Fig. 3. The bomb dropped on Hiroshima. This device was 120 in. Fig. 4. The bomb dropped on Nagasaki. This device was 128 in. long
long and weighed 7000 lb. and weighed 10,000 lb.

As can be seen from Figures 3 and 4, the bombs used in World War II were rather cumbersome. Advances in the technology of shaping chemical high explosives used for the implosion of plutonium have made it possible to construct bombs in which the lump of plutonium is only about as large as a golf ball; the overall diameter of such a bomb is only about a foot, and yet it releases an amount of energy in the kiloton range.

Much higher yields can be achieved by taking advantage of nuclear fusion. Binding energy of light nuclei is relatively low – when two such light nuclei are made to fuse together to form a heavier nucleus, energy will be released. Fusion is the opposite of fission: in the former two nuclei merge into one, in the latter one nucleus splits into two. Furthermore, the energy released in fusion is strong energy while the energy released in fission is Coulomb energy. The strong force favors fusion since, by merging, the nuclei reduce their surface area. The Coulomb repulsion between the two nuclei opposes fusion, but in the case of light nuclei the strong force overcomes this opposition.

The heat given off by the Sun is due to a fusion reaction called hydrogen burning: hydrogen nuclei fuse together to make helium nuclei. This reaction involves several intermediate steps, which were discovered by theoretical calculations by Hans Bethe. This reaction cannot by duplicated on Earth because it will only proceed at extremely high temperatures and pressures, such as are found near the centre of the Sun. However, some fusion reaction involving deuterium and tritium (the isotopes 2H and 3H) can be made to work on Earth:

2H + 2H → 3He + n(1)
2H + 2H → 3H + p(2)
2H + 3H → 4He + n(3)
5 2H → 3He + 4He + p + 2n + 24.3 MeV

The net results of the three reactions taken together is disappearance of five 2H nuclei and formation of 3He, 4He, one free proton , and two neutrons, with the release of 24.3 MeV. The amount of energy released per nucleon of reactant is 24.3 MeV per 10 nucleons = 2.43 MeV per nucleon, whereas for the fission of uranium the energy released per nucleon of reactant is 200 MeV per 235 nucleons = 0.85 MeV per nucleon. Thus the fusion of a given mass of 2H will yield about three times as much energy as the fission of an equal mass of 235U.

The reactions (1)-(3) are called thermonuclear because they will proceed only at very high temperature and pressure. The requisite temperatures and pressures are attained at the place of explosion of an A-bomb. Hence, the fusion reactions can be initiated by exploding a fission bomb next to a mass of heavy hydrogen; this results in self-sustained explosive “burning” of the hydrogen nuclei. This so-called H-bomb is really a fission-fusion device in which fission triggers fusion. What is more, the fusion reactions release a large number of energetic neutrons which can be used to further enhance the violence of the explosion: the trick is to surround the fusion bomb by a blanket of cheap, natural uranium, consisting of mainly 238U; although this isotope will not maintain a chain reaction, it will fission when exposed to the large flux of neutrons from the fusion. This kind of H-bomb is a fission-fusion-fission device; typically one-half of the total energy yield is due to fusion, one-half is due to fission. The fission of a large amount of uranium leaves behind a residue of highly radioactive fission products; hence fission-fusion-fission bombs are dirty, i.e., they generate a large volume of radioactive fallout.

The total energy yield of an H-bomb is of the order of one or several megatons – roughly a thousand times the yield of an A-bomb. Explosions of up to 60 megatons have been tried with great success, and there seems to be no limit to the suicidal madness that nature will let us get away with.

That brings me to the end of my presentation. Thank you for your attention and if you have any questions, I’ll be glad to answer them.