|Fig. 1: An industrial sized 25-ton blast door for a shelter. (Source: Wikimedia Commons)|
Mutual assured destruction (MAD), the use full-scale use of nuclear weapons by two or many nations at war, was a common fear amongst civilians, politicians, and combatants at both sides of the Cold War. The Cold War also caused a rise in the fallout shelter market.  During the Cold War era, individuals had the opportunity to invest in a fallout shelters ranging from prices of $500 to $2500, meet their needs, and offer protection.  In order to determine which fallout shelters are suitable for protection, several factors have to be taken into consideration when constructing or purchasing a fallout shelter. Factors such as the effects and aftermath of nuclear weapons must be understood prior to designing fallout shelter components (i.e. entrances, exits, ventilation, closures) that offer several forms of protection (i.e. overpressure, radiation).  This paper will define criteria that fallout shelters aim to protect against, and elucidate this by analyzing fallout shelters marketed and tested during the Cold War era.
Nuclear weapons have been used twice in warfare history, and their effects on their targets have been extensively studied. These weapons generate a destructive blast, accompanied with thermal pulses of intense light and heat radiation, from a series of nuclear reactions within the bomb used. These blasts usually produce 50- psi overpressures accompanied with 1000-mph winds that are capable of destroying buildings, and are capable of being amplified if the blast wave were to strike a "re-entrant corner."  If the blast were to occur in the air and not touch the ground, it would not produce a crater, but produce small airborne radioactive particles that remain in the atmosphere for days to years until rain or snow dissolves and brings it to the earth's surface. On the other hand, near-surface and surfaces blasts which are similar to air blast produce and an abundance of radioactive dust from pulverized material from the formed crater which lofts the atmosphere before settling on its own. As a result, fallout shelters must be designed to withstand the initial shocks of air, as well as prevent the high speed wind- borne debris from entering the shelter through their entrances. An example of a tough 25-ton blast door capable of withstanding high over pressures ands blasts is seen to the right in Fig 1.
Aside from wind, the shelter must be capable of protecting against the initial neutron and gamma radiation emitted, especially for weapons that have overpressures above 30 psi and are in the range of ten to a few hundred kilotons. This is a detrimental factor for construction since it determines the roof and entryway thickness of fallout shelters. Furthermore, shelters must also be designed to contain "rattle space" to prevent seismic shock waves produced from the blast to damage the shelter.  Lastly, these shelters must be designed to withstand and prevent damage or fires caused by increasing temperatures, as well as carefully designed to prevent rubble buildup near entrances.
|Fig. 2: A demonstration of penetration power for α, β, and γ radiation. (Source: Wikimedia Commons)|
As previously mentioned, radioactive dust comes in two forms that "fallout" from the sky according to their sizes. The smallest fallout particles are very microscopic and tend to loft in the atmosphere for weeks to years before reaching the ground by rain or snow; fortunately, these particles have harmless levels of radiation upon arrival. On the other hand, larger fallout particles that settle within the first few hours to first two days are more dangerous that fallout shelters are required to ensure protection. Fallout can further be classified as alpha and beta particles, which classifies the radiation they emitted.  Alpha particles are identical to nuclei of helium atom and are given off by fallout debris; alpha particles have poor penetrating power and can be halted by 1 to 3 inches of air.  On the other hand, beta particles are high-speed electrons given off by radioactive fallout that can penetrate approximately 10 feet of air or about 1/8th inch of water, wood, or human tissue. The penetration powers for these types of radiation can be seen in figure 2 on the left. As a result, caution must be taken with beta particles in order to prevent radiation burns. In order for a fallout shelter to provide effective protection against radiation a method of quantifying radiation must be used.
The amount of radiation received from exposure to gamma rays and x rays is quantified by the units of Roentgen (R), which is used in dosimeters to measure the amount of radiation received. On the other hand, dose rate meters are used to measure the dose rate by recording the instantaneous amount of Roentgen being received per hour. A high-fallout area would have a dose rate near 1000 R/hr, 488 R/hr , and 100 R/hr after 1 hr, 2hr, and 7 hrs respectively after explosion.  It would take about 2 weeks or less for a 1000 R/hr dose rate (1 hr after explosion) to be decay to 1 R/hr. 
