Part 3

is, “How do you get it going?” The heavy hydrogens, deuterium and tritium, are suitable substances if somehow they could be heated hot enough and kept hot. This problem is a little bit like the job of making a fire at 20 degrees below zero in the mountains with green wood which is covered with ice and with very little kindling. Today, scientists tell us that such a fire can probably be kindled.

Once you get the fire going, of course, you can pile on the wood and make a very sizeable conflagration. In the same way with the hydrogen bomb, more heavy hydrogen can be used and a bigger explosion obtained. It has been called an open-ended weapon, meaning that more materials can be added and a bigger explosion obtained.

The phrase that goes to the very heart of the problem is “very little kindling,” which is another way of illustrating the difficulty of lighting a fire in a high wind when you have only one match. We know that to ignite deuterium, by far the cheaper and more abundant of the two H-bomb elements, a temperature comparable to those existing in the interior of the sun, some 20,000,000 degrees centigrade, is necessary. This temperature can be realized on earth only in the explosion of an A-bomb. We also know that the wartime model A-bombs generated a temperature of about 50,000,000 degrees, more than enough to light a deuterium fire. The trouble lies in the extremely short time interval, of the order of a millionth of a second (microsecond), and a fraction thereof, during which the A-bomb is held together before it flies apart. In the words of Professor Bacher, we must make our green, ice-covered wood catch fire in the subzero mountain weather before the “very little kindling” we have is burned up.

The times at which deuterium will ignite at any given temperature, in both its gaseous and its liquid form, are widely known among nuclear scientists everywhere, including Russia, through publication in official scientific literature of a well-known formula, originally worked out by two European scientists as far back as 1929, and more recently improved upon by Professor George Gamow and Professor Teller. By this formula, derived from actual experiments, it is known that deuterium in its gaseous form will require as long as 128 seconds to ignite at a temperature of 50,000,000 degrees centigrade, well above 100,000,000 times longer than the time in which our little kindling is used up. This obviously rules out deuterium in its natural gaseous form as material for an H-bomb.

How about liquid deuterium? We know that the more atoms there are per unit volume (namely, the greater the density), the faster is the time of the reaction. The increase in the speed of the reaction (in this case the ignition of the deuterium) is directly proportional to the square of the density. For example, if the density, (that is, the number of atoms per unit volume) is increased by a factor of 10, the time of ignition will be speeded up by the square of 10, or 100 times faster. Since liquid deuterium has a density nearly 800 times that of gaseous deuterium, this means that liquid deuterium (which must be maintained at a temperature of 423 degrees below zero Fahrenheit at a pressure above one atmosphere) would ignite 640,000 times faster (namely, in 1/640,000th part of the time) than its gaseous form. Arithmetic shows that the ignition time for liquid deuterium at 50,000,000 degrees centigrade will be 200 microseconds, still 200 times longer than the period in which our kindling is consumed.

The same formula also reveals the time it would take liquid deuterium to ignite at higher temperatures, the increase of which shortens the ignition time. These figures show that the ignition time for liquid deuterium at 75,000,000 degrees centigrade is 40 microseconds. At 100,000,000 degrees the time is 30 microseconds; at 150,000,000 degrees, 15 microseconds; and at 200,000,000 degrees on the centigrade scale, about 4.8 millionths of a second. Doubling the temperature speeds up the ignition time for liquid deuterium by a factor of about six.

The problem thus is a dual one: to raise the temperature at which the A-bomb explodes, and to extend the time before the A-bomb flies apart. It is also obvious that if the liquid deuterium is to be ignited at all, it must be done before the bomb has disintegrated—that is, during the incredibly short time interval before it expands into a cloud of vapor and gas, since by then the deuterium would no longer be liquid.

Can we increase the A-bomb’s temperature fourfold to 200,000,000 degrees and literally make time stand still while it holds together for nearly five millionths of a second? To get a better understanding of the problem we must take a closer look at what takes place inside the A-bomb during the infinitesimal interval in which it comes to life.

