In the atomic bomb only pure U-235, or plutonium, is used.
In both the controlled and the uncontrolled reactions a minimum amount of material, known as the “critical mass,” must be used, as otherwise too many neutrons would escape and the nuclear fire would thus be extinguished, as would an ordinary fire for lack of oxygen. In the atomic bomb two masses, each less than a critical mass, which together equal or exceed it, are brought in contact at a predetermined instant. The uncontrolled reaction then comes automatically, since, in the absence of any control, the neutrons, which cannot escape to the outside, build up at an unbelievable rate.
Whereas the fission process for the release of nuclear energy entails making little ones out of big ones, the fusion process involves making big ones out of little ones. In both processes the products weigh less than the original materials, the loss of mass coming out in the form of energy. According to the generally accepted hypothesis, the fusion process is the one operating in the sun and the stars of the same family. The radiant energy given off by them, it is believed, is the result of the fusion of four hydrogen atoms into one atom of helium, two of the protons losing their positive charge, thus becoming neutrons. Since a helium atom weighs nearly eight tenths of one per cent less than the total weight of the four hydrogen atoms, the loss of mass is thus nearly eight times that produced by fission, with a corresponding eightfold increase in the amount of energy liberated. This process, using light hydrogen, is not feasible on earth.
The nuclei of all atoms are thus vast storage depots of cosmic energy. We must think of them as cosmic safe-deposit vaults, in which the Creator of the universe, if you will, deposited at the time of creation most of the energy in the universe for safekeeping. The sun and the other giant stars that give light have, as it were, drawing accounts in this “First National Bank and Trust Company of the Universe,” whereas we on this little planet of ours in the cosmic hinterland are much too poor to have such a bank account. So we have been forced all these years we have been on earth to subsist on small handouts from our close neighbor the sun, which squanders millions all over space, but can spare us only nickels, dimes, and quarters (depending on the seasons of the year) for a cup of coffee and a sandwich. We are thus in the true sense of the word cosmic beggars, living off the bounty of a distant relative.
The discovery of fission in 1939 meant that after a million years of exclusive dependence on the sun we had suddenly managed to open a modest drawing account of our own in this bank of the cosmos. We were enabled to do it by stumbling upon two special master keys to five of the cosmic vaults. One of these keys we call fission; the other, which allows us entry into a much richer chamber of the vault, we call fusion. We can get a lot of the stored-up cosmic treasure by using the key to the fission vaults alone, but, as with our terrestrial bank vaults, which generally require two keys before they can be opened, it is not possible to use the key to the fusion vault unless we first use the fission key.
Except for the payment of our heat and light bill, the sun gives us nothing directly in cash. Instead it deposits a very small pittance in the plants, which serve as its major terrestrial banks. The animals then rob the plants and we rob them both. When we eat the food we live by we thus actually eat sunshine.
The sun makes its deposits in the plant through an agent named chlorophyll, the stuff that makes the grass green. Chlorophyll has the uncanny ability to catch sunbeams and to hand them over to the plant. A chemical supergenius inside the plant changes the sunlight energy into chemical energy, just as a bank teller changes bills into silver. With this chemical energy at their disposal, a great number of devilishly clever chemists in the plants’ chemical factory go to work building up many substances to serve as vaults in which to store up a large part of the energy, using only part of it for their own subsistence.
The building materials used by these chemists inside the plants consist mainly of carbon-dioxide gas from the atmosphere, and water from the soil, plus small amounts of minerals either supplied by the good earth or by fertilizers. Carbon dioxide, by the way, composed of one atom of carbon and two atoms of oxygen, is the stuff you exhale. In solid form it is what we know as dry ice, used in efforts to make rain. It is present in the atmosphere in large amounts.
Out of the carbon dioxide and water the chemists in the plants build cellulose, starch, sugar, fat, proteins, vitamins, and scores of other substances, all of which serve as vaults for the sun’s rays caught by the chlorophyll. The biggest vaults of all, storing most of the energy, are the cellulose, sugars and starches, fats and proteins. There the stored energy remains until it is released by processes we call burning or digestion, both of which, as we shall see, are different terms for the same chemical reaction. When we burn wood, or the petrified ancient wood we know as coal, we burn largely the cellulose, the chief component of the solid part of plants. When we eat the plants, or the animals in whom the plant tissues are transformed into flesh by the solar energy stored within them, it is the sugars, starches, fats, and proteins that give us the energy we live by.
In the process of burning wood or coal the large cellulose vaults, composed of carbon, hydrogen, and oxygen, are broken up, thus allowing the original solar energy, stored up within them as chemical energy, to escape in the form of heat and light. This is the same heat and light deposited there by the sun many years before—in the case of coal, some two hundred million years back. The process of burning thus transforms the chemical energy in the plants back to its original form of light and radiant heat energy. The complex carbon and hydrogen units in the cellulose are broken up, each freed carbon atom uniting within two oxygen atoms in the air to form carbon dioxide again, while two hydrogen atoms unite with one of oxygen to form water. Thus we see that the cellulose vaults are broken up once more into the original building bricks out of which the chemists in the plants had fashioned them.