Nuclear Friday: Salting the Fields

Last week I wrote a bit about radiological terrorism and dismissed it as not very dangerous. But there have been have been proposed nuclear weapons that used radiological contamination as an intentional means of killing, and that could render the areas attacked unusable for decades.

This is similar in effect to the story of the Romans pouring salt into the fields around Carthage so that the city would be difficult to rebuild. Many historians think that didn’t really happen, that it was a story to ease the concerns of war-weary Romans, to assure them that Carthage would never attack again. But whether the story is true or not, it gave rise to the idea of a “salted bomb”, a nuclear weapon that could render an area uninhabitable for years or even decades.

The Colbalt Bomb:

This is the best known and most investigated form of salted bomb. Leo Szilard, a former Manhattan Project physicist, proposed that such a device could be created in 1950. He didn’t really want to build such a device, he was using the cobalt bomb as an example of how runaway nuclear weapons research could lead to weapons that could end human civilization. Most other scientists dismissed the idea that a cobalt bomb would really be that  dangerous. They were right. Adding a cobalt to a fission bomb of the types available in 1950 wouldn’t really add much to the effects of an airburst as the cobalt would be dispersed too thinly. Still, the idea of the cobalt bomb entered the public imagination even if it wasn’t really possible to make one.

But in just a few short years it would be possible to make a serious cobalt bomb. The first hydrogen bombs came along in less than four years. The most common H-bomb design of that era used a small fission primary to start a much more energetic reaction in the fusion secondary. The secondary needed to be made out of something very strong that absorbed neutrons. Teller and Ulam came up with the idea of using uranium 238 as the outer part of the fusion chamber. This was a brilliant idea because U-238 is a waste product of a nuclear weapons infrastructure, so useless by itself that it’s sometimes called depleted uranium. It is not (very) radioactive, and will undergo fission only with great difficulty. But as the jacket of a fusion secondary U-238 will be overloaded with so many neutrons that it have plenty of fission reactions and add to the explosive force of the bomb.

This type of bomb construction made enough neutrons for a truly dangerous cobalt bomb to be possible. If the jacket were made out of cobalt 59, a non-radioactive isotope of cobalt, or if Co-59 were alloyed with U-238, the cobalt would absorb neutrons and become cobalt 60, one of the most deadly radioisotopes. Co-60 turns into nickel by beta decay accompanied by strong gamma ray emission. Using cobalt the jacket would reduce the blast of the bomb because there would be no or less fission in the secondary, but you might want the blast to be less. Bigger blasts suck the fallout into the stem of the mushroom cloud and spread the fallout to lower concentrations. Cobalt bombs need high concentrations of fallout.

Studies of cobalt bombs showed that affected areas could be rendered uninhabitable for 50-75 years. A low altitude blast from an inefficient early 60’s bomb might render an area uninhabitable for 6-9 months by comparison. And remember, cobalt is a biologically active metal. It’s a micronutrient used as a coenzyme by all life on land. So even after fifty years when there’s not much Co-60 around, crops and livestock could have problems, and there would be health effects for humans.

No one ever built a cobalt bomb. The UK did put a small amount of cobalt in one of their test devices to see if cobalt bombs really would be as bad as they seemed to be, and they were.

The Doomsday Device:

A hypothetical set of large cobalt bombs first explored by Herman Kahn in his 1960 book On Thermonuclear War. By 1960 it had become clear that hydrogen bombs could be scaled up to any size you want simply by adding more stages. Thus a cobalt jacketed bomb could be made so large that it would not matter where it was detonated or how the winds blew. Such bombs  could spread so much fallout that they could kill so many by direct radiation and kill so many more by ecological degradation that humanity would likely go extinct.

Kahn did not want such a thing to be built. He was using the doomsday device as a reductio ad absurdum to argue for stronger conventional forces so that nations would be less likely to rely on nuclear weapons, and also to encourage the US to focus on a “counterforce” nuclear strategy. And US nuclear strategy did indeed move in a counterforce direction starting in the late sixties.

Even so, both the US and Soviet Union investigated doomsday devices. The Soviets had a few plans for jacketed bombs almost the size of the ships they would be carried in. In the US, Teller made proposals for GNOMON and SUNDIAL, multi-stage bombs buried underground with 1 and 10 gigaton yields. No jackets required. With this kind of Earth shattering kaboom neutrons absorbed into the natural surrounding materials will be sufficient.

No one ever really considered building such a device for reasons that are all too obvious.


I haven’t seen this movie and I don’t really like James Bond movies, but salted bombs came up in both the book and movie. Auric Goldfinger has been stockpiling gold and plans to make his gold much more valuable by turning the gold in Fort Knox radioactive for 57 years with a cobalt-iodine nuclear device.  Goldfinger would then control the world economy as now he would have the largest gold reserve.

This is somewhat based in fact. There was some speculation that a small fusion bomb with a  gold jacket could spread around strongly radioactive gold-198. This would be very good for area denial, but only for a few weeks, not years.

And there’s a big difference between contaminating gold and making it radioactive. A cobalt device could make anyone want to stay away from Fort Knox for 57 years, but it wouldn’t make the gold radioactive. And any practical cobalt bomb would simply flatten the whole area as well as making it radioactive.

The only way to make the gold radioactive would be to get it in the flux of a neutron bomb. But the low-end yield of a neutron bomb is 1 kiloton. That will blow apart Fort Knox and only make the closest layers of gold radioactive.

Bonus fun (sad?)  link:

Here’s a cool comic from 1954 set in a post-salted bomb era. I figure humans have it coming most of the time, but sometimes robots make me cry.





2 thoughts on “Nuclear Friday: Salting the Fields

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