This life history of the A-bomb is an incredible tale, from the time its inner mechanisms are set in motion until its metamorphosis into a great ball of fire. As explained earlier, the A-bomb’s explosion takes place through a process akin to spontaneous combustion as soon as a certain minimum amount (critical mass) of either one of two fissionable (combustible) elements—uranium 235 or plutonium—is assembled in one unit. The most obvious way it takes place is by bringing together two pieces of uranium 235 (U-235), or plutonium, each less than a critical mass, firing one of these into the other with a gun mechanism, thus creating a critical mass at the last minute. If, for example, the critical mass at which spontaneous combustion takes place is ten kilograms (the actual figure is a top secret), then the firing of a piece of one kilogram into another of nine kilograms would bring together a critical mass that would explode faster than the eye could wink—in fact, some thousands of times faster than TNT.

Just as an ordinary fire needs oxygen, so does an atomic fire require the tremendously powerful atomic particles known as neutrons. Unlike oxygen, however, neutrons do not exist in a free state in nature. Their habitat is the nuclei, or hearts, of the atoms. How, then, does the spontaneous combustion of the critical mass of U-235 or plutonium begin? All we need is a single neutron to start things going, and this one neutron may be supplied in one of several ways. It can come from the nucleus of an atom in the atmosphere, or inside the bomb, shattered by a powerful cosmic ray that comes from outside the earth. Or the emanation from some radioactive element in the atmosphere, or from one introduced into the body of the bomb, may split the first U-235 or plutonium atom, knock out two neutrons, and thus start a chain reaction of self-multiplying neutrons.

To understand the chain reaction requires only a little arithmetic. The first atom split releases, on the average, two neutrons, which split two atoms, which release four neutrons, which split four atoms, which release eight neutrons, and so on, in a geometric progression that, as can be seen, doubles itself at each successive step. Arithmetic shows that anything that is multiplied by two at every step will reach a 1,000 (in round numbers) in the first ten steps, and will multiply itself by a 1,000 at every ten steps thereafter, reaching a million in twenty steps, a billion in thirty, a trillion in forty, and so on. It can thus be seen that after seventy generations of self-multiplying neutrons the astronomical figure of two billion trillion (2 followed by 21 zeros) atoms have been split.

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.

We would cut this loss by more than half because a kilogram of tritium would yield about two and a half times the explosive power of plutonium. But even this advantage would soon be lost, since tritium decays at the rate of fifty per cent every twelve years, so that a kilogram produced in 1951 would decay to only half a kilogram by 1963. Plutonium, on the other hand, can be stored indefinitely without any significant loss, since it changes slowly (at the rate of fifty per cent every twenty-five thousand years) into the other fissionable element, U-235, which in turn decays to one half in no less than nine hundred million years. What is more, plutonium, if the day comes when we can beat our swords into plowshares, will become one of the most valuable fuels for industrial power, the propulsion of ships, globe-circling airplanes, and even, someday, interplanetary rockets. It holds enormous potentialities as one of the major power sources of the twenty-first century. Tritium, on the other hand, can be used only as an agent of terrible destruction. It will yield its energy in a fraction of a millionth of a second or not at all. The only other possible uses it may have would be as a research tool for probing the structure of the atom, and as a potential new agent in medicine, in which it may be used for its radiations.

How much tritium would it take to make an H-bomb 1,000 times the power of the wartime model A-bombs? Since tritium has about 2.5 times the power per given weight of U-235 or plutonium, it would take 400 kilograms (about 1,880 quarts of the liquid form) of tritium to make a bomb that would equal the power of 1,000 kilograms of plutonium. Such a bomb, we can see, would have to be made at the sacrifice of 32,000 kilograms of plutonium. In other words, we would be getting a return, in terms of energy content, of 1,000 kilograms for an investment of 32,000. And we would be losing fully half of even this small return every twelve years.

How many A-bombs would we be sacrificing through this investment? On the basis of Professor Oliphant’s estimate that the critical mass of an A-bomb is between 10 and 30 kilograms, we would sacrifice at least 1,066, and possibly as many as 3,200, if we take the lower figure. And we must not forget that a bomb a thousand times the power will produce only ten times the destructiveness by blast and thirty times the damage by fire of an A-bomb of the old-fashioned variety.

These cold facts make it clear that a tritium bomb, particularly one a thousand times the power of the A-bomb, is completely out of the picture.

But, one may ask, if a deuterium bomb is not possible and a tritium bomb is not feasible, and these are the only two substances that can possibly be used at all, isn’t all this talk about a superbomb sheer moonshine? And if so, how explain President Truman’s directive “to continue” work on it?

