Lessons Learned - Nuclear Devices & Nuclear Threats

On 16 July 1945, the scientists assigned to the Manhattan Project culminated years of work when they detonated a plutonium weapon in the New Mexico desert and ushered the world into the age of atomic weapons. The implosion weapon used during the “Trinity Test” was one of two designs developed during the project: (a) an implosion weapon with plutonium in its core; and (b) a gun-type weapon that used highly enriched uranium (HEU).

Nuclear Devices – Fissile Material & Design During the Manhattan Project, the implosion weapon was more challenging to design as it required that a conventional explosive force uniformly and rapidly crush or implode a plutonium core to create a supercritical mass when triggered. Conversely, the design of the gun-type weapon was more intuitive; with two subcritical pieces of HEU on either end of a tube or gun barrel, one piece would explosively shoot down the tube and collide with the other to create a nuclear detonation. Although the gun-type design was inefficient and largely replaced by an implosion design using HEU, it still remains an option for creating a crude, inefficient, yet functional, improvised device for a terrorist group with access to a sufficient amount of HEU.

The most challenging element of the Manhattan Project was the production of the fissile material – HEU and plutonium – for the atomic weapon core. Enriching uranium involves processing natural uranium to separate uranium-235 (U-235) from uranium-238 (U-238). Natural uranium is approximately one atom of U-235 for every 139 atoms of U-238. Enriching uranium to weapons grade requires that the material be approximately 90 percent U-235 and, as such, is a large-scale industrial process that requires a variety of equipment and multiple production phases.

Plutonium, though, does not exist in nature. It is manmade in a nuclear reactor by bombarding uranium with neutron particles, then extracting the plutonium and separating the other materials. This process also is industrial in scale and difficult to undertake. Obtaining special nuclear materials in the quantities needed to make an atomic weapon is an impediment to building a nuclear device, but the physics behind making a crude device for someone who possesses the necessary materials is not as complicated as many would think.

The United States designed these weapons with 1940s technology. It is very unlikely that a terrorist group could produce special nuclear material on their own; however, very limited quantities of these materials have been available on the black market. As nuclear weapons proliferate in more countries, especially those with less-stable governments and those that have supported terrorist activities in the past, the likelihood that fissile materials may fall into the hands of a terrorist group may increase.

Modeling the Nuclear Threat The probability of nuclear terrorism in a U.S. city may still be lower than threats by other attack methods, but the potential consequences merit serious preparations. Modeling efforts conducted by the federal government have resulted in detailed guidance regarding what a ground-level detonation of a crude nuclear device might look like in a modern U.S. city. These results depict a much different scenario than what would have occurred had the former Soviet Union launched an attack against the United States during the Cold War. The weapons that would have been launched by the Soviets would likely have been sophisticated thermonuclear devices with yields measured in megatons (millions of tons of TNT), rather than a crude atomic weapon with a yield measured in kilotons. Cold War weapons would likely have detonated at an altitude referred to as the optimal height of the blast – that is, where the blast effects would cause the greatest amount of damage to a given target.

Modeling efforts had to derive this information because there have been few nuclear tests at ground level and none within the confines of a modern city with steel and concrete buildings. Sturdy, well-constructed buildings would mitigate many of the immediate effects of the detonation, but the proximity to the ground would result in much higher levels of dangerous fallout than if the same device were detonated in the air. Although these factors would minimize the immediate effects of an improvised nuclear attack compared to a Cold War-type attack, the casualties would still be much greater than any previous single event in U.S. history.

These new modeling efforts helped a federal interagency committee led by the executive office of the president create a 2010 document, entitled “Planning Guidance for Response to a Nuclear Detonation,” which clearly explains what the aftermath of an improvised nuclear detonation might look like. This document provides a logical way to understand the consequences and to assist in building a proper response strategy. Federal planning guidance assumes that terrorists most likely would use a low-yield, 10-kiloton nuclear device. This is consistent with the Federal Emergency Management Agency’s (FEMA) National Planning Scenario One: A 10-kiloton improvised nuclear device detonated within a major metropolitan city.

