THE MAKING OF THE H-BOMB

FOR four years the hydrogen bomb grew in secret and silence, stirring like a quickened fetus in the guarded laboratories. Few qualified physicists, U.S. or foreign, cared to talk about it. They knew that their science would soon give monstrous birth, but they had been warned to keep quiet. When the pictures of the bomb's fury hit the public last week, not many laymen remembered that the scientists long ago predicted what was likely to happen (TIME, Feb. 13, 1950).

"Fusion" of light elements, on which the hydrogen bomb depends, is the senior source of nuclear energy. More than 20 years ago, at Cambridge University, Physicists John D. Cockcroft and Ernest T. S. Walton shot hydrogen nuclei (protons) from a primitive high-voltage machine at a lithium target. A few of the protons hit lithium nuclei. The product of each such reaction: two atoms of helium and 17.3 million electron-volts of energy.

That experiment in 1932 was man's first taste of nuclear energy, but it was like the quick-fading taste of a single grain of sugar. Since most of the protons missed their targets, the hydrogen-lithium reaction gave a net loss of energy, and no one knew how to improve its efficiency. Other reactions of light elements yielded theoretical energy too, but all of them were overshadowed by the wartime development of atom-splitting uranium fission.

The scientists, however, did not forget fusion. Graven on their minds was a curious set of facts: when the elements are arranged in series according to their atomic weights, the atoms of those in the center of the series are lighter than they "should be." So when an atom of uranium (the heaviest natural element) splits into two fragments and a few loose neutrons, all the pieces, added together, weigh less than the original uranium atom. By Einstein's famous equation (E = Mc2), this loss of weight shows up as the energy that powers uranium bombs.

OMINOUS'PROSPECT

A similar thing happens at the light end of the series. If light atoms, e.g., hydrogen, are packed together into a larger atom, it weighs less than the pieces that form it. Here again, the loss of weight shows up as energy. A little figuring told the physicists that a given amount of a light element, forced to fuse, would yield more energy than the same amount of uranium. Besides, light elements are plentiful, while uranium is scarce.

This was an exciting and ominous prospect, but the trouble with fusion reactions is that they are not self-starting; uranium fission is. When a sufficient amount (critical mass) of U-235 is assembled, a single, slow-moving neutron can start an atom-splitting chain reaction in it and make the whole chunck explode. Light elements are not so accommodating. Their atoms must be slammed together violently to make them group into larger atoms and yield energy.

Except for such demonstrations as the 1932 Cockcroft-Walton experiment, the only way to get a fusion reaction is to raise the temperature. The hotter a material gets, the faster its atoms move. If it gets hot enough, they may hit one another so hard that they combine into larger atoms, yielding the energy of fusion.

Ordinary-high temperatures, attainable by chemical means, are not nearly high enough, but the center of an exploding uranium-fission bomb (more than 50,000,000DEG C.) is as hot or hotter than the interior of the sun. Before the first atom bomb exploded, physicists were speculating as to whether atom bombs might serve as "detonators" to start fusion explosions.

The temperature of an atom bomb at the instant of explosion is fabulously high, but as the fireball expands, it cools off rapidly. If it cools too fast, any fusion reaction that it has started will die out. But if the high temperature lasts long enough, it will "ignite" the light elements. Then the fusion reaction will continue, generating energy to keep the materials hot until a large part of the light-element charge has been fired.

In the early postwar period, the prospects for fusion did not look very good. The available light elements--lithium, ordinary hydrogen and deuterium (heavy hydrogen)--seemed to require more heat than could be provided by the first atom bombs. The third hydrogen isotope, tritium, looked more promising. A mixture of tritium (H^3) and deuterium (H^2) will ignite at a comparatively low temperature, turning into helium (He4) and a free neutron, and giving a big yield of energy.

