Effects of Nuclear Earth-Penetrator and Other Weapons

Committee on the Effects of Nuclear Earth-Penetrator
and Other Weapons, National Research Council

ISBN: 0-309-09673-1, 150 pages, 8 1/2 x 11, paperback (2005)

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Underground facilities are used extensively by many nations to conceal and protect
strategic military functions and weapons’ stockpiles. Because of their depth and hardened
status, however, many of these strategic hard and deeply buried targets could only be put
at risk by conventional or nuclear earth penetrating weapons (EPW). Recently, an
engineering feasibility study, the robust nuclear earth penetrator program, was started by
DOE and DOD to determine if a more effective EPW could be designed using major
components of existing nuclear weapons. This activity has created some controversy
about, among other things, the level of collateral damage that would ensue if such a
weapon were used. To help clarify this issue, the Congress, in P.L. 107-314, directed the
Secretary of Defense to request from the NRC a study of the anticipated health and
environmental effects of nuclear earth-penetrators and other weapons and the effect of
both conventional and nuclear weapons against the storage of biological and chemical
weapons. This report provides the results of those analyses. Based on detailed
numerical calculations, the report presents a series of findings comparing the
effectiveness and expected collateral damage of nuclear EPW and surface nuclear
weapons under a variety of conditions.
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 Summary

The tasking for the Committee on the Effects of Nuclear Earth-Penetrator and Other Weapons,
stated in Section 1033 of the Bob Stump National Defense Authorization Act for Fiscal Year 2003
(Public Law 107-314), is included in Chapter 1 of this report. The charge requests an examination of the
anticipated health and environmental effects of (1) a nuclear earth-penetrator weapon that would enhance
ground-shock coupling to destroy deep underground or other hard targets, (2) a nonpenetrating nuclear
weapon that would also be used against deeply buried or other hard targets, and (3) conventional
weapons used against facilities for the storage or production of weapons of mass destruction. Study of
the effects on civilian populations and on U.S. military personnel who carry out operations or battle
damage assessment in the target area is specified.

To provide a more complete analysis of the issues, the committee expanded its study to consider the
effects of nuclear weapons used against facilities for the storage of chemical or biological agents. It also
considered the effects of nuclear bursts that can be described as locally fallout-free, because the weapon
is detonated well above the ground surface.

The committee received many briefings from a wide variety of public and government sources and
reviewed classified reports from the Department of Defense (DOD) and the Department of Energy
(DOE). Although this report is unclassified, the committee also produced a separate classified annex,
which does not modify any of the study’s conclusions but provides supporting material.


CONCLUSIONS
The committee’s major conclusions are the following:

Conclusion 1. Many of the more important strategic hard and deeply buried targets are beyond the reach of conventional explosive penetrating weapons and can be held at risk of destruction only with nuclear weapons. Many—but not all—known and/or identified hard and deeply buried targets can be held at risk of destruction by one or a few nuclear weapons.

Conclusion 2. Nuclear earth-penetrator weapons (EPWs) with a depth of penetration of 3 meters capture most of the advantage associated with the coupling of ground shock. While additional depth of penetration increases ground-shock coupling, it also increases the uncertainty of EPW survival. To hold at risk hard and deeply buried targets, the nuclear yield must be increased with increasing depth of the target. The calculated limit for holding hard and deeply buried targets at risk of destruction with high probability using a nuclear EPW is approximately 200 meters for a 300 kiloton weapon and 300 meters for a 1 megaton weapon.

Conclusion 3. Current experience and empirical predictions indicate that earth-penetrator weapons cannot penetrate to depths required for total containment of the effects of a nuclear explosion.

Conclusion 4. For the same yield and weather conditions, the number of casualties from an earthpenetrator weapon detonated at a few meters depth is, for all practical purposes, equal to that from a surface burst of the same weapon yield. (Any reduction in casualties due to the use of an EPW is attributable primarily to the reduction in yield made possible by the greater ground shock produced by buried bursts.)

