Grant Elliott- History and Use of Tactical Nuclear Weapons in Earth Penetrating Applications

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History and Use of Tactical Nuclear Weapons in Earth Penetrating Applications

Grant Elliott∗

MIT Program in Science, Technology, and Society

(Dated: May, 2005)

1. INTRODUCTION

The 1960s saw the rise of a new kind of de-

structive force - the tactical nuclear weapon. Un-

like strategic nuclear weapons, designed to at-

tack carefully calculated targets from great dis-

tance, the comparatively small tactical weapons

are meant to be used by soldiers in battlefield en-

vironments. As such, tactical weapons blur the

line between conventional and nuclear weapons

by making the use of nuclear capability a fea-

sible option. While this flexibility offers con-

siderably increased options to military planners,

opponents fear that the fallout of even a smallnuclear weapon makes its use unwise and, more

importantly, that battlefield nuclear capability

will lead to increased global proliferation.

Today, tactical nuclear weapons remain a con-

tentious subject as the United States considers

the development of a new earth penetrating nu-

clear weapon intended for destruction of hard-

ened underground targets. This possibility raises

both technical and political questions. First, can

a weapon be developed which not only destroys

the intended target but can also penetrate deeply

∗Electronic address: [email protected]

enough to contain its own fallout and destroy

any chemical or biological agents the target may

contain? Opponents argue that both clauses are

difficult, if not impossible, to achieve. Second,

at the risk of renewing the arms race or ham-

pering international disarmament efforts, do the

abilities afforded by such a weapon warrant thepotential political damage?

We primarily consider the technical issues in-

volved in the nuclear earth penetrator debate.

Before doing so, however, we investigate the his-

tories of tactical nuclear weapons and conven-

tional earth penetrators. We then consider the

three purported abilities of nuclear earth pene-trators - destructive power, fallout containment,

and chemical and biological agent destruction -

and assess the likelihood of successfully deploy-

ing a weapon capable of each.

2. TACTICAL NUCLEAR WEAPONS

First, it is important to understand what char-

acterizes a tactical nuclear weapon. The clear-

est definition is based on intended use. While

strategic weapons, like those installed in Inter-

continental Ballistic Missiles (ICBMs), generally

target missile silos or cities, tactical weapons

are intended to be retargeted with the changing



2

conditions of war. Unfortunately, this definition

is largely subjective and attempts to formulate

objective criteria reveal that the line between

strategic and tactical weapons is not as clear as

one would expect.Obvious possibilities for classification include

range and yield. Intercontinental capabil-

ity seem a unique characteristic of strategic

weapons, but at least three nuclear states -

France, India, and Pakistan - have no nuclear

weapons with intercontinental capability, al-

though their arsenals are clearly used for strate-gic purposes [5]. Yield as a criterion is even more

troubling. Tactical nuclear weapons are often re-

ferred to as low yield, a term which extends as

far as one megaton and would include the bulk

of strategically deployed nuclear weapons.

Just as the line is blurred between strate-

gic and tactical weapons, disarmament ac-tivists fear a blurring of the line between tacti-

cal nuclear weapons and conventional weapons.

While strategic nuclear weapons, owing to their

tremendous (and lasting) destructive power,

have become largely unusable, a battlefield vari-

ant offers pinpoint attacks with less risk of mas-

sive retaliatory strikes. With mutually assureddestruction less of a hinderance to use, critics of

tactical nuclear weapons fear that their existence

will inevitably lead to extensive proliferation and

potentially even wartime use.

Tactical nuclear weapons may be deployed in

missile warheads, gravity bombs, demolition mu-

FIG. 1: The Davy Crockett W54 warhead, launchable from a

recoilless rifle. Photo from [4]

nitions, artillery shells, or even land-mines [5].

Today, the United States arsenal includes ap-

proximately 1,300 tactical gravity bombs and

320 cruise missiles, though other forms have been

deployed and subsequently withdrawn (See Sec-

tion 3). Consider, for instance, the W54 Davy

Crockett, a 51 pound warhead launched from

a recoilless rifle with a maximum range of two

miles. The Davy Crockett (depicted in Figure

1) was intended for use against troop formations

[4]. A variant, the B54, was used in the Spe-

cial Atomic Demolition Munition (SADM), a 163

pound bomb with a yield of under one kiloton[4]. The SADM was developed for the Navy in

the 1960’s and intended for deployment by a two

man parachute team, who would set a timer and

be recovered by submarine. These examples ex-

emplify the wide range of forms a nuclear weapon

could take in tactical use, most of which have



3

been thought of as upgrades to existing conven-

tional capability rather than fundamentally new

destructive power.

This same pattern has repeated in recent years

as most discussion of nuclear earth penetratingweapons similarly fails to acknowledge the fun-

damental differences associated with adding nu-

clear capabilities to the battlefield. Even their

colloquial name “nuclear bunker busters” implies

that such a weapon is merely a nuclear modi-

fication of the existing weaponry for attacking

underground targets (see Section 4.1).

3. TACTICAL NUCLEAR WEAPON

DISARMAMENT

As with strategic weapons, the current distri-

bution of tactical nuclear weapons is weighted

heavily towards the United States and Russia,

each of which maintains an arsenal including

several thousand such weapons. The United

Kingdom and China also have several hundred

each. The history of tactical nuclear weapon dis-

armament, like strategic weapon disarmament,

therefore consists primarily of talks between the

United States and Russia.

