Nuclear bunker buster

Subsidence craters left over after underground nuclear (test) explosions

Bunker-busting nuclear weapons are a type of nuclear weapon that would be designed to penetrate into soil, rock or concrete to deliver a nuclear warhead. These weapons would be used to destroy hardened, underground military bunkers buried deep in the ground usually under 25 to 100 meters or more of concrete. These weapons would in theory limit the amount of radioactive nuclear fallout by confining the explosion underground. Warhead yield and weapon design has changed periodically throughout the history of the design of such weapons.

Methods of operation

Penetration by explosive force

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The Russian Tsar Bomba was designed to ablate entire mountains to destroy deeply buried targets, such as Cheyenne Mountain. Estimates place the size of the resultant explosion as high as 64km

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 thick (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.

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).

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 a nuclear munitions is fallout. Nuclear bunker busters are not believed to be free of fallout. Instead, the reasoning is that fallout will be contained within the shielding of the target attacked. However, underground nuclear testing has revealed a "chimney" or "smokestack" effect, whereby fallout "leaks" through the roof of the cavity created by the charge.

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 .3 and 340 kt, containing the resultant blast is viewed as improbable.

It is worth noting that in the United States arsenal, weapons which penetrate as much as 30 meters have been developed, but only against shallow targets.

Politically, as well, such nuclear bunker busters are unpopular. In addition to fallout, use of nuclear weapons, even low-yield weapons, is a violation of the no first use policy some nations have agreed to. Furthermore, developing new nuclear weapons is prohibited by the Comprehensive Test Ban Treaty.

Lastly, the requirement to use nuclear weapons in this role is questionable, since very effective conventional ground penetration weapons were designed by the British aerodynamic engineer Barnes Wallis in the 1940s, and these were shown to be 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, but rather to slide under a target and dig it up, thus negating any possible hardening. The destruction of targets such as the V2 complex at Wizernes, or the V3 guns at Mimoyecques 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 lbs) and casing (constructed from 8 in artillery shells) to penetrate 20 feet of concrete, and more than 100 feet of 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. The Soviets developed a nuclear weapon (the "Tsar Bomba"), with a demonstrated yield of 50 MT, and possibly with the potential for yield of 100 MT. Such a weapon was speculated to have been designed solely to defeat deeply buried installations, e.g., Cheyenne Mountain (others have suggested it was a propaganda weapon). To counter this, the United States made nuclear retaliation possible from the air via Air Force One.

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Mountainous terrain in Afghanistan

The weapon was revisited 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 alotted budget to develop it, and tests were conducted by the Air Force Research Laboratory. Such complexes were not found.

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

• Thermobaric weapon
• Tsar Bomba
• Fail-deadly
• No first use
• Nuclear strategy

References

1. Barrier Penetration Tests, Moore, R. T. National Bureau of Standards, ASIN B0006CHZT6
2. Penetration Resistance of Concrete: A Review, James R. Clifton, The Physical Security and Stockpile Directorate, Defense Nuclear Agency, ASIN B0006E76U2
3. U.S. Nuclear Weapons: Changes In Policy And Force Structure, Woolf, Amy F., ISBN 1594542341
4. Nuclear Weapon Initiatives: Low-yield R&D, Advanced Concepts, Earth Penetrators, Test Readiness, Ernest, Jonathan V., et al, ISBN 1594542031

External links

• Union of Concerned Scientists article on Earth Penetrating Weapons (EPW)
• New Scientist article