The hell bomb

by William L. Laurence

At this point let us hold our breath and get set to believe what at first glance may appear to be unbelievable. The time it takes to split these two billion trillion atoms is no more than one millionth of a second (one microsecond). If we keep this time element in mind we can arrive at a clear understanding of the tremendous problem involved in exploding an A- or an H-bomb.

And while we are recovering from the first shock we may as well get set for another. That unimaginable figure of two billion trillion atoms represents the splitting (explosion) of no more than one gram (1/28th of an ounce) of U-235, or plutonium.

Now, the energy released in the splitting of one gram of U-235 is equivalent in power to the explosive force of 20 tons of TNT, or two old-fashioned blockbusters. Since we know from President Truman’s announcement following the bombing of Hiroshima that the wartime A-bomb “had more power than 20,000 tons of TNT,” it means that the atoms in an entire kilogram (1,000 grams) of U-235 or plutonium must have been split. In other words, after the A-bomb had reached a power of 20 tons of TNT, it had to be kept together long enough to increase its power a thousandfold to 20,000 tons. This, as we have seen, requires only ten more steps. It can also be seen that it is these ten final crucial steps that make all the difference between a bomb equal to only two blockbusters, which would have been a most miserable two-billion-dollar fiasco, and an atomic bomb equal in power to two thousand blockbusters.

With the aid of these facts we are at last in a position to grasp the enormousness of the problem that confronted our A-bomb designers at Los Alamos and is confronting them again today. It can be seen that for a bomb to multiply itself from 20 to 20,000 tons in ten steps by doubling its power at every step, it has to pass successively the stages of 40, 80, 160, 320, and so on, until it reaches an explosive power of 2,500 tons at the seventh step. Yet it still has to be held together for three more steps, during which it reaches the enormous power of 5,000 and 10,000 tons of TNT, without exploding.

Here was an irresistible force, and the problem was to surround it with an immovable body, or at least a body that would remain immovable long enough for the chain reaction to take just ten additional steps following the first seventy. There is only one fact of nature that makes this possible, or even thinkable—the last ten steps from 20 to 20,000 tons take only one tenth of a millionth of a second. The problem thus was to find a body that would remain immovable against an irresistible force for no longer than one tenth of a microsecond, 100 billionths of a second.

This immovable body is known technically as a “tamper,” which pits inertia against an irresistible force that builds up in 100 billionths of a second from an explosive power of 20 tons of TNT to 20,000 tons. The very inertia of the tamper delays the expansion of the active substance and makes for a longer-lasting, more energetic, and more efficient explosion. The tamper, which also serves as a reflector of neutrons, must be a material of very high density. Since gold has the fifth highest density of all the elements (next only to osmium, iridium, platinum, and rhenium), at one time the use of part of our huge gold hoard at Fort Knox was seriously considered.

With these facts and figures in mind, it becomes clear that an H-bomb made of deuterium alone is not feasible. It is certainly out of the question with an A-bomb of the Hiroshima or Nagasaki types, which generate a temperature of about 50,000,000 degrees, since, as we have seen, it would take fully 200 microseconds to ignite it at that temperature. It is one thing to devise a tamper that would hold back a force of 20 tons for 100 billionths of a second, and thus allow it to build up to 20,000 tons. It is quite another matter to devise an immovable body that would hold back an irresistible force of 20,000 tons for a time interval 2,000 times larger, particularly if one remembers that in another tenth of a microsecond the irresistible force would increase again by 1,000 to 20,000,000 tons. Obviously this is impossible, for if it were possible we would have a superbomb without any need for hydrogen of any kind.

It is known that we have developed a much more efficient A-bomb, which, as Senator Edwin C. Johnson of Colorado has inadvertently blurted out, “has six times the effectiveness of the bomb that was dropped over Nagasaki.” We are further informed by Dr. Bacher that “significant improvements” in atomic bombs since the war “have resulted in more powerful bombs and in a more efficient use of the valuable fissionable material.” It is conceivable and even probable that the improvements, among other things, include better tampers that delay the new A-bombs long enough to fission two, four, or even eight times as many atoms as in the wartime models. But since, as we have seen, the ten steps of the final stages require only an average of 10 billionths of a second per step, increasing the power of the new models even to 160,000 tons (eight times the power of the Hiroshima type) would take only three steps, in an elapsed time of no more than 30 billionths of a second. And even if we assume that the improved bomb generates a temperature of 200,000,000 degrees, it would still be too cold to ignite the deuterium during the interval of its brief existence, since, as we have seen, it would take 4.8 microseconds to ignite it at that temperature. In fact, calculations indicate that it would require a temperature in the neighborhood of 400,000,000 degrees to ignite deuterium in the time interval during which the assembly of the improved A-bomb appears to be held together, which, as may be surmised from the known data, is within the range of 1.2 microseconds.

From all this it may be concluded with practical certainty that an H-bomb of deuterium only is out of the question. Equally good, though entirely different, reasons also rule out an H-bomb using only tritium as its explosive element.

There are several important reasons why an H-bomb made of tritium alone is not feasible. The most important by far, which alone excludes it from any serious consideration, is the staggering cost we would have to pay in terms of priceless A-bomb material, as each kilogram of tritium produced would exact the sacrifice of eighty times that amount in plutonium. The reason for this is simple. Both plutonium and tritium have to be created with the neutrons released in the splitting of U-235, each atom of plutonium and each atom of tritium made requiring one neutron. Since an atom of plutonium has a weight of 239 atomic mass units, whereas an atom of tritium has an atomic weight of only three, it can be seen that a kilogram, or any given weight, of tritium would contain eighty times as many atoms as a corresponding weight of plutonium, and hence would require eighty times as many neutrons to produce. In other words, we would be buying each kilogram of tritium at a sacrifice of eighty kilograms of plutonium, which, of course, would mean a considerable reduction in our potential stockpile of plutonium bombs.