While the INF Treaty of 1987 began the pro-cess of eliminating intermediate and shorter-

range strategic missiles [9], the disarmament

movement did not move into the realm of true

tactical weapons for several more years. Fol-

lowing an attempted Soviet coup in August of

1991 and amidst increased concern over the se-

curity of Soviet nuclear weapons, the Bush ad-

ministration announced dramatic reductions in

the United States nuclear arsenal. In addition

to reductions in strategic ICBMs, then President

Bush’s September 27th address stated that theUnited States would “eliminate its entire world-

wide inventory of ground-launched short range...

nuclear weapons” and “withdraw all tactical nu-

clear weapons from its surface ships, attack sub-

marines, as well as those nuclear weapons asso-

ciated with our land-based naval aircraft.” [11]

Further, the Soviet Union was urged to do the

same, destroying both its short range nuclear

warheads and artillery and its nuclear air defense

and land mines - programs which, as the Presi-

dent pointed out, had already been discontinued

in the United States. This proposal of unilateral

reductions has come to be known as the Presi-

dential Nuclear Initiative (PNI) and is the most

significant tactical nuclear disarmament effort to

date.

What President Bush proposed was not a to-

tal elimination of tactical nuclear weapons; only

nuclear artillery shells and short-range ballistic

missiles were slated for destruction. The remain-

ing tactical weapons were simply withdrawn. So-viet President Gorbachev’s response contained

equally varied wording. Nuclear artillery am-

munition and warheads for tactical missiles were

slated for destruction, while nuclear warheads for

anti-air missiles, nuclear mines, tactical nuclear

weapons on surface ships and submarines, and



4

ground based naval aviation nuclear weapons

were to be removed and placed in storage in

central bases [5]. He went on to claim that in

many Soviet states, this process had begun sev-

eral years prior.By the end of 1992, the unilateral reductions

appeared largely successful. President Bush

had confirmed the worldwide withdrawal of the

United State’s ground and sea-based tactical nu-

clear weapons and Kazakhstan, Belarus, and

Ukraine had completed transfers of all tactical

nuclear weapons to Russia [5].Critics are quick to point out several prob-

lems with the initiative, however. Since it is not

legally binding, either side can withdraw with-

out notice. Further, with no verification mech-

anism, the initiative leads to great uncertainty

as to actual arsenals and disarmament progress.

Finally, the initiative makes no attempt to re-strict research and development of new tactical

weapons, only to reduce the number of deployed

weapons. Openings of this nature enable devel-

opment efforts like the W61-11 (See Section 4.2)

or the Bush Administration’s Robust Nuclear

Earth Penetrator Initiative. Ultimately, these

critics claim that an agreement of this nature,made in good faith, should be used as a step-

ping stone to a binding treaty agreement, rather

than as a replacement.

To the chagrin of such critics, the issue of tac-

tical weapon disarmament has since been largely

ignored in favor of reducing strategic arsenals. In

fact, several policy-making bodies in the United

States have called for increasing the arsenal of

tactical nuclear weapons. Although the Nu-

clear Posture Review (NPR) produced by the

Department of Defense in 2002 proposed “elimi-nating the capability to deploy nuclear weapons

on naval surface ships” [24], it also promoted

the development of “improved earth penetrating

weapons (EPWs) to counter the increased use

by potential adversaries of hardened and deeply

buried facilities” [5]. It further called for the Na-

tional Nuclear Security Administration to per-

form a feasibility study of such weapons. This

move came several years after the quiet intro-

duction of the B61-11, a gravity bomb designed

for earth penetration and described in detail in

Section 4.2. This proposed development of a

Robust Nuclear Earth Penetrator could involve

a repackaging of an existing weapon or, even

worse according its many critics, the develop-

ment of a new low yield weapon. Such a move

would not only counteract recent disarmament

successes, but could also require that the United

States return to nuclear testing. Such testing

was ended by Congress in 1994 and is forbidden

by the Comprehensive Test Ban Treaty (CTBT),

which has been ratified by the United States, but

is not yet in force.

Finally, though not explicitly a move towards

disarmament, the International Court of Justice

ruled in 1996 that the use of nuclear weapons is

against international law unless a nation’s sur-



5

vival is at risk [19]. Such a ruling could arguably

include all use of tactical nuclear weapons. Fur-

ther, the Nuclear Posture Review [24], in justi-

fying the need for nuclear earth penetrators, im-

plies that they are needed to combat several non-nuclear states. Historically, conventional pene-

trators like the GBU-28 and BLU-118, have seen

most of their use in Iraq and Afghanistan. In ad-

dition to being against United States policy, the

justification of a nuclear attack on a non-nuclear

state would be extremely difficult in light of the

International Court’s ruling.

4. EARTH PENETRATING WEAPONS

In principle at least, an earth penetrating

weapon is simple. A long slender heavy tube is

dropped from high altitude and penetrates the

ground by several meters. An explosion below

the ground surface results in a seismic shock

wave that reflects off the earth-air boundary.

This coupling effect results in considerably more

energy being transferred to the underground tar-

get. In an above ground explosion, most energy

is instead reflected off the earth-air boundary.

As a result, an explosion at the surface must beboth very large and very well targeted to effec-

tively damage a hardened underground target.

