Cobalt bomb
A cobalt bomb is a type of "salted bomb": a nuclear weapon designed to produce enhanced amounts of radioactive fallout, intended to contaminate a large area with radioactive material, potentially for the purpose of radiological warfare, mutual assured destruction or as doomsday devices. There is no firm evidence that such a device has ever been built or tested.
For cancer radiation treatments delivered from a device with a cobalt-60 isotope source, see cobalt therapy.History[edit]
The concept of a cobalt bomb was originally described in a radio program by physicist Leó Szilárd on February 26, 1950.[1] 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.[2][3]
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.[4] In Russia, the triple "taiga" nuclear salvo test, as part of the preliminary March 1971 Pechora–Kama Canal project, produced relatively high amounts of cobalt-60 (60Co or Co-60) from the steel that surrounded the Taiga devices, with this fusion-generated neutron activation product being responsible for about half of the gamma dose in 2011 at the test site. The high percentage contribution is largely because the devices primarily used fusion rather than fission reactions, so the quantity of gamma-emitting caesium-137 fallout was comparatively low. Photosynthesizing vegetation exists all around the lake that was formed.[5][6]
In 2015, a page from an apparent Russian nuclear torpedo design was leaked. The design was titled "Oceanic Multipurpose System Status-6", later given the official name Poseidon.[7][8][9][10] The document stated the torpedo would create "wide areas of radioactive contamination, rendering them unusable for military, economic or other activity for a long time." Its payload would be "many tens of megatons in yield". Russian government newspaper Rossiiskaya Gazeta speculated that the warhead would be a cobalt bomb. It is not known whether the Status-6 is a real project, or whether it is Russian disinformation.[11][12] In 2018 the Pentagon's annual Nuclear Posture Review stated Russia is developing a system called the "Status-6 Oceanic Multipurpose System". If Status-6 does exist, it is not publicly known whether the leaked 2015 design is accurate, nor whether the 2015 claim that the torpedo might be a cobalt bomb is genuine.[12] Amongst other comments on it, Edward Moore Geist wrote a paper in which he says that "Russian decision makers would have little confidence that these areas would be in the intended locations"[13] and Russian military experts are cited as saying that "robotic torpedoes could have other purposes, such as delivering deep-sea equipment or installing surveillance devices."[11]
Fallout from cobalt bombs in comparison to other nuclear weapons[edit]
Fission products are more deadly than neutron-activated cobalt in the first few weeks following detonation. After one to six months, the fission products from even a large-yield thermonuclear weapon decay to levels tolerable by humans. The large-yield two-stage (a fission trigger/primary with a fusion–fission secondary) thermonuclear weapon is thus automatically a weapon of radiological warfare, but its fallout decays much more rapidly than that of a cobalt bomb. A cobalt bomb's fallout on the other hand would render affected areas effectively stuck in this interim state for decades: habitable, but not safe for constant habitation.
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 product fallout radiation levels drop 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.[14]
Complete 100% conversion into Co-60 is unlikely; a 1957 British experiment at Maralinga showed that Co-59's neutron absorption ability was much lower than predicted, resulting in a very limited formation of Co-60 isotope in practice.
In addition, fallout is not deposited evenly throughout the path downwind from a detonation, so some areas would be relatively unaffected by fallout and the Earth would not be universally rendered lifeless by a cobalt bomb.[15] 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.[16] However, nuclear devices exploded at high altitudes result in much more widespread but slower fallout, especially for dirty or cobalt-like weapons. The radioactive isotopes are caught in the natural global meteorological processes which, because of the extraordinary hardiness of the isotope, will cycle many times throughout the condensation and evaporation process, resulting in global spread and the effective destruction of usable water for plants, land animals, humans, and sea life.
For the type of radiation given by a cobalt bomb, the dosage measured in sievert (Sv) and gray (Gy) can be treated as equivalent. This is because the relevant harmful radiation from cobalt-60 is gamma rays. When converting between sievert and gray for gamma rays, the radiation type weighting factor will be 1, and the radiation will be a highly penetrating radiation spread evenly over the body so the tissue type weighting factor will also be 1.
Assume a cobalt bomb deposits intense fallout causing a dose rate of 10 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[17]). People in well-built shelters would be safe due to radiation shielding.