In fallout shelters, the primary radiation of concern is the previously mentioned initial gamma radiation (i.e. highly penetrating nitrogen capture gamma rays, fission product gamma rays, and neutrons) and not the beta components of fallout radiation.  Effective shielding material has a high surface density to intervene between the source of radiation and the object receiving radiation and thus reduce fallout radiation received.  A method of measuring gamma-ray attenuation is through "tenth-value thickness" and the "protection factor" (PF) concepts. The tenth-value thickness is the thickness of a material that is required to reduce the radiation transmitted through the material by a factor (referred to as the PF) of 10.  For gamma rays, the tenth-value thickness for lead, concrete, and packed earth, are 0.4, 2.5, and 3.6 inches respectively.  The tenth-value thickness can also be applied to surface density as well, in order to quantify the amount of weight of material needed. By applying these concepts to materials, it can be concluded that materials of higher atomic number (i.e. lead) or density offer the best protection for radiation, and that the quantity of shielding material can be adjusted to prevent gamma ray penetration.
|Fig. 3: Operation Plumbbob's Mushroom Cloud (Source: Wikimedia Commons)|
All of the previously mentioned factors, and additional (i.e. ventilation, food rations, cost, generators, etc.) not mentioned in this paper, were taken into consideration by the Office of Civil and Defense Mobilization pamphlet on instructions on how to construct their own "Family Fallout Shelter."  These fallout shelters can be constructed onto the basements of new constructions for $250 to $500 per space.  These basement shelters are underground to reduce radiation transmission, and employ concrete framework for support and improving hardness to above 10 psi. Packages such as the FCDA Family Shelter Mark I were tested and marketed of withstanding blasts up to 65 psi, which is greater than the average overpressure previously mentioned of 50 psi. More elaborate packages, such as those tested in Operation Plumbbob in Nevada, consisted of concrete arch structure with 16-ft spans and 8 inch wall thicknesses endured being subjected to overpressures produced by a 36-kT tower shot with overpressures between 50-200 psi.  The mushroom cloud of the 36-kT shot can be seen in figure 3. If the previously defined values of tenth-value thicknesses are applied to these arch structures, the walls would have a total PF of 1000. Within the same project, 7-ft-diam, 10-gauge, galvanized, multi-plate corrugated culvert buried under 5-10 feet of soil (1016 to 10166 PF) managed to survive overpressures greater than 245 psi with no deformation and negligible radiation (from a 100,000 R gamma neutron dose exposure) were recorded.  These structures prove to show that careful building consideration can provide protection to a nuclear bomb's blast and radiation. However, the factor that heavily influences survival is the individual within the fallout shelter.
These shelters help people survive the initial effects of nuclear blasts, but the survival of people within the shelter is heavily dependent on themselves before, during, and after a nuclear explosion. Before the blast, these individuals must decide whether they'd like to invest in a project to help secure themselves and their family, and how much to invest as well.  During the blast or fallout, the individuals within the fallout must be cautious of coming out at an appropriate time when radiation levels are low. After leaving the shelter, these survivors must also be cognizant of radiation around them by measuring their surrounds, and protecting themselves against residual beta radiation. By doing individuals can prevent receiving a lethal dose of 450 R, and survive with a tolerable radiation dose of no more than 100 R at 6 R per day. Aside from radiation impacting survival, psychological behavior and fear of radiation after a nuclear blast can impact an individual or society's survival.  A survivor must be able to cope with stress and put effort in reestablishing food production and production of vital necessities if they're in a nuclear holocaust, or in escaping the blast zone to areas of low radiation if their surrounding area wasn't impacted. By applying the previously mentioned mentality, survivors can endure the nuclear blast and ensure their survival.
© Joel Dominguez. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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