This is a guest post on behalf of ACW reader and occasional contributor Chris Camp.
Everyone knows that there are basically two paths to a bomb, right?
You either build a bunch of centrifuges and enrich uranium to 93% or you build a few reactors and extract plutonium from the spent fuel. If you’re old enough, you might also mention gaseous diffusion as an alternative to centrifuges, but once again pretty much no one does that anymore. This common understanding has led to certain materials and technologies being watched very carefully to detect proliferation. For example, fluorine compatible seals, special bearings, and maraging steel tubes all point to centrifuges, while someone attempting to obtain ultra high purity graphite and Tributyl-phosphate points to a plutonium program. All of these items have other uses of course, but they serve as “tripwires” for potential proliferation.
While everyone knows the standard routes to the bomb, history is littered with other paths that have been tried and either rejected or overtaken by more efficient systems. However, like the title says, just because these technologies are antiquated doesn’t mean they don’t work and couldn’t pop up again in someone’s nuclear program.
Let’s start with the one that a lot of people know I’m going to mention because it’s been actually used by an aspiring nuclear weapons state. When UN inspectors arrived in Iraq in 1991 after the first Gulf War they were surprised to find Calutrons (or Baghdadtrons as the Iraqis called them) operating to produce enriched uranium. A Calutron is basically a circular particle accelerator in which atoms of a material are accelerated by magnets. In the case of uranium, the lighter U235 turns more sharply around the corners than the heavier U238 allowing separation and enrichment. This technology was used by the US during the Manhattan project to produce the final weapons grade material used in the Little Boy bomb, but was quickly scrapped when the gaseous diffusion technology came on line. An interesting historical tidbit is that the windings on the magnets were made of silver from the US treasury since copper was in such short supply. For an in-depth discussion of Calutron technology and the Iraqi program in particular see http://nuclearweaponarchive.org/Iraq/andre/ISRI-95-03.pdf
The other uranium enrichment technology to come out of the Manhattan Project was gaseous thermal diffusion. In this method three pipes of different materials are placed one inside the other in concentric fashion. Steam at 545 degrees F and 100 psi flowed through the innermost pipe, uranium hexafloride gas in the middle pipe and water at 155 degrees F through the outermost pipe. The heavier U238 tends to flow towards the cold pipe and downwards towards the bottom of the annular space while the lighter U235 tends to flow upwards along the hot pipe. The separation efficiency wasn’t great, from natural uranium at 0.7% U235 the thermal diffusion plant increases enrichment to about 0.9%. This slightly enriched uranium was fed into the gaseous diffusion plant which took it up to about 23% enrichment and that product was then fed to the calutrons which took it to a final 90% U235 material which was used in the Little Boy bomb. The thermal diffusion plant, code named S-50, at Oak Ridge wasn’t planned for and came about as a result of difficulties with the gaseous diffusion plant, code named K-25. The K-25 plant was an enormous undertaking, and for many years was the largest building in the world. Due to problems with the diffusion membranes the plant wasn’t ready on time, but it’s massive powerhouse was, and so as an interim measure the S-50 plant was built to make the most of the limited functioning of K-25. Once the gaseous diffusion plant got it’s bugs worked out it replaced both thermal and electromagnetic separation technologies and was the exclusive means of enriching uranium for many years. One place where thermal separation did find ongoing use is in the separation of tritium from deuterium after it has been irradiated in a reactor.
Ok, on the topic of uranium, everyone knows that it has two isotopes, U235 and U238, but only the first one is usable in bombs, right? Actually, no. On both counts. Uranium actually has a number of isotopes, and two of them, U235 and U233 are effective for bomb cores (U238 is also used in weapons, and forms the third stage of a classic three stage thermonuclear weapon, producing as much as half the yield, but that’s for another article). U233 has a half-life of a bit over 159,000 years and so all of the natural isotope has decayed from the earth’s crust. However, it can be bred in a reactor from the element thorium which is approximately four times as abundant as Uranium. During the 1950s the US had not yet found domestic sources of uranium and so extensive work was done on the thorium fuel cycle and U233 weapons before uranium reserves were discovered in the southwest. Approximately 2 tons of U233 were produced and a bomb partially fueled by U233 was tested during the TEAPOT MET shot at the Nevada Test Site on April 15, 1955. The MET stood for Military Effects Test and was designed to measure blast and radiation effects of a standard sized weapon. The Department of Defense agreed to the test of the U233 device on the condition that it would produce the same yield as the standard plutonium bomb to within 10%. Los Alamos promised that it would, but instead of the anticipated 33kt, the MET shot only produced 22kt which invalidated most of the measurements DOD was trying to accomplish. Oops.
Uranium 233 is actually a better material for nuclear cores than U235 from a physics perspective, but it has one major drawback. U233 is almost always contaminated with small amounts of U232 which is a gamma emitter and therefore more dangerous to handle and easier to detect than U235 or plutonium cores. Several attempts at a thorium to U233 breeder reactor have been built, but as yet the technology has not caught on. India, which is very poor in domestic uranium sources has been the leader in thorium fuel cycle technology, but so far it has not shown commercial success.
Another historical footnote that could pop up again is the hydride bomb. The idea here is that instead of using uranium or plutonium metal in the core you instead use uranium hydride. The hydrogen atoms will tend to slow down the neutrons flying about inside the core and theoretically reduce the amount of fissile material needed in the bomb and it’s size. The idea came out of Los Alamos during the late 1940s, but the tradeoff in lower efficiency was felt to outweigh any advantage and the idea was shelved One man who was not ready to give up on hydride was Edward Teller and when he left Los Alamos to found the competing Lawrence Livermore lab he took the concept with him. LLNLs first two devices were both hydride bombs. Unfortunately, both these devices fizzled, the first of them not producing enough energy to destroy the tower it sat on. The second test produced an underwhelming 200-ton yield. However, it should be kept in mind that both devices were pushing the lower boundary of fissionable material that would still form a critical mass.
The final technology that could pop up again someday is the early plutonium reprocessing chemistry. While most of you are probably familiar with the PUREX (PlUtonium URanium EXtraction) used today, there were several systems that preceded it. The process that produced the early plutonium bombs used bismuth phosphate to extract the uranium and plutonium from the dissolved fuel rods. This was replaced in 1952 by the REDOX (REDuction / OXidation) process which used methyl isobutyl ketone as a solvent. Both processes have their disadvantages (explosive ones in the case of REDOX) they share an advantage in that they don’t use Tributyl phosphate which is internationally regulated.
It’s been said that the stone age didn’t end for a lack of stones, and likewise the technologies I’ve mentioned above weren’t discarded because they don’t work. In most cases something better came along and replaced them. As the Baghdadtrons demonstrated they can pop up at any time if we forget to look for them!