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The X-10 Reactor
The X-10 reactor ran from 1942 until 1963. In 1966 it was declared a national landmark. This was not the first reactor, or even the second, though it is arguably the first reactor to be put to practical use. This was, in actuality, a very simple device. It has been described as a piece of twenty first century technology, using twentieth (or even nineteenth) century methods. So what high tech materials, and high tech design features were incorporated into this reactor? Well, the whole thing was constructed of concrete, graphite blocks, pitch (yes pitch, like what you use on your roof), and piping. There was not a mechanical moving part in the whole device, and such a structure could have been built a hundred, or even a thousand or more years ago, had humanity understood the physics involved. A gun, a mechanical alarm clock, or even a doorbell is more complicated than a graphite reactor. At it's most basic, this type of nuclear reactor is simply a collection of regularly spaced tubes, set in concrete, and eventually in carbon (graphite). It is not too far fetched to imagine the early English, or even the Romans building such a device, had they known the theory. Certainly they had the building techniques.
To make the whole thing work, fuel must be inserted. The fuel used in this reactor is natural uranium. Much has been made of enriched uranium, and various types of nuclear materials for weapons, and power generation. The most common reactor in use today is the light water reactor, which requires reactor grade uranium. Reactor grade uranium is enriched to about 3% U235. Natural uranium only has 0.7% U235, and can not be used in light water reactors. Nuclear bombs require 90% enrichment or better, as do the breeder type reactors, and the fast reactors. Though they have many disadvantages, the one great advantage of the graphite moderated reactor is that it can use regular, non enriched uranium. This was the type of fuel used in this first reactor; really, there was no choice. There would not be sufficient enriched uranium for months, of not years, and that would be used for bomb making. The only other type of reactor which can use natural uranium for fuel is the heavy water reactor, and heavy water is exceptionally difficult to make in the quantities required.
X-10 was not designed to produce electricity, nor were the other early reactors designed for such a task. So why build them? These early reactors were all designed as a part of the Manhattan Project, to try and build a nuclear bomb. While certain parts of the project labored to separate the small amounts of uranium 235 from the U238 of which most of uranium consists, some scientists theorized that a new element, with an atomic number of 94, would make a great nuclear explosive. This element did not exist in nature; but could be made in a nuclear reactor. What made this such an intriguing possibility, was the idea that this element could be made in a pure isotopic form in a reactor, and then separated by simple chemical means, rather than by the painstakingly slow, and energy intensive methods being used to separate uranium. So this is what the early reactors were designed to do, including the X-10 reactor at Oak Ridge. This new element was eventually to be named plutonium. This is the element that makes up the pits (or cores) of the vast majority of today's nuclear weapons.
Still, I may have overstated things just a bit. Though the design of this type of reactor is simple, the fuel, and some of the materials used are a bit fussy. The fuel slugs used must be absolutely pure, though they do not need more than the natural 0.7% concentration of U235. Also, the graphite used must be very pure, and free from boron contamination, which was a problem that plagued early American reactor building, and caused the German nuclear program to abandon graphite moderated reactors altogether, and go off on a failed attempt to build a heavy water reactor. Happily for the world, the Germans never were able to produce enough heavy water to build their reactor.
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