The previous chapters present the results of the work carried out to address the committee’s charge, including a literature review, calculations, and analysis of information presented by many experts. The process led the committee to reach several conclusions. Listed below are the nine conclusions that the committee believes are most important to addressing the issues raised in the charge, followed by additional conclusions grouped by general topic.
MOST IMPORTANT CONCLUSIONS
Conclusion 1. Many of the more important strategic hard and deeply buried targets (HDBTs) 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 earth-penetrator 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.
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.
Hard and Deeply Buried Targets and Nuclear Earth-Penetrator Weapons
In potential adversary nations, there is a large (2,000) and growing number of identified, strategically important facilities that are sheltered in underground bunkers.
Because of the limitations on the penetration depth of penetrating weapons in the stockpile today, as well as those of the robust nuclear earth penetrator (RNEP) weapon currently under study, effects of their use would not be contained.1 Depending on many variables, including weapon yield, proximity of the target to urban areas, distribution of the civilian population and warning time available for sheltering or evacuation, ambient wind profile, and other weather conditions, this study’s calculations of numbers of deaths and serious injuries resulting from attacks on representative targets near or in urban areas range from less than 103 to greater than 106.
Nuclear EPWs (300 kilotons to 1 megaton) can hold at risk HDBTs of interest (1 kilobar hard) at up to ~100 meters to 300 meters depth of burst in granite with high probability of damage (PD greater than 0.95) if delivered with high precision.
Targets buried up to 85 meters can be held at risk (PD greater than 0.95) by precision low-yield nuclear weapons (less than 10 kilotons).
A ~100 kiloton weapon with moderate accuracy (~100 meter circular error probable (CEP)) detonated at its fallout-free height of burst (HOB)—that is, with no local fallout in the absence of rain—can be highly effective (PD greater than 0.95) against surface or near-surface, moderately hard (~500 pounds per square inch (psi) overpressure) point targets. To be highly effective against targets that are harder than about 2000 psi, detonation must occur lower than the fallout-free HOB, regardless of the yield or accuracy of the weapon. For a surface-burst weapon, the yield required to destroy with high probability (PD greater than 0.95) very hard surface or near-surface point targets (e.g., missile silos requiring 1,000 psi to 5,000 psi overpressure) is highly dependent on accuracy (e.g., 250 kilotons requires a CEP of few tens of meters; 1 megaton requires a CEP of ~100 meters). The importance of the accuracy of weapon delivery (CEP) increases with increasing target depth of burial up to about 150 meters. Beyond this depth, the importance of accuracy diminishes relative to that of increased yield.
Differences in assumptions regarding sheltering and evacuation of the population can alter the estimated number of casualties by a factor of 2 to 8. 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, and for targets outside cities, fatalities from exposure to fallout can vary by more than a factor of 100, depending on population distribution and which way the wind blows. National leaders can attempt to minimize casualties by choosing time of day and timing attacks for favorable forecasted wind patterns, but the predictability of weather is limited and there may be constraints on the ability to wait until forecasts are favorable.
Credible empirical equations are available for estimating depth of penetration and resulting EPW axial deceleration. Maximum depths of penetration are estimated to be between 7 meters for medium-strength rock to 70 meters for silty clay.
The greatest uncertainty regarding EPW survival concerns the heterogeneous nature of target geologies. Rugged EPW designs will enhance EPW survivability. A depth of penetration of about 3 meters achieves most of the benefits of effective energy coupling, and limiting detonation to that depth avoids the uncertainties associated with geologies below that depth.
Collateral damage from a nuclear earth-penetrator weapon cannot be avoided entirely, but it could potentially be reduced by new design concepts combining deeper-penetration, lower-yield, and low-fission-fraction nuclear design for reduced radioactivity. To achieve such reductions, innovative concepts must be developed for achieving combinations of penetration depth and yield combinations that would substantially diminish the radioactive fallout from a nuclear earth-penetrator weapon attack on an HDBT. Use of such a weapon with reduced yield requires more precise and reliable intelligence and greater delivery accuracy than for an above-surface nuclear burst of comparable military effectiveness.
Chemical and Biological Facilities
To destroy chemical or biological agents in an attack, a weapon must detonate essentially within the chamber in which the agent is stored. In general it must detonate in flight, having penetrated through the protective cover of the underground storage. Similarly, if the agents are not in a single room, but are in adjacent tunnels, no more than a region of a single tunnel could be irradiated by a single nuclear explosion.
If chemical or biological agents are released in an EPW attack, there are several ways to reduce casualties and fatalities. Protective clothing and masks can protect people in the high-hazard areas. Those regions can also be treated chemically to decontaminate them. Chemical agents can be rendered inert by exposure to sunlight, heat, or rain, and neutralizing injections exist in some cases. All exposed biological agents are eventually destroyed by ultraviolet exposure, and the effects of many are preventable by vaccines or are treatable by other medical countermeasures. However, the level of “cleanliness” required by military and public health officials for occupancy of a contaminated area is still under substantial discussion and debate.
Given the same target, using conventional rather than nuclear weapons to destroy a chemical storage facility (surface or buried) most likely will cause fewer casualties in either a populated or an unpopulated area, even if there is a potential for release of the chemicals. The same is not necessarily true for a biological storage facility.
The codes and models used to estimate environmental and health effects were designed for Cold War scenarios, have many limitations, and are often ill-suited for today’s national security environment. In particular, the mind-set in designing tools was often based on particular warfighting modes, involving massive nuclear exchanges or tactical encounters. Modern tools need to emphasize the effects of single releases of weapons of mass destruction in a variety of urban or rural environments characterized by detailed meteorology and terrain.
Documentation of the widely used codes and models is sparse, and so users often do not understand the assumptions underlying the calculations. In particular, the range of applicability and of uncertainty is often left to the user’s imagination.
In the calculations for this study, the probability of finding, identifying, and characterizing the target; weapons system survival and arrival at the target; and weapon penetration and detonation are assumed to be 1.0. These assumptions are recognized as unrealistic. In addition, many cautions are needed regarding conclusions based on the model runs.
The model runs show the following:
For urban targets, the use of a nuclear EPW is calculated to reduce casualties by a factor of ~2 to 10 relative to an aboveground nuclear burst whose yield is 25 times larger than that of the EPW. Estimates of this factor can vary by up to 4 to 8, depending on assumptions.
For rural targets, the use of a nuclear EPW is estimated to reduce casualties by a factor of 10 to 100 relative to an aboveground nuclear burst, with the variability in the modeling results extending into the 102 range.
At least four key sensitivities affect estimates of military effectiveness and casualties associated with use of a nuclear EPW:
Target location, especially urban versus rural, as illustrated above;
Accuracy of weapons delivery (CEP) and precise knowledge of target location and structure, as military effectiveness depends closely on a combination of CEP and yield;
Functionality on penetration, especially as influenced by target heterogeneity and its uncertainty (e.g., local geology, or complex structures in urban areas); and
Estimates of the source, transport, and influence on populations of the effects of a nuclear explosion; the estimates are highly variable (by factors of up to ~101 to 103, depending on assumptions).
Current analytical tools have an overall propagated uncertainty of about one order of magnitude (factor of 10) in the estimated casualties. This conclusion is based 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.