Conventionally, hydrogen is not, and will NEVER be, an “alternative” fuel because there is no place on Earth that it can be sourced in a usable form. Sure, it’s a common element on Earth, but it’s all locked up in the form of water and other molecules. You can produce hydrogen from water, but only by reversing the very reaction that would later be used to extract energy from the hydrogen thus generated. And since inefficiencies will exist at every step along the way, you will need to use more energy in making hydrogen this way than could ever be recovered by burning it. In practice, hydrogen is currently produced from methane by a reaction that requires considerable heat input and which yields carbon dioxide as a by-product – the very gas that alternative energy proponents want to get away from.
And then there is handling. How do you store and transport hydrogen as a fuel? There are basically three options: (1) as a compressed gas, (2) as a cryogenic liquid, and (3) as a metal hydride (solid or liquid).
Compressed gases are not especially convenient to handle and require fairly heavy containment vessels. In addition, compressed gases are fairly low density. Cryogenic liquids are higher in density, but require fairly sophisticated containers and must continuously vent to maintain their low temperatures and pressures.
That leaves metal hydrides. As a storage medium, only half of the desired hydrogen gas would need to be stored as a hydride, since the reaction between the hydride and water would produce the desired gas, along with an equivalent of metal hydroxide. Liquids of course are easier to handle and it would be nice if a suitable metal hydride were available in liquid form. Unfortunatley, the only ones that come to mind are fairly viscious and bulky (e.g. diisobutylaluminum hydride, or DIBAL). Realistically, we are left to choose from the various solid forms.
So how do we choose a metal hydride for use as a storage medium for hydrogen gas? Something like potassium hydride would work fine, but note how far down potassium appears on the periodic table. Potassium has an atomic mass of 39 compared to 1 for hydrogen. So one unit of KH is only 2.5% “hydride,” with the remaining 97.5% of the mass being potassium. What we want is a light metal. Lithium hydride (LiH) is probably the best choice. Lithium has a mass of only 6.94, so lithium hydride is 12.5% “hydride” by mass, with the remaining 87.5% of the mass being the metal. It’s not great, but certainly better than potassium hydride. Diborane (B2H6) has an even better hydride:metal mass ratio, but it tends to be messy, forming gels and such. Diborane also has some toxicity issues and is a gas.
So that’s that. If you want to store and transport hydrogen for use in a combustion reaction, one of the better ways to do it might be to utilize lithium hydride. The relevant equations are shown below.
So then … what the Hell does all of THAT have to do with hydrogen bombs? With hydrogen bombs, these same problems of storage and transport arise, and the same solutions are very nearly applied … but with a glorious and amazing twist!
In the case of a hydrogen bomb, what is actually desired are the heavy isotopes of hydrogen: Deuterium (D, with one neutron) and tritium (T, with two neutrons). Deuterium is stable and readily obtained as it occurs naturally on Earth. Tritium, on the other hand, is dangerously radioactive with a half-life of 12.32 years. It is only obtained from nuclear reactions. The basic method of storage, however, is exactly as outlined above for the storage of conventional hydrogen – as a hydride of lithium. In this case, it is lithium deuteride, or LiD.
So where does the tritium come from? This is the magical part. 😀In a thermonuclear device (i.e. a hydrogen bomb), a primary fission reaction of uranium or plutonium is initiated with conventional explosives. This results in a release of X-rays which are then reflected and directed through various materials, yielding neutrons in the process. These neutrons ultimately arrive at the lithium deuteride fuel, where a further nuclear reaction of the lithium occurs, yielding the required tritium.
In other words, used in a nuclear device, lithium deuteride is more than a simple source of heavy hydrogen bound to a metal. The lithium metal itself is ALSO a source of heavy hydrogen, AND it is a source of the least stable yet required isotope, tritium. With a half-life of just over twelve years, tritium would need to be replaced in the device every so many years to maintain potency if it were present in the device.
So that’s it. Lithium deuteride, as used in a hydrogen bomb, is a most amazing fuel, and it almost feels as if nature herself has conspired to make it so. The lithium metal serves as a masked form of tritium. It is stable and has an indefinite self-life. When bombarded with neutrons, however, the tritium is unmasked and able to undergo nuclear fusion with the deuterium component of LiD, resulting of course in a very large explosion.
The history of the development of the hydrogen bomb (of all nuclear devices, actually) is pretty amazing. One part of that history that I can’t leave unmentioned here has to do with the lithium deuteride, and in particular the isotope of lithium.
The Teller-Ulam design was initially tested in Operation Ivy, with Ivy Mike being the first successful detonation of such a device. In Operation Ivy, the device actually did use cryogenic deuterium as the fuel, since the goal was to test the concept rather than make a practical device. Together with it’s elaborate system of dewars and pipes necessary to handle the cryogenic deuterium, Mike was 82 tons!Following Operation Ivy was Operation Castle, with Castle Bravo being the first test of a dry fuel (lithium deuteride) device (March 1, 1954; 60 years ago last month). The yield of this device was 15 megatons, about three times the expected yield of about 5 megatons. Why?
Natural lithium on Earth is a mixture of lithium-6 and lithium-7. Only 7.5% of it is normally lithium-6. For the Castle Bravo device, the lithium used to prepare the lithium deuteride fuel was enriched in lithium-6 (40% instead of 7.5%). In the design of the Bravo device and in the calculations of its theoretical yield, the scientists at Los Alamos considered only the lithium-6, expecting the lithium-7 to be more or less inert. It wasn’t! Lithium-7, it turns out, undergoes nuclear decay very similar to that of lithium-6 when exposed to energetic neutrons, yielding both tritium and a new neutron. As a result, more tritium was available for fusion in Castle Bravo than was expected. In addition, the extra neutron from lithium-7 decay enhanced the “neutron flux,” increasing the fission efficiency of the uranium tamper in the device.Being a relatively crude device (the first ever dry fuel hydrogen bomb) with triple the anticipated yield, Castle Bravo was relatively “dirty” and produced substantial fallout. Detonated at Bikini Atoll, Marshall Islands, the fallout contaminated a number of inhabited islands. Additionally, the fallout led to at least one death on a Japanese fishing vessel, and contaminated a number of vessels involved in the test. A number of crew members of these vessels received radiation burns during the test.
For their intended use, hydrogen bombs are of course not things to be admired. But it’s hard not to appreciate their sheer efficiency and the chemistry and physics of their design and function. 🙂
When I first published this post, I inadvertently showed the reaction of lithium metal, and not lithium hydride. Additionally, the lithium metal reaction I showed wasn’t balanced. Doh!
I’ve corrected that, but I thought I would point out here why lithium hydride is preferable to lithium metal. Shown below are (a) the reaction of lithium metal with water (top, this time a balanced reaction), and (b) the reaction of lithium hydride with water (bottom). Note that when lithium metal is used, twice as much is needed and it produces twice as much LiOH waste. Using lithium hydride instead of lithium metal is more efficient.