Conclusion 5. The yield required of a nuclear weapon to destroy a hard and deeply buried target is reduced by a factor of 15 to 25 by enhanced ground-shock coupling if the weapon is detonated a few meters below the surface.

Conclusion 6. For attacks near or in densely populated urban areas using nuclear earth-penetrator weapons on hard and deeply buried targets (HDBTs), the number of casualties can range from thousands to more than a million, depending primarily on weapon yield. For attacks on HDBTs in remote, lightly populated areas, casualties can range from as few as hundreds at low weapon yields to hundreds of thousands at high yields and with unfavorable winds.

Conclusion 7. For urban targets, civilian casualties from a nuclear earth-penetrator weapon are reduced by a factor of 2 to 10 compared with those from a surface burst having 25 times the yield.
[FNC: Merging conclusions 5, 6, and 7, we infer that our current EPW, the B61-11, will not protect the civilians.  It will kill at least hundreds (1/10 of "thousands") and maybe more than half a million (1/2 of "more than a million") people.]

Conclusion 8. In an attack on a chemical or biological weapons facility, the explosive power of conventional weapons is not likely to be effective in destroying the agent. However, the BLU-118B thermobaric bomb, if detonated within the chamber, may be able to destroy the agent. An attack by a nuclear weapon would be effective in destroying the agent only if detonated in the chamber where
agents are stored.

Conclusion 9. In an attack with a nuclear weapon on a chemical weapons facility, civilian deaths from the effects of the nuclear weapon itself are likely to be much greater than civilian deaths from dispersal of the chemical agents. In contrast, if the target is a biological weapons facility, release of as little as 0.1 kilogram of anthrax spores will result in a calculated number of fatalities that is comparable on average to the number calculated for a 3 kiloton nuclear earth-penetrator weapon.

Additional conclusions are presented in Chapter 9 of this report.
The committee notes that although some scenarios show substantial nuclear-radiation-induced
fatalities, military operational guidance is to attack targets in ways to minimize collateral effects.
Calculated numbers of fatalities to be expected from an attack on an HDBT might be reduced by
operational planning and employment tactics. Assuming that other strategic considerations permit, the
operational commander could warn of a nuclear attack on an HDBT or could time such an attack to take
advantage of wind conditions that would reduce expected casualties from acute and latent effects of
fallout by factors of up to 100, assuming that the wind conditions were known well enough and were
stable and that defenses against the attack could not be mobilized. However, a nuclear weapon burst in
a densely populated urban environment will always result in a large number of casualties.
BACKGROUND
Potential U.S. adversaries worldwide are using underground facilities to conceal and protect leaders,
military and industrial personnel, weapons, equipment, and various other assets and activities. These
facilities include hardened surface bunkers and tunnel facilities deep underground. Specifically, many
underground command, control, and communications (C3) complexes and missile tunnels are between
100 and 400 meters below the surface, with the majority less than 250 meters deep. A few are as deep as
500 or even 700 meters, in competent granite or limestone rock.
The activities in such underground facilities pose a potentially serious threat to U.S. national
security. As a generic term, “hard and deeply buried targets” refers to all types of intentionally hardened
targets, either aboveground or belowground. The DOD estimates that 10,000 HDBTs exist in the
territory of potential adversaries worldwide. Of the estimated 10,000 HDBTs, about 20 percent have a
major strategic function; of that 20 percent, about half are near or in urban areas. More than 100 HDBTs
could be candidates for targeting with a robust nuclear earth penetrator (RNEP) weapon, if one were
developed (the Robust Nuclear Earth Penetrator program is currently an engineering feasibility study).
(In Chapter 3, see the section entitled “Current Robust Nuclear Earth Penetrator Program.”)
Although much of the congressional discussion in this area has been about the RNEP weapon, a
more general term is “earth-penetrator weapon.” The EPW is designed to detonate below the ground
surface after surviving the extremely high shock and structural loading environments that result during
impact and penetration.
The DOE national laboratories and DOD laboratories have maintained EPW programs and testing
activities since the 1960s, resulting in more than 1,000 representative non-nuclear penetration tests that
are recorded in the Sandia National Laboratories Earth Penetration Database. Penetration tests have
been conducted at various impact angles, angles of attack, and velocities into undisturbed geologic
targets to provide insight into how the physical properties of a penetrator affect its ability to penetrate.
The greatest uncertainty in predicting EPW depth of penetration and structural survival of the
weapon until detonation arises from the inherently heterogeneous nature of the local subsurface geology.
This uncertainty can be countered to some degree by designing an EPW to be as rugged as possible,
consistent with mission and system requirements. Calculated penetration depths depend on the mechanical
properties of the earth materials at the target point. For example, for the same penetrator and velocity,
calculations give penetration depths of 100 meters in a silty clay soil, 30 meters in low-strength rock,
and 12 meters in medium-strength rock; and the maximum depth in soil can vary by ±20 percent. Deeper
EPW penetration is generally better for target destruction because the ground-shock coupling increases
with deeper depth of burst (DOB), although most of the advantage is obtained in the first few meters.
The current nuclear EPW is the B61-11, which uses the B61-7 nuclear weapon components and was developed to replace the B53 gravity bomb. The Robust Nuclear Earth Penetrator program is an engineering feasibility study to determine if it is possible to design an earth-penetrator weapon system that uses the major components of an existing weapon system and can hold at risk of destruction a significantly larger number of HDBTs than could the B61-11.