Earth penetrating weapons are therefore not de-

signed to penetrate deeply enough to reach the

target, but simply deeply enough to benefit from

coupling with the ground.

4.1. Conventional Earth Penetrators

Among the most successful earth penetrating

weapons to date is the GBU-28, developed dur-

ing the Gulf War to destroy Iraqi command cen-

ters unreachable by conventional weapons. The

weapon measures nineteen feet in length and

only 14.5 inches in diameter and weighs 5000

pounds, including 650 pounds of tritonal ex-

plosive. Originally laser guided, it has since

been enhanced with Global Positioning Satellite

(GPS) guidance, so that it may be used at higher

altitude or with poor visibility [12]. Penetration

ability is estimated at 100 feet (30m) in hard

sand or 20 feet (6m) in concrete. The GBU-28

proved extremely successful in the Gulf War.

A number of new technologies, in addition to

GPS guidance, have improved the abilities of

conventional earth penetrators. First, depleted238U , a byproduct of nuclear power generation,

may be used in the weapon case. Uranium has

a density twice that of iron and a Brinell hard-

ness a full order of magnitude larger. Weapons

made with depleted Uranium can therefore pen-

etrate considerably deeper than weapons made

from iron.

Secondly, the advent of hard target smart fuses

(HTSFs) [13] has led to weapons with dramat-

ically improved detonation timing. In contrast

to fuses on early earth penetrators, which con-

sisted of contact fuses with timers, HTSFs are

accelerometer based fuses which trigger detona-



6

tion after passing through a certain number of

hard layers or voids. Such fuses are useful both

in earth penetration, where they allow detona-

tion at optimal penetration depths for ground

coupling, as well as for bombing buildings, wherethey may count floors before detonating.

Today, the BLU-118 has surpassed the GBU-

28 in effectiveness. By using thermobaric explo-

sions which fire in sequence, the BLU-118 is able

to produce sustained and powerful shock waves,

in contrast to the single sharp shockwave pro-

duced by the GBU-28. Unfortunately, its pene-

tration ability remains limited to several meters

of rock [17] and, even with coupling effects, it

is unable to destroy targets buried hundreds of

meters under the surface. Nuclear penetrators

would produce much larger shock waves, possi-

bly making them the only weapons capable of

destroying deeply buried targets.

4.2. Nuclear Earth Penetrators

The application of nuclear weapons in hard

target penetration was realized early on. The

Mk-8 Light Case, also known as the Elsie, was

developed for the Navy in the early 1950’s and

later adapted for the Air Force as the Mk-11. Itwas a simple gun-type gravity bomb much like

Little Boy repackaged in an earth penetrating

case [2] and wired with a delayed action fuse for

earth penetration. Penetration claims vary, but

it has been claimed to achieve depths of 22 ft (7

m) in concrete and between 40 and 90 ft (12 to

27 m) in hard sand [14]. Interestingly, after more

than 50 years, earth penetrators are still limited

to depths very close to these.

For the next twenty years, there was little per-

ceived need for earth penetrators. As nuclearweapons improved, the most powerful bombs of

the day could be detonated at ground level and

be more effective at destroying underground tar-

gets than streamlined earth penetrators. The

tremendous degree of collateral damage these

weapons would cause was seen as almost irrel-

evant by comparison to the nuclear war thatwould have warranted their use.

One such weapon was the B53 gravity bomb,

first deployed in 1962 and still considered us-

able against bunkers until 1997, when it was

supplanted by the B61-11 (see below). Origi-

nally conceived of as a “city buster,” the B53

had a yield of nine megatons, making it the mostpowerful weapon in the United States arsenal at

the time of its introduction [6]. The B53, be-

ing a large bomb, can be delivered by a B-52

bomber, a vulnerable low-flying plane, but not

by the more stealthy B-2. In the absence of nu-

clear war, the tremendous collateral damage and

difficulty of delivery make the B53 effectively un-usable. As a result, its use as a tactical weapon

produces no credible threat.

The W86, a nuclear penetrator based on the

B61 bomb, was developed in the late 1970’s for

the Carter administration, but the project was

canceled when the primary target of the Persh-



7

ing II missile was switched from underground

bunkers to surface-based missile launchers [2].

Instead, the W85, with a dialable yield from 5

to 50kT and also based on the W61, was in-

stalled on a maneuverable reentry vehicle andmounted in Pershing II missiles where it served

an intermediate range strategic role. Ironically,

following the withdrawal of these missiles in ac-

cordance with the INF treaty (see Section 3), the

W85 warheads were modified into B61-11 earth

penetrators [16] (see below).

During the Reagan administration, researchbegan on an earth penetrating ICBM warhead.

This would require a steerable reentry vehicle

that could impact the ground at a sufficiently

slowed speed and at the proper angle. The

project was abandoned after several years, but

an offshoot project to develop a cruise missile

carrying a B61 bomb persisted until the ColdWar ended and the need for such a capability

vanished [2].

When the Nuclear Posture Review of 1994

recommended replacing the B53, largely citing

safety issues with the weapon’s outdated secu-

rity measures, funds were requested to begin the

development of an earth penetrating form of theB61, widely considered the most versatile nu-

clear weapon in the United States’ arsenal. The

resulting weapon was the B61-11, essentially a

B61-7 in an earth penetrating package. The

yield is assumed to be dialable from .3 to 340 kT

[2], matching that of the B61-7. Unfortunately,

the B61-11’s penetrating ability is no better than

that of its conventional forerunners, with a pen-

etration depth of seven meters.

