You Might Want to Know: How are fusion and fission bombs different? Why are fusion bombs so much more powerful?
You Might Want to Know: How are fusion and fission bombs different? Why are fusion bombs so much more powerful?
Fission bombs work by causing neutrons to split some of the heaviest atoms on earth--like thorium with 90 protons in its nucleus, uranium with 92, plutonium with 94. Neutrons are sub-atomic particles that have no charge and thus are not pushed back against by the particles in the nuclei of atoms that have a positive charge, the protons. When the atoms of these three heavy elements split, some neutrons will fly off that can split other atoms. The newly released neutrons are what can produce a multiplying chain reaction. They are what make these heavy elements “fissile.”
The released neutrons are flying very fast. Very fast. Not so short of the speed of light. When a fission bomb explodes, the neutrons will have done all their work in the fissile fuel in less than a millionth of a second. All the explosive energy of a fission bomb—like the one that fell on Hiroshima--is generated in that tiny amount of time.
The fissioning of the heavy atoms produces new elements. The new elements--also known as “fission products”--have fewer protons in their nuclei than the atoms of fissile fuel did. The different numbers of protons are what makes them into different elements. Added together, these “fission products” will also have less mass than did the atoms of the fissile fuel. The missing mass will have been released as energy.
Fusion bombs work by taking atoms of the lightest element on earth—hydrogen, which has a single proton in its nucleus--and compressing and heating them terrifically, so terrifically that the positively charged protons overcome their natural tendency to repel each other and fuse to create another element, helium, which has two protons in its nucleus. The helium atom will have less mass than the two hydrogen atoms that were fused to make it. That missing mass will have been released as energy. Fusion reactions that use isotopes of hydrogen that have an extra neutron in them—deuterium with two neutrons and tritium with three neutrons—also release very energetic neutrons, which can go on to cause more fission in any fissile fuel that is available.
Fusion bombs, commonly referred to as “hydrogen” bombs, usually yield much much more than fission bombs, hundreds of times more. Why is that?
The first atomic bombs, the ones dropped on Japan, were fission bombs. Already the scientists of the Manhattan Project knew that, in theory, fusion bombs were possible. But they figured that the only way they would be able to create the heat and pressure needed to start a fusion reaction would be with a fission bomb. First they’d have to make a fission bomb that worked. They showed that they had done this in the first-ever atomic explosion at Trinity on July 16, 1945, and in the bombs that fell on Hiroshima and Nagasaki in August 1945.
But how could you contain and focus the heat and pressure of the fission bomb so it would compress the fusion fuel and not just blast it apart and scatter it?
One way, they realized, would be to put some fusion fuel into a space left at the center of an imploding fission bomb.
In 1951, in the Item test in Operation Greenhouse at our Pacific Proving Ground, they injected some deuterium and tritium (D-T) gas into the center of a fission bomb just before setting it off. When the device was imploded and went super-critical, the heat and pressure produced at its core for a very brief time was enough to fuse the atoms of D-T gas. This nearly doubled usual the yield of the weapon, to forty-five kilotons. Later, this gas-injection process, called “boosting,” came to be employed in almost all our fission bombs.
In boosted bombs, fusion releases some added energy but most of the added energy comes from the extra fission that is produced by the very energetic neutrons that the fusion releases. These neutrons can initiate more fission in the fissile fuel.
Boosting would turn out to be a very good way to increase the yield and efficiency—the amount of energy released from a given amount of fissile fuel--of fission bombs. But there was limit to how much D-T gas could be inserted into the core of a fission bomb and thus a limit to how much more energy you could get from boosting. Was there a way of getting around this limitation?
This was the technical problem that had defeated the scientists who had been working on “the Super” since such a bomb had first been imagined. The design breakthrough came in March 1951, a little more than a month before Operation Greenhouse began. In the new design, a “primary” fission bomb would be used to create radiation that would compress a “secondary” that contained the fusion fuel. The explosion of the secondary would release more energy and more neutrons. The neutrons would cause more fission in any fissile fuel that had been located in and around the secondary, all of which would also release neutrons. The breakthrough allowed construction of the first “staged” or “true” fusion weapon by the scientists and engineers at Los Alamos National Laboratory, led by the physicist Richard Garwin.
These bombs weren’t just fusion bombs, then. They were fission-fusion-fission bombs.
The first explosion of such a staged hydrogen device took place at our Pacific Proving Ground in the Ivy Mike test on November 1, 1951. It was a “device,” much too big to be delivered as a bomb. Ivy Mike yielded ten megatons, 666 times what the Hiroshima bomb had yielded, more than forty-four times what the largest boosted test explosion had yielded. Two and a half years later, on March 1, 1954, the Castle Bravo tried out a new “dry” fusion fuel that yielded fifteen megatons, half again what Mike had yielded, a thousand times Hiroshima. The new fuel made deliverable bombs possible.
Just how big could these bombs get then?
Next: How big can they get?
Thanks, John.
Your writing alone though is quite clear. But I wonder if you could include some diagrams -- the fission-fusion-fission discussion was a bit confusing. Thanks.