NUCLEAR EARTH-PENETRATOR WEAPON
Although conventional high-explosive weapons can penetrate at least as deep as a nuclear EPW can, if not deeper, conventional weapons are not likely to be effective against targets that the penetrator cannot reach. For destroying targets near the surface, however, either nuclear or conventional weapons may be effective. Because of the radiation doses and much higher temperatures associated with their detonation, nuclear weapons are expected to be more effective than conventional weapons at destroying biological or chemical agents.

The major advantage of an EPW over a surface or aboveground burst is the effectiveness with which energy is transmitted into the ground. The ground-shock-coupled energy of an earth-penetrator weapon
approaches 50 percent with increasing depth of burst, and is effectively fully coupled at a scaled DOB of about 2.3 m/Y1/3 (where m is depth of burst in meters and Y is yield in kilotons).1 The ground-shockcoupling factor has already risen to 15 to 25 for a 300 kiloton EPW at 3 meters’ depth of burst (scaled DOB of about 0.5 m/Y1/3). Calculations indicate that such a weapon is capable of severely damaging tunnels in a competent granite site down to depths of around 150 meters with a 0.95 probability. A nonpenetrating nuclear weapon capable of causing the same damage would have a yield of about 6 megatons. To be fully contained (i.e., with no venting of radioactive gases), a 300 kiloton weapon would have to be detonated at the bottom of a carefully stemmed emplacement hole about 800 meters deep. Because the practical penetration depth for an EPW is a few meters—a small fraction of the depth for full containment—there will be blast, thermal, initial nuclear radiation, and fallout effects from use of an EPW.
The effectiveness of nuclear weapons against deeply buried targets can be estimated by calculating
the intensity of the ground shock in the vicinity of the buried target in relation to the hardness of the
target. There is a reasonably extensive experimental database, Effects Manual Number 1 (EM-1),2
covering the various physics regimes governing the energy-coupling process. Uncertainties associated
with estimates of energy coupling into the ground are far greater for near-surface airbursts than for
buried bursts, and they depend on how well the actual burst location and details of weapon energy output
are known.
The Defense Threat Reduction Agency and DOE have invested considerable resources to develop
computational methods for predicting the ground-shock environments at depth from both high-explosive
and nuclear bursts. This is a complicated problem owing to various shock-attenuation mechanisms—
such as inelastic effects, hysteresis, fracture, and dilatation—and geometric effects due to divergence of
the stress waves and the presence of layers, interfaces, faults, and joints throughout the target area. The
directly applicable U.S. experimental database, EM-1, is limited to the results of data on eight underground
nuclear tests in which tunnels of various construction types were exposed to damaging groundshock
levels of nuclear bursts in a few types of rock geologies. Only two of these tests were dedicated
to experiments on engineered structures in competent granite geology. The others were add-on experiments
to underground nuclear tests conducted for different purposes on engineered structures in relatively
soft tuff geology.
Calculations show that both surface-burst and earth-penetrating nuclear weapons must be delivered
with high accuracy in order to have a high probability of destroying hard and deeply buried targets. For
example, a circular error probable (CEP) of less than 60 meters is needed for a 1 megaton contact burst
for targets of at most 125 meters’ depth to be held at risk with a 0.95 probability of severe damage. For
an EPW, a yield of 300 kilotons eases the accuracy requirements to a CEP of 110 meters or less, with
targets potentially as deep as 225 meters held at risk with a 0.95 probability of severe damage.