The disadvantage of tritium is that it does not exist in nature. It has to be made at fantastic cost in nuclear reactors. Optimistic physicists hoped that a small priming of tritium would ignite large amounts of light elements that are not so hard to come by. Pessimists feared that too much tritium would be required. They pointed out that each atom of tritium manufactured in a nuclear reactor costs about one atom of U-235 or plutonium, which could be used to better advantage, they thought, in old-style fission bombs.

The optimists won the argument, and a tritium-production program got under way. The great Savannah River plant (cost: $1.5 billion) was largely built for this purpose. As things finally turned out, it may not have been necessary.

GUESSING GAME During the last month or so, there has been a storm of guessing about how hydrogen bombs are made. Every non-insider's guess is surely wrong in some particular. In the early days of nuclear energy, only two main ingredients, U-235 and plutonium, were available to the bombmakers, and both behaved about the same. Now the situation is more complicated. Many light isotopes are suitable for fusion, and under the conditions in an exploding bomb, they may react with one another in many different ways. They also react with the products, e.g., neutrons, given off by the fission detonator, and with materials in the casing of the bomb. As the temperature changes, their behavior changes too. So a diagram describing the behavior of a fusion bomb can give only a few of the possible ingredients and tell only a few of the ways in which they may react.

The main trends of H-bomb development, however, are clear to all. An early step was to force the temperature of the fission detonator (atom bomb) as high as possible. One way to do this is to make the fission reaction more efficient. The early bombs "burned" only a fraction of their fissionable material. As they were improved, they burned more of it and reached higher temperatures. The improved bombs, even though not designed with hydrogen bombs in mind, were therefore more effective as detonators.

FUSION BOOST Another trick available to researchers is to place in the fissionable core a small amount of highly reactive tritium, perhaps mixed with deuterium. Both the isotopes are light gases, and so they can be highly compressed and confined inside the metal. They can also be dispersed through it in some chemical or mechanical way. When the detonator explodes in such a rig, the tritium reacts, turning into helium and raising the temperature of the explosion. Such "fusion-boosted" detonators are much discussed among hydrogen-bomb connoisseurs. The long series of "nuclear devices" that the Atomic Energy Commission tested in the Pacific and Nevada may have included many experiments with fusion boosting.

The purpose of the detonator, boosted or not, is to ignite the main charge of light elements. What this charge may contain has not been announced, and the possibilities are numerous. With some oversimplification, charges can be grouped in two categories: "wet" and "dry."

In a "wet" bomb, the main charge is made up of liquefied hydrogen isotopes: tritium and deuterium. The precious tritium is the most reactive. It combines readily with deuterium, and the energy that results raises the temperature sufficiently to make deuterium nuclei combine in pairs, forming helium and giving off more energy.

Since deuterium is comparatively cheap and easily obtained, a practical "wet" bomb should contain very little tritium. But even the best of this type is cumbersome and impractical. Liquefied hydrogen isotopes must be kept under high pressure at a temperature close to absolute zero. They must be carefully insulated. If held for long periods, they must be cooled mechanically to keep them from vaporizing and rupturing their container. Outside scientists say that the "device" exploded on Eniwetok in 1952 was "wet," and that it weighed, with its necessary insulation and cooling equipment, more than 65 tons. If so, it could not have been a droppable bomb.

"Dry" bombs (the March 1 explosion may have been the first of them) use chemical forces instead of cold and pressure to keep their volatile hydrogen crammed into a small space. Their main charge is lithium hydride, a chemical compound containing one atom of lithium and one of hydrogen. Since it is a stable solid that needs no unusual treatment, its use eliminates the troubles connected with liquid hydrogen. It is the key to what airmen call a "transportable" H-bomb.

THE BIG QUESTION

Plain lithium hydride, which can be bought on the open market, is probably not the kind that the bomb-builders use. Natural lithium contains two isotopes, L17 and L16, which behave differently in a fusion reaction. Most guessers believe that L16 is the preferred isotope. The hydrogen in the compound is probably deuterium (H^2). So the compound may be described as "lithium-six deuteride."