It should be noted that the B61-11 was de-

veloped despite a prohibition of developing lowyield nuclear weapons (less than five kilotons)

in United States weapons labs. This prohibition

was included in the 1994 defense authorization

bill and signed into law by President Clinton [15].

The Department of Defense insists that the B61-

11 is not a new weapon, but rather a repackaging

of the old B61-7 physics package with new fusingand a new case, so this restriction does not ap-

ply to it. Many critics disagree with this claim,

arguing that the weapon provides a new capabil-

ity and is therefore a new weapon. To say oth-

erwise, they argue, undermines nonproliferation

efforts. Consequently, the quiet development of

the B61-11 was loudly protested by groups suchas the Bulletin of the Atomic Scientists [2] and

the Los Alamos Study Group [19].

Further, development and deployment of the

B61-11 were rushed to completion, much to the

chagrin of the project’s critics. In April of 2005,

the B61-11 was referenced by the Pentagon with

respect to a suspected chemical weapons factoryin Libya. One spokesman pointed out that while

no conventional weapon could destroy the plant

from the air, the B61-11, which would be de-

ployed before the factory’s completion, would be

able to [19]. The thinly veiled threat, criticized

in diplomatic circles as a violation of the United



8

States’ policy against first strikes, was later re-

tracted by the Pentagon.

Most recently, the current Bush administra-

tion has pushed the Robust Nuclear Earth Pen-

etrator initiative with the intent of developinga weapon to supplant the B61-11 in destroying

hardened underground targets. Following the

2002 Nuclear Posture Review, a study was be-

gun in April 2002 under the Department of De-

fense and the Department of Energy to evaluate

the production of a new weapon with penetrat-

ing ability superior to that of B61-11. While the

project originally proposed the development of

a new low yield weapon, it is increasingly turn-

ing to modification of existing bombs. Livermore

and Los Alamos are taking part in the project,

modifying the B83 and B61 respectively.

5. FEASIBILITY OF NUCLEAR EARTH

PENETRATORS

5.1. Penetration and Destructive Capability

While most hardened underground targets are

buried at a depth of several hundred meters,

an earth penetrator need not reach nearly this

depth. As with a conventional weapon, theshockwave from a below-ground nuclear explo-

sion is reflected off the earth-air boundary and

the resulting coupling enhances the destruc-

tive power of the shockwave. As calculated

by the National Academies Committee on the

Effects of Nuclear Earth-Penetrator and Other

Weapons [20], producing an equivalent under-

ground shockwave to that created by an earth

penetrating weapon would require the contact

burst of a weapon with yield tens of times higher.

Figure 2 presents the effective yield factor (ascompared to a contact burst) as a function of the

ratio of depth of burst to weapon yield. For in-

stance, a 300kT weapon detonated at just 3 me-

ters below ground has a ground-shock coupling

factor of 20, indicating that the contact burst of

a 6MT weapon would produce the same shock-

wave damage below ground. The size of these

equivalency factors offers some insight into the

importance of underground detonation.

The vibrational effects of these shockwaves

may be sufficient to devastate nearby tunnel

networks and heavy equipment. Forden’s anal-

ysis [10] of the 1960 GNOME shot (a 3.1kT

weapon detonated underground by the Plow-

share Project with the intent of testing peace-

ful applications of nuclear weapons) reveals that

a 1kT weapon detonated at a depth of just

30m could effectively destroy heavy equipment

(large fermentation tanks, generators, and mo-

tors) within approximately 160m of the blast.

Objects of this size are sensitive to extremelylow frequency vibrations, on the order of 10Hz,

which the GNOME shot data reveals to be

prevalent in a nuclear explosion. Lighter equip-

ment, requiring higher frequencies to cause ex-

tensive damage, is far less susceptible to shock-

wave damage at these distances.



9

FIG. 2: Effective yield factors due to coupling for nuclear weapon detonation below ground. Yields are normalized to a contact

burst. Note that negative depth corresponds to above ground detonation. Figure adapted from [20]

In a nuclear airburst, however, it is the emit-

ted heat and light (soft x-rays) that are respon-

sible for most of the resulting devastation. As

Eden [8] describes in Whole World on Fire, a

nuclear detonation causes a firestorm that ex-

pands rapidly and can consume areas far from

the blast zone of the explosion. These effects

are equally significant in an underground explo-

sion where they first serve to vaporize a large

spherical cavity in the rock. Beyond this cav-

ity, the intense heat causes a network of cracks

and ruptures to expand horizontally outward as

well as slightly downward. Examination of the

rupture zone formed after underground nuclear

testing reveals that much of the rock is charred

[10], possibly from a firestorm like those that fol-

low airbursts.

While a facility on a platform suspended by

shock absorbers is far less susceptible to shock-

wave damage (Forden points out that this tech-

nique is used in Minuteman missile silos), it re-

mains highly vulnerable to rupture. However,since the rupture zone does not extend far ver-

tically, burying a facility at several hundred me-

ters is sufficient to avoid damage from this effect.

The rupture zone, particularly with the possibil-

ity of a firestorm, remains effective against shal-

low tunnel networks, such as those leading into



10

underground bunkers and storage facilities.

