deeply buried or strengthened sites. Additionally, other conventional weapons such as thermobaric weapons and napalm (as in the Vietnam War) have proved effective in defeating buried targets. Development of bunker-busting weapons As early as 1944, the 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 7 metres (23 feet) 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 guns at Mimoyecques or with the first operational use of the Tallboy, one bored through a hillside and exploded in the Saumur rail tunnel about 18m (60 ft) below, completely blocking it; show that these weapons could destroy any hardened or deeply excavated installation, and modern targeting techniques allied with multiple strikes could unquestionably 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) and casing (constructed from barrels of surplus 203 mm howitzers) to penetrate 20 feet of concrete, and more than 100 feet of earth. The B61 Mod 11, which first entered military service 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 20–100 feet 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 out of this Cold War context. The weapon was revisited in the post-Cold War during the 2001 U.S. 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. While a nuclear penetrator (the "Robust Nuclear Earth Penetrator", or "RNEP") was never built, the DOE was allotted budget to develop it, and tests were conducted by the Air Force Research Laboratory. Such complexes were not found. As well, it has been stated * that Iran may have such deeply buried bunkers to guard its nuclear program. The Bush administration removed its request for funding of the weapon in October 2005. Additionally, US Senator Pete Domenici announced funding for the nuclear bunker-buster has been dropped from the Department of Energy's fiscal 2006 budget at the department's request. While the project for the RNEP seems to be in fact cancelled, Jane's Information Group speculates work may continue under another name. See also Footnotes | |||||||
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