A cobalt bomb is a theoretical type of "salted bomb": a nuclear weapon designed to produce enhanced amounts of radioactive fallout, intended to contaminate a large area with radioactive material. The concept of a cobalt bomb was originally described in a radio program by physicist Leó Szilárd on February 26, 1950. His intent was not to propose that such a weapon be built, but to show that nuclear weapon technology would soon reach the point where it could end human life on Earth, a doomsday device. Such "salted" weapons were requested by the U.S. Air Force and seriously investigated, but not deployed. In the 1964 edition of the DOD/AEC book The Effects of Nuclear Weapons, a new section titled Radiological warfare clarified the "Doomsday device" issue.
As far as is publicly known, no cobalt bombs have ever been built. The Operation Antler/Round 1 test by the British at the Tadje site in the Maralinga range in Australia on September 14, 1957 tested a bomb using cobalt pellets as a radiochemical tracer for estimating yield. This was considered a failure and the experiment was not repeated. Furthermore the triple "taiga" nuclear salvo test, as part of the preliminary March 1971 Pechora–Kama Canal project, produced substantial amounts of Co-60, with this fusion generated neutron activation product being responsible for about half of the gamma dose now (2011) at the test site, with photosynthesizing vegetation existing all around the lake that was formed.
A cobalt bomb could be made by placing a quantity of ordinary cobalt metal (59Co) inside a nuclear bomb. When the bomb explodes, the neutrons produced by the explosion would transmute the cobalt to the radioactive isotope cobalt-60 (60Co), which would be vaporized by the explosion. The cobalt would then condense and fall back to Earth with the dust and debris from the explosion, contaminating the ground.
The deposited cobalt-60 would have a half-life of 5.27 years, decaying into 60Ni. The nickel nucleus is activated, and emits two gamma rays with energies of 1.17 and 1.33 MeV, hence the overall nuclear equation of the reaction is:
27Co + n → 60
27Co → 60
28Ni + e− + gamma rays.
Nickel-60 is a stable isotope and undergoes no further decays after emitting the gamma rays.
The 5.27 year half life of the 60Co is long enough to allow it to settle out before significant decay has occurred, and to render it impractical to wait in shelters for it to decay, yet short enough that intense radiation is produced. Many isotopes are more radioactive (gold-198, tantalum-182, zinc-65, sodium-24, and many more), but they would decay faster, possibly allowing some population to survive in shelters.
In a fission bomb, it has been suggested, the weapon's tamper could be made of cobalt. In a fusion bomb the radiation case around the weapon, normally made of 238U, could be made of cobalt. These changes would reduce the explosive power (yield) of the weapon somewhat.
Fallout from cobalt bombs vs other nuclear weapons
Fission products are more deadly than neutron-activated cobalt in the first few months following detonation. After one to six months, the fission products from a thermonuclear weapon decay to levels tolerable by humans. The conventional three-stage thermonuclear weapon is thus automatically a weapon of radiological warfare, but its fallout decays much more rapidly than that of a cobalt bomb. Areas irradiated by fallout from even a large-yield thermonuclear weapon begin to increasingly become habitable again after one to six months; a cobalt bomb's fallout on the other hand would render affected areas effectively stuck in this interim state for decades, of habitable, but not safely so under constant habitation conditions.
Initially, gamma radiation from the fission products of an equivalent size fission-fusion-fission bomb are much more intense than Co-60: 15,000 times more intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1 month; and about equal at 6 months. Thereafter fission drops off rapidly so that Co-60 fallout is 8 times more intense than fission at 1 year and 150 times more intense at 5 years. The very long-lived isotopes produced by fission would overtake the 60Co again after about 75 years.
Another important point in considering the effects of cobalt bombs is that deposition of fallout is not even throughout the path downwind from a detonation, so that there are going to be areas relatively unaffected by fallout and places where there is unusually intense fallout, so that the Earth would not be universally rendered lifeless by a cobalt bomb.  The fallout and devastation following a nuclear detonation does not scale upwards linearly with the explosive yield (equivalent to tons of TNT). As a result, the concept of "overkill" - the idea that one can simply estimate the destruction and fallout created by a thermonuclear weapon of the size postulated by Leo Szilard's "cobalt bomb" thought experiment by extrapolating from the effects of thermonuclear weapons of smaller yields - is fallacious.