With this in mind, one must consider how deep

a nuclear weapon can realistically penetrate. In

order to preserve the integrity of the weapon

payload, the impact velocity must be limited.In the case of a nuclear weapon, this limit is

approximately three kilometers per second [22].

For these speeds, the theory of long-rod pene-

tration approximates the ratio of the maximum

penetration depth, D, to penetrator length, L,

as proportional to penetrator density and weakly

dependent on penetrator yield strength:

D

L≈

ρ pρt

lnY pY t

(1)

in which ρ is density, Y is yield strength and p

and t refer to penetrator and target [22]. From

long-rod penetration, the ratio of penetration

depth to weapon length is typically on the or-

der of ten for iron penetrators impacting hard

rock. This is typically an overestimate.

A much more complete model of penetration

is given by Young’s Empirical Equation [20]

D = αK sSN mA

0.7

(V s − 30.5) (2)

where D is penetration depth (m), α =

0.0000175, m is penetrator mass (kg), A is cross-

sectional area (m2

), V s is impact velocity (m/s),N is a function of nose parameters, K s is a scal-

ing factor dependent on m and the type of target

material (soil or rock), and S is an empirical con-

stant dependent on the target material. When

considering the penetration depth of a weapon,

Young’s empirical formula (2) is generally used.

Long rod penetration (1) is used as an order of

magnitude approximation to understand the ap-

proximate effects of changing casing materials or

targets.

Using Young’s formula (2), Table I estimatesthe maximum penetration depth in various

mediums of three variants of the W86 [20]. As

described in 4.2, the W86 nuclear earth penetra-

tor was designed in the 1970s but never built.

Calculations reveal that the optimized design of

the W86 would be capable of penetrating about

seven meters in hard rock. The B61-11 earthpenetrator, with a dialable yield from 300 tons

to 340 kilotons, is also claimed to achieve pene-

tration depths of about seven meters.

Earth penetration may generally be improved

in two ways: increasing the length of the weapon

or increasing the impact velocity. Weapon size

is strictly limited by delivery capability. Indeed,

one advantage of the B61-11 is that its size allows

for delivery by B-2A bombers or F-16 fighters

[2]. Impact velocity for a gravity bomb is gen-

erally limited to the terminal velocity achieved

from free fall. In theory, a rocket could be fired

before impact to increase this speed, but the

sudden deceleration at impact must not be suf-ficient to destroy the payload. Nuclear weapons

are particularly sensitive to this impact due to

the importance of maintaining a symmetrical pit

and properly timing the conventional explosives

to form a spherical inward shockwave. Conse-

quently, deceleration is limited to approximately



11

Weapon Target S N m (kg) A (m2) V s (m/s) D (m)

Strategic EPW Medium Strength Rock 0.76 0.85 411 0.059 700 3.7

Strategic EPW Low Strength Ro ck 1.30 0.85 411 0.059 1200 11.1

Strategic EPW Silty Clay 8.00 0.85 411 0.059 1500 86

Low-Yield EPW Medium Strength Rock 0.76 1.00 184 0.022 1000 7.2

Low-Yield EPW Low Strength Rock 1.30 1.00 184 0.022 1500 18.7

Low-Yield EPW Silty Clay 8.00 1.00 1 84 0.022 1500 115

Optimized EPW Medium Strength Rock 0.76 1.00 2668 0.107 500 7.5

Optimized EPW Low Strength Rock 1.30 1.00 2668 0.107 500 12.8

Optimized EPW Silty Clay 8.00 1.00 2668 0.107 500 79

TABLE I: Estimated penetration depths of W86 earth penetrator variants, calculated using Young’s empiricle formula (2).

Table adapted from [20].

10,000g [20]. In practice this means that increas-

ing impact velocity is not a possibility. (In fact,some weapons must use parachutes for this rea-

son, particularly when fused for laydown.)

Ultimately we may conclude that hard rock

penetration may be feasible to a depth of several

dozen meters, (The Robust Earth Penetrator

Initiative aims for 80 feet of penetration, about

25 meters [17].) We will see in section 5.2 that

even this generous estimate of future capabilities

is insufficient for containing fallout. However,

these seemingly miniscule depths (as compared

to the several hundred meters below ground that

one expects to find targets) are sufficient to en-

able large degrees of ground coupling, producingdevastating shockwaves. While this shockwave

may damage large equipment hundreds of me-

ters below, rupturing ground and flash fire at the

level of the burst may be effective at destroying

tunnels and other surface linkages to the buried

target.

5.2. Fallout

Airburst nuclear explosions are associated with

large amounts of radioactive fallout over large ar-

eas, particularly in dust within their character-

istic mushroom clouds. These radioactive par-

ticles, measuring 10µm to 20µm are dispersed

by wind and may take months to settle globally,

leading to the term world-wide fallout. Conceiv-

ably, an underground detonation could reduce

this fallout by restricting it to within the cavity.

In addition to worldwide fallout, surface and

underground detonations produce local fallout

composed of larger particles ranging from 100µm

to several mm. These larger particles settle

quickly - on the order of days - and settle overtens to hundreds of kilometers. Underground

detonations are further affected by base surge,

a fluid flow of large solid particles outward from

the crater. [22]

Nuclear tests in the Nevada desert were largely

conducted underground, providing a consider-



12

FIG. 3: Cavity formation from detonation of a 1kT nuclear weapon at various depths (indicated in feet) below ground level.