COLLATERAL EFFECTS
The primary goal for any nuclear weapon is the deterrence of a potential adversary by the ability to
hold the adversary’s most-valued assets at risk of destruction. To contribute to deterrence, the weapon should be capable of defeating those assets. The use of a weapon to accomplish the goal of target defeat or destruction will have accompanying collateral effects that, in the case of nuclear weapons, can be extremely large.
Modeling collateral effects is a multistage process. Estimated first are deaths and serious injuries
due to “prompt” (i.e., occurring immediately after detonation) effects—air blast, thermal effects, and initial nuclear radiation. Second, the downwind transport and deposition of radioactive material produced by the explosion are modeled. Third, the dose from external radiation from ground-deposited fallout is calculated. Fourth, the health effects of exposure to radiation are estimated in those populations that survive the prompt effects of the explosion.
Two computer programs are in wide use to model collateral effects. The Hazard Prediction and
Assessment Capability (HPAC) code was developed by the Defense Threat Reduction Agency and its predecessor agencies to analyze nuclear, chemical, and biological releases for military studies and operational planning. The K-Division Defense Nuclear Agency Fallout Code (KDFOC) was developed
by Lawrence Livermore National Laboratory to model fallout from nonweapon Plowshare tests, which
involved nuclear explosives designed to produce craters with minimal fallout. Both computer codes are
calibrated to available data from nuclear tests conducted at the Nevada Test Site. They differ somewhat, including their treatment of prompt casualties due to blast and radiation, wind transport, and the prediction of casualties associated with a given level of radiation from fallout.
Fallout is a long-studied and experimentally measured feature of many nuclear weapons tests. When
a nuclear weapon is exploded underground, a sphere of extremely hot, high-pressure gases is formed,
which includes vaporized weapon residues and ground materials, that is the equivalent of the fireball in
an airburst or surface burst. If the subsurface burst is at a shallow depth, the pressure of the explosion,
uncompensated by similar pressure above the surface, will throw rock, soil, and weapon material into
the air.
Fallout is determined primarily by the fission yield of the weapon, the amount and constitution
(hence activation) of entrained mass, the injection height distribution, the particle size distribution, and
subsequent atmospheric transport. Surface geology is critical. The prediction of fallout for shallow
buried bursts is uncertain because the United States has performed only three tests at depths shallower
than 20 scaled meters, and none of these tests was in rock. Another feature of a buried or surface burst
is the base surge. The base surge begins to form as the growth of the crater stops and entrained material
in the column begins to fall and expand radially along the ground surface. For depths of burst of 2 to
3 scaled meters, the fraction of activity in the base surge is typically less than a few percent of the total
activity.
Presumably, nuclear EPWs would not be used for surface and near-surface point targets, especially
if other options were available that were effective and could ameliorate the collateral damage due to
fallout. Calculations have been done for the so-called fallout-free height of burst (HOB). The falloutfree
HOB, as its name implies, is sufficiently high that the fireball produced by the nuclear explosion
does not touch the ground surface. In the absence of rain, the explosion therefore is not expected to
generate significant local fallout, because no surface material is activated, entrained, lofted, or dispersed,
and the weapon residues are present in the form of fine particles that will remain airborne for
weeks or years. For a 1 megaton weapon the fallout-free HOB is about 900 meters. The nuclear weapons at both Hiroshima and Nagasaki were detonated above the fallout-free HOB and produced no significant local fallout.
Thermal radiation from the fireball may make fire a significant collateral effect, especially for
airburst and surface-burst nuclear weapons. The potential for fire damage depends on the nature of the
burst and the surroundings. Fires can be an indirect effect of destruction caused by a blast wave, which can upset stoves, furnaces, gas lines, and so on.
The committee asked Lawrence Livermore National Laboratory (LLNL) and the Defense Threat
Reduction Agency (DTRA) to run several scenarios involving three typical targets, a range of yields,
and both surface and EPW-depth bursts. Once differences in input variables are removed, the LLNL and
DTRA results are comparable within the uncertainties in the estimated parameters. The results of these
calculations for several scenarios and weapons yields are presented in Chapter 6 and form the basis for
several of the committee’s conclusions. In addition to the conclusions stated above, significant results
include the following:
1. Any reduction in the number of casualties owing to the use of a nuclear earth-penetrator weapon
compared with the number of casualties from a surface burst is due primarily to the reduction in yield by a factor of about 25 that is made possible by the greater coupling of the released energy to the ground shock for a buried detonation.
2. For rural targets, the use of a nuclear earth-penetrator weapon is estimated to reduce casualties by a factor of 10 to 100 relative to a nuclear surface burst of equivalent probability of damage.
3. Wind patterns can have an enormous effect on the number of casualties resulting from fallout.
For targets in large urban centers, fatalities from acute and latent effects from fallout can vary by more than a factor of 10. For targets outside cities, fatalities from fallout can vary by more than a factor of 100, depending on population distribution and wind direction.
4. The estimated number of deaths and injuries resulting from a nuclear attack depends on many
variables, including weapon yield and design, depth of burst, weather conditions, and population distribution and sheltering during and after the attack. The estimated number of casualties ranges over four orders of magnitude—from hundreds to over a million—depending on the combination of assumptions used.
The committee advises readers to keep in mind that the foregoing are the results of model calculations
and that they have significant uncertainty due to uncertainties in the physical model inputs (e.g.,
the definition of the source term), boundary conditions (e.g., weather conditions and population distribution),
and the paucity of relevant experience against which the modeled results can be validated. For
each of the model calculations, a range of boundary conditions has been assumed. Uncertainty inevitably
exists in such calculations, and the scale of these uncertainties is essential to understanding the results of
the calculations and the findings of this committee. The uncertainties are of three types: scenario
uncertainty, data uncertainty, and conceptual model uncertainty.