What happens when a charge of lithium-six deuteride is ignited is almost anyone's guess. A great many reactions are possible, and many must surely take place (see diagram). The main reaction is the combination of L16 with H^2, forming two atoms of helium (He4) and giving off a flood of energy. Since helium is the final product, the well-designed bomb should produce as much of it as possible, but side reactions are likely. Neutrons from the reacting plutonium are apt to hit lithium atoms, turning them into helium and tritium (H^3). Tritium may hit deuterium, yielding helium and a free neutron. The bomb-com-pounders may include other ingredients (e.g., lithium seven and ordinary hydrogen), and these will react in characteristic ways.

One big question: How much original tritium must the dry bomb contain? It may be possible to use none of it except in the boosted detonator, but some guessers believe that a small amount of tritium in the main charge is needed to promote the reaction. It will tend to re-create itself, acting like a chemical catalyst. Other guessers think that free neutrons from the detonator will create enough tritium (by combining with lithium six) to keep the reaction going at full speed.

It may even be possible to get along with no tritium in the detonator. A highly efficient fusion bomb may raise the temperature high enough to ignite the lithium hydride. Or perhaps it may, by "implosion." cause the fusion of a core made of deuterium alone.

If a fusion bomb can be made really "dry," with no tritium at all. a new era of nuclear energy has arrived. Every fission bomb in the world's stockpiles can then be upgraded into an H-bomb, with hundreds or thousands of times its original power. They will have to be reworked slightly and surrounded by a reasonable amount of lithium-six deuteride.

This task should be no strain on any bomb-possessing nation. Lithium is abundant, and its L16 isotope (7.9% of the total) is not hard to separate. Deuterium is found in nature as about 1/5,000 of the hydrogen in water. As nuclear prices go, it is cheap and easy to obtain. Measured by its explosive effect, lithium-six deuteride is cheap indeed. One pound, if all of it reacts, has the explosive effect of 23,000 tons of TNT. Any desired amount can be used in a single bomb. Twenty-two tons of it, efficiently fired, would be equivalent in explosive force to one billion tons of TNT.

Bombs of this size may never be assembled. Even if considerably smaller, they would be hard to deliver, and they would "overbomb" a small area, digging a deep crater instead of spreading their killing effect over the living film that covers the surface of an inhabited region. There is a way, however, of getting around this disadvantage.

Unlike plutonium bombs, whose fission products are naturally radioactive, a lithium-six deuteride bomb is only a moderate producer of radioactive contamination. Its end product, helium, is not radioactive at all. The detonator yields the normal products of fission, but they are no worse than those of an old-style atom bomb. Side reactions may produce radioactive isotopes, but they can be minimized. Apparently, they were minimized effectively in the H-bomb that exploded in the Marshall Islands on March 1.

Though that explosion was 750 times as violent as a Hiroshima-type A-bomb (TIME, March 22), the radioactive contamination was not in proportion. Its effect at a distance was little, if at all, greater than that of earlier A-bomb tests. The Japanese fishermen who were burned by "death ash" were apparently victims of a local concentration of contaminated pulverized coral. Some of their burns, according to AEC Chairman Strauss, came from the chemical action of the ash. He probably meant that the coral, chiefly-calcium carbonate, had been turned by heat to quicklime, which sears human skin.

There is no reason to believe, however, that the radioactive aftereffect of the hydrogen bomb cannot be increased, if thai is what the designers want to do. First step would be the addition of an ingredient that yields free neutrons (L17 might be a good one). Next step would be to surround the bomb with 3 casing of an element that absorbs neutrons and becomes radioactive. Such a doctored H-bomb might poison a whole country.

Few scientists feel cheerful about the H-bomb. It looks like too ready a tool of destruction. They have only one reassuring opinion. At the present state of the art, they say, there is no chance that even the most monstrous bomb will get out of control, set fire to the ocean's hydrogen and turn the earth into a short-lived star. The H-bomb's ingredients must be pure and carefully selected, but the ocean is a mess of many nonreactive elements. Less than one-ninth of it is hydrogen, and the safe kind of hydrogen at that.

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