Notice that, for this relatively low yield weapon, a large crater forms at detonation levels less than approximately 550ft (170m),

accompanied by radioactive fallout. Figure from [22]

able knowledge base for understanding cavity

formation. Figure 3 shows the effects of detonat-

ing a 1kT nuclear weapon at various depths. No-

tice that a depth of 550ft (170m) is necessary to

prevent the ejection of radioactive material into

the surrounding environment [22]. At depths

measured in tens of meters, the fireball itself is

likely to breach the surface. Like a low level air-

burst, local fallout will likely be extremely high.

Further understanding of the effects of rela-

tively shallow depth detonation comes from the

Plowshare program, initiated with the intent

of applying nuclear weapons to peaceful uses.

The previously mentioned GNOME shot was the

first Plowshare detonation. It was followed by

the more famous Sedan test, performed in the

Nevada test site with a 104kT weapon at a depth

of more than 600 feet (180 m). The Sedan test

was intended to determine the efficacy of using

nuclear weapons for excavation and the resulting

crater, shown in Figure 4, is a testament to the

weapon’s ability to move earth. The ejection of

12 million tons of rock produced a crater 320 feet

(100 m) deep and 1,280 feet (370 m) across and

the radioactive debris plume reached an altitude

of 12,000 feet (3.7 km).

Nelson [22] claims that roughly half of the ra-

dioactivity produced in the Plowshare tests was



13

released, with the other half contained in the

cavity. This release took the form of local fall-

out and a base surge which spread for more

than a mile. Nelson also points out that even

if a weapon were somehow capable of burrow-ing sufficiently far as to contain its fallout (re-

call the estimate of 550 ft for a 1kT weapon),

it would likely vent radioactive material through

its entry hole. The earlier mentioned GNOME

shot demonstrated this phenomenon when its en-

try shaft failed to collapse properly and instead

vented radioactive steam for several hours.

Certainly, the Sedan test must be viewed in

light of the fact that the configuration of the

weapon was intended to produce a crater, but

the weapon involved had a yield less than the

maximum yield of a W61-11 and was buried at

a depth several orders of magnitude deeper than

the W61-11 is capable of reaching.

In light of this evidence, it is unlikely that a

nuclear earth penetrator may be used without

producing extensive local fallout, though world-

wide fallout may be limited. Lindon Brooks,

chief of the National Nuclear Security Admin-

istration, understands this and maintains that

the Bush administration does as well. In testi-mony on March 2, 2005, he stated, “I really must

apologize for the my lack of precision if we in the

administration have suggested that it was possi-

ble to have a bomb that penetrated far enough

to trap all fallout... I don’t believe the laws of

physics will ever let that be true.” [3]

FIG. 4: The Sedan crater produced by a 104kT nuclear

weapon burried 635 feet below ground. The crater is 320

feet deep and 1,280 feet across. The radioactive dust cloud

reached an altitude of 12,000 feet.

5.3. Chemical and Biological Agent Destruction

A common claim of some proponents of nuclear

earth penetrators is increased ability to destroy

chemical and biological agents stored in under-ground facilities targeted by the weapon. Since

the coupled shockwave produced by the explo-

sion would likely rupture storage or fermentation

tanks, it is important to understand the effect on

these agents if an attack is to be planned on such

a facility.

A nuclear weapon could conceivably neutralizeagents through intense radiation or heat. As de-

scribed in section 5.1, the nuclear earth penetra-

tor is also quite distant from its target, reaching

only tens of meters (generously) into the ground.

Nelson [21] points out that neutron and gamma

radiation produced by a nuclear explosion can-



14

800K 1000K 1500K

Sarin 20s 1s 0.01s

VX 0.7s .01s .0001s

Mustard 0.03s 0.002s 0.00005s

TABLE II: Necessary times at various temperatures to neu-

tralize 99.999% of chemical agents. Table from [18].

not penetrate more than a few meters through

rock and is therefore impotent against chemical

or biological agents in a facility below. Extreme

temperature is thus the most feasible means by

which agents may be neutralized.

We first consider the temperatures and timesnecessary to destroy chemical and biological

weapon samples. Table II summarize this in-

formation for several chemical agents of partic-

ular concern. Neutralizing chemical agents is

generally far more difficult than destroying bi-

ological agents, and consequently 1,500K, the

temperature necessary to neutralize 99.999% of sarin in 10ms, is often quoted as the necessary

temperature for destroying chemical and biolog-

ical agents. Levi [18] points out that non-viral,

non-spore-forming organisms can be 99.999% de-

stroyed in milliseconds at temperatures below

400K (120oC). Even for spore-forming organisms

such as anthrax, temperatures of 510K (240

o

C)may achieve 99.999% neutralization in five mil-

liseconds. The destruction of spores, however, is

critically dependent on spore conditions. The

temperature quoted above corresponds to the

worst case scenario of dry heat and dry spores.

From this, Levi [18] argues that a conventional

fuel air explosive (FAE) could be quite effective

in destroying biological weapons, but this argu-

ment only holds if an FAE can be introduced

to the system, as is likely not the case for a

buried facility. Levi also grants that a facilityof this nature may not contain sufficient oxygen

for consumption by a fuel air explosive. A con-

ventional weapon of this nature is a reasonable

option for attacks on above ground plants that

must be penetrated then neutralized, such as

the suspected chemical weapons plant in Libya

which was threatened with attack from a B61-

11 (see Section 4.2). It may also be a reasonable

follow-up to a nuclear attack on a buried facility.