CHEMICAL AND BIOLOGICAL AGENTS
The committee’s task included examination of the use of conventional weapons against facilities for the storage or production of weapons of mass destruction. The committee addressed the ability of
conventional and nuclear earth-penetrator weapons to effectively destroy buried production facilities,
stores, and weapons containing chemical agents and biological agents. The Department of Defense
Global Strike Mission requires the capability to deliver rapid, extended-range, precision kinetic (nuclear
and conventional) and nonkinetic weapons in support of theater and national objectives. Many conventional
high-explosive weapons are currently available and under development to support this mission.
Sufficient knowledge from intelligence assessments is of paramount importance to both weapon
choice and targeting. Some of the key elements for selecting the weapon type and its impact point(s)
include knowing the placement of the storage containers for chemical or biological agents; knowing
whether the agents are in production or, if already produced, the type of storage containers and the
material of which they are constructed; and knowing the amount of agent in containers. This information
is in addition to knowing the depth of the facility and its structure. If agents are in deeply buried facilities
that are crushed or rendered unusable, and the fracture zone created by the explosion does not open a
channel between the facility and the surface, the probability of agent ejection after impact and detonation
is very low.
Biological and chemical agents degrade after they are released into the air. The atmospheric degradation
of agents occurs as a result of several mechanisms, such as photochemical reactions, exposure to
radiant energy, and atmospheric chemistry. Biological agents are especially susceptible to atmospheric
degradation, as their viability decreases depending on levels of radiant solar energy, oxygen, relative
humidity, temperature, ozone, and hydroxyl radicals. Chemical agents decompose mainly owing to
photochemical processes in the atmosphere, such as reactions with ozone, hydroxyls, and industrial
pollutants. Both decay and decomposition are more pronounced during the daytime owing to ultraviolet
(UV) radiation and the increase in reactivity with atmospheric components. Therefore, the amount of
exposure to solar energy generally tends to determine the rates of degradation. Also, a key factor in the
loss of viability or toxicity is the length of exposure to these atmospheric elements and conditions.
Even if large amounts of chemical agents were released, substantial lethal areas would result only
under very stable meteorological conditions. The agents differ in how they disperse, but exposure to rain
and sunlight reduces their effectiveness. In the case of biological agents, only spores are relatively
immune to destruction by UV rays.
Therefore, for a daytime attack, biological agents such as smallpox and tularemia are of relatively
low danger except in the immediate vicinity of the explosion, and then for only a short period. Anthrax
spores and those of other disease agents are more UV-resistant and can withstand high temperatures.
Nevertheless, the data that the Centers for Disease Control and Prevention gathered from the anthrax
experience a few years ago and from areas in the United States where anthrax is endemic indicate that
few cases of the disease occur from wide exposure to spores after they have entered the ground. Not all
chemical agents (VX, mustard, lewisite) aerosolize. They also are similar to anthrax spores in being
unaffected by UV. If they are ejected following an explosion, they contaminate the immediately
surrounding ground area.