Example of radiation levels vs. time
Assume a cobalt bomb deposits intense fallout causing a dose rate of 10 sieverts (Sv) per hour. At this dose rate, any unsheltered person exposed to the fallout would receive a lethal dose in about 30 minutes (assuming a median lethal dose of 5 Sv). People in well-built shelters would be safe due to radiation shielding.
After one half-life of 5.27 years, only half of the cobalt-60 will have decayed, and the dose rate in the affected area would be 5 Sv/hour. At this dose rate, a person exposed to the radiation would receive a lethal dose in 1 hour.
After 10 half-lives (about 53 years), the dose rate would have decayed to around 10 mSv/hour. At this point, a healthy person could spend 1 to 4 days exposed to the fallout with no immediate effects.
After 20 half-lives (about 105 years), the dose rate would have decayed to around 10 μSv/hour. At this stage, humans could remain unsheltered full-time since their yearly radiation dose would be about 80 mSv. However, this yearly dose rate is on the order of 30 times greater than the peacetime exposure rate of 2.5 mSv/year. As a result, the rate of cancer incidence in the survivor population would likely increase.
After 27 half-lives (about 142 years), the dose rate from cobalt-60 would have decayed to less than 1 mSv/year and could be considered negligible.
However it must be kept in mind that the prior section's treatise neglects the effects of remediation by humans and is for illustrative purposes only, in all likely scenarios a clean-up of selected important contaminated areas would be conducted, with the use of lead glass window lined excavators and bulldozers, similar to those employed in the Lake Chagan project. By skimming off the thin layer of fallout on the topsoil surface and burying it in the likes of a deep trench along with isolating it from ground water sources, the gamma air dose is cut by orders of magnitude. The decontamination after the Goiânia accident in Brazil 1987 and the possibility of a "dirty bomb" with Co-60, which has similarities with the environment that one would be faced with after a nuclear yielding cobalt bomb's fallout had settled, has prompted the investigation of "Sequestration Coatings" and cheap liquid phase sorbents for Co-60 that would further aid in decontamination.
The concept of cobalt bombs has been used in a number of works of apocalyptic fiction.
- The 1952 short story by Fritz Lieber, The Moon is Green, describes the catastrophic consequences of a war fought with cobalt bombs.
- Similarly, the 1954 science fiction short story "Exhibit Piece" by Philip K. Dick ends with the newspaper headline "RUSSIA REVEALS COBALT BOMB; TOTAL WORLD DESTRUCTION AHEAD".
- In the 1957 novel On the Beach by Nevil Shute (and the films based on it), the source of a global contamination of radioactive material is the detonation of cobalt bombs in the Northern Hemisphere.
- The 1959 novel Level 7, by Mordecai Roshwald, involves the use of weapons intended to exterminate the populace by permanently contaminating the surface of the earth.
- In the 1960 book On Thermonuclear War, nuclear theorist Herman Kahn mentions cobalt weapons with the implication that they're militarily irrelevant or irresponsible (there's a chapter largely devoted to the drastically destabilizing nature of attempts to use nuclear blackmail with "doomsday machines" such as cobalt bombs or other radiological weapons). Herman Kahn was one of the main influences for Stanley Kubrick's film "Dr. Strangelove, or How I Learned to Relax and Love the Bomb"
- In the 1964 satirical apocalyptic film Dr. Strangelove, the Soviet Union had established a secret nuclear deterrent comprising 50 buried cobalt bombs, more specifically as the "Cobalt Thorium G doomsday machine."
- The mutant human New Yorkers in the 1970 post-apocalyptic film Beneath the Planet of the Apes pray to an 'Alpha-Omega' bomb, which Colonel Taylor explains to Brent is a doomsday weapon with a cobalt casing; Taylor detonates the bomb at the end of the film, after which a narrator states that the planet "is now dead." In the next film of the series Escape from the Planet of the Apes (1971), chimpanzee scientist Zira states she saw "the rim of the earth melt" from space when the bomb went off.
- In the 1968 film Countdown, the astronaut (Chiz) played by Robert Duvall makes reference to a "cobalt bomb" or salted bomb as it then pertained to the cold war tensions between the United States and the former Soviet Union.