Based on data from the Ranier underground

test of a 1.7kT weapon, the air temperature

in the cavity left by detonation is estimated as

1,500K for the first several minutes following the

explosion. This temperature decays to ambi-ent within two to three cavity radii [21]. Not

surprisingly, the nuclear penetrator will neither

heat nor irradiate agents at the level of the facil-

ity. However, any materials ejected through the

chimney formed by the explosion will be exposed

to these extreme temperatures for several mil-

liseconds, which should be adequate to destroy

most biological agents. Destruction of chemi-

cal weapons is much less certain, but these are

generally considered more easily contained and

therefore pose a somewhat smaller (but still very

significant) risk to the nearby populace.

It is of course possible that agents will remain



15

in the facility, liberated from their storage tanks

but not ejected through the hot cavity. These

unneutralized agents will leak out over time, pos-

ing a threat. As such, a nuclear detonation is far

from a complete solution in the destruction of anunderground agent storage facility and its con-

tents.

When considering the issue of neutralizing

chemical and biological agents, this analysis re-

veals that the two must be treated very differ-

ently. Many critics of nuclear penetrators dis-

miss their neutralizing ability as absurd, citing

the necessary sustained temperatures to destroy

a chemical weapon such as sarin, but neglect-

ing to mention the much cooler temperatures

needed to destroy biological weapons. As with

all weapon systems, an imperfect ability is af-

forded by a nuclear earth penetrator and it is

critical that military planners have an accurate

assessment of the actual capabilities of these

weapons prior to considering their use. The use

of this weapon on a facility under the assump-

tion that it can neutralize an agent stored there

could potentially be disastrous, but at the same

time the potential use in neutralization should

not be ignored. As such, intelligence gatheringis crucial to the effective use of a nuclear pen-

etrator. Ultimately, a nuclear earth penetrator

may provide only part of the solution. Follow-up

attacks with fuel air explosives or similar conven-

tional weapons may be necessary to completely

decontaminate an area.

6. CONCLUSIONS

Evidence is strong that a nuclear earth pene-

trator such as the W61-11 or the proposed Ro-

bust Nuclear Earth Penetrator can effectively

damage hardened underground facilities through

a coupled shockwave and destroy shallow tunnel

networks through ground rupture. While this

is not unlike conventional penetrators like the

GBU-28 or BLU-118, a nuclear penetrator can

be expected to function on much more deeply

buried targets. Ground coupling makes the use

of small nuclear weapons effective so that fallout

may be minimized, but not eliminated, as the

weapon simply cannot penetrate deeply enough

to achieve complete containment. A nuclear pen-

etrator may also be capable of destroying most

biological and some chemical agents liberated by

the explosion, though such neutralization would

be generally incomplete.

Two questions arise from this analysis. First,

is a nuclear earth penetrator a usable weapon?

Second, is the gain from adding a new nuclear

penetrator to the arsenal worth the political

losses?

While a nuclear penetrator may be more ef-fective in certain situations (destroying deeply

buried targets and more completely neutralizing

biological and chemical agents) than its conven-

tional equivalent, it also introduces unavoidable

fallout and could potentially release unneutral-

ized biological or chemical agents. In attacking



16

a facility in an unpopulated region, these risks

may be acceptable. In a populated area, how-

ever, the risk of civilian casualties may make the

nuclear option unpalatable.

A low-yield tactical nuclear weapon such asthe B61-11 does make the threat of use con-

siderably more real. While an enemy state

likely recognizes that the United States can-

not attack it with standard nuclear weapons,

it may have to consider the threat of small nu-

clear weapons capable of destroying central com-

mand and weapon bunkers with minimal collat-eral damage. The existance of a credible threat

may prove more useful than the weapon itself.

Any attack, however, would likely vio-

late United States policy which, since 1978,

has explicitly forbidden the use of nuclear

weapons against signatories of the Nuclear Non-

Proliferation Treaty (with the exception of statesallied with nuclear states engaged in acts of ag-

gression) [2]. Further, it would break the “nu-

clear taboo” against the wartime use of nuclear

weapons. Such political setbacks would certainly

affect US status in global policy making and

would have a particularly harsh effect on United

States efforts towards international nuclear dis-armament.

The B61-11 nuclear earth penetrator is already

present in the United States arsenal. Clearly, it

is a superior alternative to the enormously pow-

erful B53, claimed to be capable of destroying

underground targets by detonating its 9MT pay-

load at ground level. The low level to which

its yield may be adjusted also makes the B61-

11 a credible threat. As such, it is a reasonable

weapon to have in the arsenal for consideration

in missions unachievable with conventional pen-etrators.

The need for a new nuclear earth penetra-

tor is far less clear. Some proposals call for

weapons with yields as low as ten tons. It is dif-

ficult to imagine a nuclear weapon more likely to

see actual use, assuming it can penetrate deeply

enough to be useful. This credibility is exactlywhat worries many opponents of the project,

who feel that the use of a nuclear weapon is

something to be avoided at all costs, citing im-

paired disarmament efforts and increased use of

nuclear weapons in the future.

