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Deuterium, the sole fuel in reactions 2 and 3 (see Fusion Principles), is relatively cheap (especially considering its enormous energy content) and is completely stable. Pure deuterium has been used in at least one fusion weapon test - Ivy Mike, the first true fusion weapon explosion in history (1 November 1952). It must either be highly compressed, or liquified at extremely low temperatures. This problem can be overcome by combining the deuterium chemically with lithium to form lithium deuteride, a stable solid. An additional benefit is that through reactions 5 and 6, the lithium can itself participate in the fusion reaction.
Nuclear fusion
Ulam, made the first key steps towards a workable fusion design, rendering the idea of a fusion bomb practical. [3] He noted that compression of the thermonuclear fuel before extreme heating was a practical path towards the conditions needed for fusion. [4] This spawned the idea of staging and placing a separate thermonuclear component outside the primary fission component.
Basic Principles of Staged Radiation Implosion
The concept of a thermonuclear fusion bomb ignited by a smaller fission bomb was initially suggested by Enrico Fermi to Edward Teller in 1941during the inception of the Manhattan Project. [2] This idea caused Teller to spend most of his time during the Manhattan Project attempting to figure out how to make this concept work; neglecting his assigned work on the Manhattan Project. Teller, was able to convince his colleague Stanislaw Ulam, to join him on this idea provided by Fermi.
The weapons are tested
The huge energy released by fission was used for compression and ignition of fusion fuel. Simply put, the fission explosion created a shock wave that compressed fusion fuel inside the fission envelope. Again, remember that the context of this is Ulam’s initial idea for very high efficiency fission bombs, not just hydrogen bombs. The rest explains that the goal here is very high compression of relatively large masses of material (fusion fuel). Enrico Fermi came up with the basic idea using the power of a fission bomb to ignite fusion reactions — a thermonuclear or hydrogen bomb — as early as 1941.
The 2.45 MeV and 14.1 MeV neutrons that escape from the fusion fuel can also contribute greatly to bomb yield by inducing fission in the highly compressed fusion tamper. This extra boost can release most of the explosion energy, and commonly accounts for half of the yield of large fission-fusion-fission bombs and can reach at least 85% of the total yield. When the primary explodes, the X-rays escaping from the fission trigger fill the radiation channel, the space between the bomb casing and the fusion capsule, with a photon gas. This space is filled with plastic foam, essentially just carbon and hydrogen, which becomes completely ionized and transparent as the x-rays penetrate. The inner casing and outer capsule surfaces are heated to very high temperatures. The uranium shield between the trigger and the fusion capsule, and capsule pusher/tamper, prevents the fusion fuel from becoming heated prematurely.
The largest American bombs had yields of 10 to 25 megatons and weighed up to 20 tons. Beginning in the early 1960s, however, the United States built a variety of smaller, lighter weapons that exhibited steadily improving yield-to-weight and yield-to-volume ratios. By the time nuclear testing ended in 1992, the United States had conducted 1,030 tests of weapons of every conceivable shape, size, and purpose.
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The explosion achieved a yield of 10.4 megatons (million tons), 500 times larger than the Nagasaki bomb, and it produced a crater 1,900 metres (6,240 feet) in diameter and 50 metres (164 feet) deep. The largest thermonuclear bomb ever created and detonated was the Russian Tsar Bomba (bomb-emperor in Russian), tested in 1961 in remote areas of Novaya Zemlya. It detonated 4 km above ground and achieved a yield of 50 Mt. The shock wave from the blast travelled three times around the Earth. Thermal equilibrium is established extremely rapidly, so that the temperature and energy density is uniform throughout the radiation channel. As the surface of the tamper becomes heated, it expands and ablates (blows off the fuel capsule surface).
Castle Bravo
Separating the trigger from the fuel package is a thick plug of dense material (again U or W). Experiments with particle accelerators proved the possibility of fusion reactions, but the first large fusion energy release was realised in a thermonuclear bomb. The hydrogen bomb can be considered the first successful demonstration of inertial confinement.
Biographical Memoirs: Volume 90
The fuel in the fission capsule consists of lithium deuteride that may be enriched in the Li-6 isotope (which makes up 7.5% of natural lithium). There is some tritium generated by the fission neutrons, but as noted above the contribution to bomb yield is insignificant. Far more tritium is produced by the D+D reactions, either directly by reaction 3, or by reaction 5 via the neutrons produced in reaction 2. The principles of the Teller-Ulam configuration are more easily explained with the help of the diagram below.
The rod fissions at an accelerating rate as it, and the rest of the fuel capsule continue to implode and acts as the fusion "spark plug". Combined with the high temperatures generated by the convergent shock wave, this raises the temperature of the fusion fuel around the rod high enough to initiate the fusion reaction. The fusion tamper prevents the escape of thermal radiation from the fuel, enhancing the burn efficiency considerably. The temperatures generated by fusion burning can considerably exceed that produced by fission (up to 300 million K).
At the very least, it required a familiarity with nuclear reactions at energy regimes which had never been achieved previously on Earth. It also required breaking out of several wrong ideas along the way whose wrongness was not obvious. Since the D+T reaction rate is so high, and there is large excess of deuterium, the tritium is consumed almost as fast it is produced. The 14.1 MeV neutrons also produce large amounts of tritium from Li-7 through reaction 6.
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