To find the answer let us go back for a moment to Dr. Bacher’s man in the mountains, confronted with the problem of lighting a fire with green, ice-covered wood at twenty degrees below zero with “very little kindling.” Obviously the poor fellow would be doomed to freeze to death were it not for one little item he had almost forgotten. Somewhere in his belongings he discovers a container filled with gasoline, which increases the inflammability of the wet wood to the point at which it will catch fire with a quantity of kindling that would otherwise be much too small.

Something closely analogous is true with the H-bomb. It so happens that a mixture of deuterium and tritium is the most highly inflammable atomic fuel on earth. It yields 3.5 times the energy of deuterium and about twice the energy of tritium when they are burned individually. Most important of all, the deuterium-tritium mixture, known as D-T, ignites much faster than either deuterium or tritium by themselves. For example, the D-T combination ignites 25 times faster than deuterium alone at a temperature of 100,000,000 degrees, and the ignition time is fully 37.5 times faster than for deuterium at 150,000,000 degrees.

The published technical data show that at a temperature of 50 million degrees the D-T mixture ignites in only 10 microseconds, or 20 times faster than deuterium alone. At 75 million degrees it takes only 3 microseconds, as against 40 for deuterium, while at 100 million degrees it needs only 1.2 microseconds to catch fire, a time, as we have seen, only 0.1 microsecond longer than it took the wartime A-bomb to fly apart. Since the latter held together for 1.1 microseconds at a temperature of about 50 million degrees, it is reasonable to assume that the improved and more efficient models generate a temperature at least twice as high, and that this is done by holding them together for about 1.2 microseconds.

It can thus be deduced that the only feasible H-bomb is one in which a relatively small amount of a deuterium-tritium mixture will serve as additional superkindling, to boost the kindling supplied by the improved model A-bomb, for lighting a fire with a vast quantity of deuterium. This, it appears, is the real secret of the H-bomb, which is really no secret at all, since all the deductions here presented are arrived at on the basis of data widely known to scientists everywhere, including Russia. And since it is no secret from the Russians, whom the arch-traitor Fuchs has supplied with the details still classified top secret, the American people are certainly entitled to the known facts, so vitally necessary for an intelligent understanding of one of the most important problems facing them today.

A deuterium bomb with a D-T booster would become a certainty if the temperature of the A-bomb trigger could be raised to 150 million or, better still, to 200 million degrees. At the former temperature the D-T superkindling ignites in 0.38 microseconds; at the higher temperature the ignition time goes down to as low as 0.28 microseconds. Now, the D-T mixture releases four times as much energy as plutonium, and the faster the time in which energy is released, the higher goes the temperature. Since four times as much energy is released at a rate four times faster than in the wartime model A-bomb, it is not unreasonable to assume that the temperature generated would be high enough to ignite the green wood in the bomb—its load of deuterium.

How much tritium would be required for the kindling mixture? On this we can only speculate at present. Since the D-T kindling calls for the fusion of one atom of tritium with one atom of deuterium, and the atomic weight of tritium is three as compared with two for deuterium, the weight of the two substances will be in the ratio of 3 for tritium to 2 for deuterium. Thus if the amount to be used for the kindling mixture is to be one kilogram, it will be made up of 600 grams of tritium and 400 grams of deuterium. Since, as we have seen, it would take eighty kilograms of plutonium to produce one kilogram of tritium, we would have to use up only 48 kilograms of plutonium to create the 600 grams, or the equivalent of one and a half to about five A-bombs, according to Dr. Oliphant’s estimate.

But would we need as much as 600 grams of tritium? Such an amount, mixed with 400 grams of deuterium, would yield an explosive power equal to 80,000 tons of TNT, an energy equivalent of 100 million kilowatt-hours. A twentieth part of this amount would still be equal in power to 4,000 tons of TNT, equivalent in terms of energy to 5,000,000 kilowatt-hours. Now one twentieth of 600 grams, just 30 grams of tritium, could be made at a cost of no more than 2.4 kilograms of plutonium. Thus we would be paying only one twelfth to one fourth of an A-bomb (in addition to the one used as the trigger) to get the equivalent of ten A-bombs in blasting power and of thirty times the incendiary power, which would totally devastate an area of more than 300 square miles by blast and of more than 1,200 square miles by fire.