Whether or not the Robust Nuclear Earth Pen-

etrator weapon project proceeds, the current ar-

senal already offers these risks. The most signif-

icant political damage would come as a result of

breaking the self-imposed bans on new weapon

development and nuclear testing, though modi-

fication of existing weapons could, in a superfi-

cial way, avoid these problems. The Los Alamos

Study Group claims that the development andcertification of the B61-11 without testing “un-

dercuts [the Comprehensive Test Ban Treaty]

and could provide political cover to countries

which have their own unsatisfied nuclear ambi-

tions.” [19] These concerns are reasonable and

the continuation of nuclear weapon development,



17

even in the form of modifications, would likely

have lasting effects on both nonproliferation and

disarmament efforts.

In light of recent Nuclear Posture Reviews, the

quiet development of the B61-11, and the Ro-bust Nuclear Earth Penetrator Initiative, it ap-

pears that there is a strong movement to increase

the presence of tactical nuclear weapons in the

United States arsenal. In part, this is because

a low yield weapon could potentially be used,

but more importantly it is because enemies of

the United States will believe it can be used.

Ultimately, however, the political risks of actu-

ally using even a low yield weapon are likely too

great. Like their strategic counterparts, tactical

nuclear weapons will serve a purpose of deter-

rence; the existence of an earth penetrator may

temporarily discourage the use of undergroundbunkers and storage facilities. For this purpose,

and in the event that one must be used, it is

important that these weapons be known to be

functional and effective. Regardless of the poli-

tics of their construction or use, politics must not

interfere with the understanding of the weapons’

actual capabilities.

[1] Bulletin of the Atomic Scientists, The B61 Family of

Bombs, www.thebulletin.org/article nn.php?

art ofn=jf03norris.

[2] Bulletin of the Atomic Scientists, New Bomb, No Mis-

sion , www.thebulletin.org/article.php?

art ofn=mj97mello

[3] Bulletin of the Atomic Scientists, March 2005 Archives,

www.thebulletin.org/weblog/archives/2005/03/

[4] The Brookings Institution, www.brook.edu.

[5] Center for Arms Control and Non-Proliferation,

Briefing Book on Tactical Nuclear Weapons,

www.armscontrolcenter.org/prolifproject/tnw/.

[6] Center for Defense Information United States Arsensal ,

www.cdi.org/issues/nukef&f/database/

usnukes.html#b53

[7] Congressional Research Service, “Bunker Busters”:

Sources of Confusion in the Robust Nuclear Earth Pene-

trator Debate, http://www.armscontrolcenter.org/

resources/20040922 crs rnep confusion.pdf

[8] Eden, Lynn, Whole World on Fire. Cornell University

Press, 2004.

[9] Federation of American Scientists, Nuclear Non-

Proliferation Treaty Chronology , www.fas.org/nuke/

control/npt/chron.htm.

[10] Forden, Geoffrey, USA Looks at Nuclear Role in Bunker

Busting . Jane’s Intelligence Review, March 12, 2002.

www.janes.com/press/pc020312 1.shtml

[11] George Bush Presidential Library and Mu-

seum, Address to the Nation on Reducing United

States and Soviet Nuclear Weapons, bushli-

brary.tamu.edu/research/papers/1991/

91092704.html.

[12] GlobalSecurity.org, Enhanced Guided Bomb Unit

(EGBU)-28 , www.globalsecurity.org/military/systems/

munitions/gbu-28e.htm

[13] GlobalSecurity.org Hard Target Smart Fuze [HTSF] ,

www.globalsecurity.org/military/systems/munitions/

fmu-157.htm

[14] GlobalSecurity.org MK.8 Light Casee,

www.globalsecurity.org/wmd/systems/mk8.htm

[15] GlobalSecurity.org Robust Nuclear Earth Penetrator ,

www.globalsecurity.org/wmd/systems/rnep.htm

[16] GlobalSecurity.org W85 , www.globalsecurity.org/wmd/systems/

[17] Koch, Andrew, The Bunker Buster Nightmare Goes Nu-

clear , Popular Science. October, 2002.

[18] Levi, Michael A., Written Statement of before The Na-



18

tional Academy of Sciences Study on the Effects of

Nuclear Earth-Penetrator Weapon and Other Weapons,

www.brookings.edu/views/testimony/fellows/

levi20040427.pdf

[19] Los Alamos Study Group, B61-11 Con-

cerns and Background , February 10, 1997.

www.lasg.org/archive/1997/b61-11.htm

[20] National Academies Press, Effects of Nu-

clear Earth-Penetrator and Other Weapons,

www.nap.edu/books/0309096731/html

[21] Nelson, Robert W., Nuclear “Bunker Busters”

Would More Likely Disperse Than Destroy Buried

Stockpiles of Biological and Chemical Agents,

www.princeton.edu/ rnelson/papers/agent defeat.pdf

[22] Nelson, Robert W., Low Yield Earth-Penetrating Nu-

clear Weapons, Federation of American Scientists.

www.fas.org/faspir/2001/v54n1/weapons.htm

[23] Union of Concerned Scientists, Earth Penetrating

Weapons, www.ucsusa.org/global security/

nuclear weapons/page.cfm?pageID=777.

[24] United States Department of Defense, Nuclear Posture

Review , www.defenselink.mil/execsec/adr95/npr .html.

APPENDIX A: EDITING NOTE

The formatting of this document causes it to

appear shorter than it actually is (approximately

25 standard Word pages).

Documents

Grant Elliott- History and Use of Tactical Nuclear Weapons in Earth Penetrating Applications