HAZARDS TO U.S. MILITARY PERSONNEL
The committee also addressed the hazards to U.S. military personnel from entering an area after use
of an earth-penetrator weapon. Because the committee concluded that such a weapon would produce
local fallout, the hazards are similar to those faced by troops entering an area after a surface burst. For
equivalent-target damage, because of the substantially smaller EPW yield, the local effects are reduced
significantly, but not eliminated.
Current analytical tools have an overall propagated uncertainty no smaller than one order of magnitude
(factor of 10), and likely in the range of 10 to 100, for estimates of casualties resulting from a
nuclear attack. This conclusion is founded both on evaluation of the underlying calculations (source
terms, transport models, grid resolution, and so on) and their experimental validation and on a review of
the variability in results that can be obtained for different scenarios when considering plausible ranges
in parameters.
At least three key sensitivities affect estimates of military effectiveness and casualties associated
with use of a nuclear EPW or a nuclear surface-burst weapon:
1. Target location, especially urban versus rural;
2. Accuracy of weapons delivery (circular error probable) and precise knowledge of target location
and structure, as military effectiveness depends strongly on a combination of accurate delivery and
yield; and
3. Estimates of the source, transport, and influence on populations of the effects of a nuclear
explosion, as these can be highly variable (by factors of up to about 10 to 1,000, depending on assumptions).
One additional sensitivity affects estimates of the effects of the nuclear EPW:
4. Functionality after penetration, especially as influenced by target heterogeneity and its uncertainty
(e.g., local geology or complex structures in urban areas).


NOTES
1. Scaled depth of burst (DOB) is a normalization of the actual depth (or height) of a burst based on weapon yield to that
for a 1 kiloton weapon. This is determined by DOB/Y1/3. Thus, the scaled DOB and actual DOB are the same for a 1 kiloton
EPW. For example, a 1 kiloton weapon buried 3 meters has a 3 meter scaled DOB, whereas a 300 kiloton weapon buried at
the same depth of 3 meters couples its energy to the ground as if it were a 1 kiloton weapon buried at an actual depth of about
0.45 meter; that is, 3/3001/3 = 3/6.67 = 0.45.
2. Defense Nuclear Agency. 1991. Effects Manual Number 1 (EM-1), Chapter 3, “Cratering, Ejecta and Ground Shock,”
DNA-EM-1-CH-3, Alexandria, Va., December.