One of the overarching concepts in the planning guidance document is the need toentify three distinct damage zones: the severe damage zone, the moderate damage zone, and the light damage zone. Modeling can provide only general guidance as to the geographic dimensions of these zones because no real-world testing is available. Authorities would only be able toentify the damage zones after an incident by conducting a visual inspection of the overall infrastructure damage.

The temporary blindness caused by the bright flash of light after the detonation would likely result in widespread automobile accidents, plane crashes, and other nonblast-related casualties across a wide area. There also would be a release of a strong electromagnetic pulse (EMP), which has the potential to destroy or damage electronics in the immediate area of the blast and hamper the ability to communicate. The effect of an EMP during a ground-level detonation likely would be much less severe than it would be during a high-level, above-ground detonation, but this effect may complicate communications into and out of the affected area.

The vaporized materials combined with the radioactive particles created during the nuclear detonation would travel high into the atmosphere in the resulting mushroom cloud. These materials will travel into the upper atmosphere, cool, solidify, and eventually fall back to earth – the “dangerous fallout zone” – resulting in radioactive fallout particles similar in size to grains of salt or sand. Ground-level winds are not an accurate predictor of the winds in the upper atmosphere, which can vary substantially in direction and speed from those on the ground. Modeling in U.S. cities using actual weather conditions at different times of the year has revealed various patterns and directions of travel for fallout, making advanced predictions difficult. Standard plume modeling software used to track a chemical release would not accurately predict the deposition of the fallout.

The dangerous fallout zone is not mutually exclusive to any of the damage zones, but rather overlaps the three damage zones to some degree. Dangerous fallout may extend miles beyond the light damage zone and less dangerous fallout may travel long distances. Highly radioactive particles would begin to descend within about 15 minutes of the detonation; so prompt protective actions may be necessary to avoid receiving a lethal dose of radioactivity. Fortunately, the radioactivity in these particles would decay rapidly – more than 50 percent of the radiation from these particles would be released in just the first hour – and the exposure rates would fall dramatically over a relatively short period of time. Therefore, minimizing exposure to fallout particles until assessments are complete is critical to survival in areas that may contain unsafe levels of radiation.

The Best Defense – Planning & Sheltering Unlike during the Cold War, fallout shelters are generally not pre-identified. There likely would be no advance warning for the public to shelter, rather they would initially be on their own to determine that a nuclear detonation had occurred. For those close to the detonation, this determination could be difficult, as clouds of dust and debris would obscure visibility. Resisting the urge to simply flee the area would be challenging, but remaining outdoors in the dangerous fallout zone could be immediately dangerous to life and health.

Developing response plans and educating responders as well as the public in advance about the aftermath of a nuclear detonation are critical to saving lives. Survival rates may be low for those caught near the detonation site, but proper response and preplanning could potentially save thousands of other lives in the immediate aftermath. A public education plan and prescripted public information messages, developed well in advance of an attack, could help protect the public from the unseen dangers caused by exposure to radioactive fallout.

Since it will take time toentify, map, and communicate the dangerous fallout zone, survivors must know how and where to seek adequate temporary shelter. The best shielding generally is in the cores of well-constructed buildings or below ground. Responders should quickly analyze and assess the radioactive conditions to determine whether an area is safe to work in or it is safer to continue sheltering. Since radiation is undetectable without proper instrumentation, adequate equipment that can detect and display elevated levels of radioactivity should be readily accessible across and around a city for rapid assessment before any incident – much like the fallout shelters stocked with instrumentation during the Cold War era.

Although the probability of a nuclear detonation occurring in a U.S. city is lower than other types of terrorist attacks, the dire and unique consequences from this type of incident make advance planning critical to a comprehensive all hazards preparation strategy. Should a nuclear incident occur, immediate actions must be taken at the state and local levels to save lives until federal assets can respond to the affected area. Even when federal agencies immediately mobilize resources, the state and local agencies must be prepared to take appropriate response actions to avoid unnecessary deaths.