- In the 1973 film Battle for the Planet of the Apes, set two millennia earlier than Beneath the Planet of the Apes, Governor Kolp had ordered Méndez to detonate the bomb if he and his troops failed to return from their mission to destroy the ape village; instead, Méndez created a religion around the bomb.
The concept was also used in some other works of fiction at well.
- A cobalt and iodine "atomic device" is supplied by the Chinese Communist government to Auric Goldfinger in the 1964 James Bond film Goldfinger, where he intends to detonate the bomb inside Fort Knox, rendering the USA's gold bullion reserves radioactive for 58 years.
- In Stan Lee's comic book story featuring The Mighty Thor in Journey Into Mystery 86, a mad scientist from the year 2262 named Zarrko goes back in time to the Nevada desert in the year 1962 to steal a cobalt bomb from a military weapons test site.
- Paul Erdman's 1976 novel The Crash of '79 includes Iran using cobalt bombs to attack Middle East oilfields, rendering them "totally inaccessible for at least twenty-five years".
- In the 1970s TV series The Bionic Woman ("Doomsday is Tomorrow" (episodes 13 and 14)), professor Elijah Cooper incorporates a cobalt bomb in a doomsday device in an attempt to blackmail the world into peace.
- In the 2008 series of TV programme Ultimate Force (Series 4, Episode 5), a ″slow bomb″ was stolen and set to detonate in Central London.
- In the 2009 TV series Castle (episodes "Setup" (16) and "Countdown" (17)), a cobalt bomb is built with the intention to destroy New York City.
- In the 1975 Serial, (serial5, series 13) of Doctor Who, "Revenge of the Cybermen", the Doctor, Harry Sullivan and Sarah Jane Smith are coerced, by the cybermen, to carry cobalt bombs to the centre of the planet Voga, The Planet of Gold, so that they could destroy it. In a passing reference, the Doctor claims that the Cybermen and the Daleks both continue to use the devices in spite of their having been outlawed on most planets.
- Brian Clegg. Armageddon Science: The Science of Mass Destruction. St. Martins Griffin. p. 77. ISBN 978-1-250-01649-2.
- Bhushan, K.; G. Katyal (2002). Nuclear, Biological, and Chemical Warfare. India: APH Publishing. pp. 75–77. ISBN 81-7648-312-5.
- Sublette, Carey (July 2007). "Types of nuclear weapons". FAQ. The Nuclear Weapon Archive. Retrieved 2010-02-13.
- Samuel Glasstone, The Effects of Nuclear Weapons, 1962, Revised 1964, U.S. Dept of Defense and U.S. Dept of Energy, pp.464–5. This section was removed from later editions, but, according to Glasstone in 1978, not because it was inaccurate or because the weapons had changed.
- "1.6 Cobalt Bombs and other Salted Bombs". Nuclearweaponarchive.org. Retrieved February 10, 2011.
- Radiological investigations at the “Taiga” nuclear explosion site: Site description and in situ measurements V Ramzaev, V Repin, A Medvedev, E Khramtsov… - Journal of environmental …, 2011 - Elsevier
- Radiological investigations at the “Taiga” nuclear explosion site, part II: man-made γ-ray emitting radionuclides in the ground and the resultant kerma rate in air V Ramzaev, V Repin, A Medvedev, E Khramtsov… - Journal of environmental …, 2012 - Elsevier
- "Nuclear Weapons FAQ: 1.6".
- Samuel Glasstone; Philip J. Dolan, eds. (1977). "The Effects of Nuclear Weapons" (PDF) (3rd ed.). Washington, D.C.: United States Department of Defense and Department of Energy.
- Martin, Brian (December 1982). "The global health effects of nuclear war". Current Affairs Bulletin 59 (7): 14–26.
- Russia Today documentary that visits the lake and shows archival footage.
- "IAEA Joint Division, Questions & Answers. How can we reduce contamination of arable land, once deposition (fallout) of radioactive material has ceased?".
- International Atomic Energy Agency International Atomic Enmergy Agency, 2000 - Technology & Engineering - restoration of environments with radioactive residues : papers and discussions, 697 pages
- Sequestration Coating Performance Requirements for Mitigation of Contamination from a Radiological Dispersion Device- 9067
- Fritz Lieber (1952). "The Moon is Green". Gutenburg Project.