Graphite lattice arrangements

By the time he heard from Fermi, Szilard had seen still farther and reahzed that small spheres of uranium arranged within blocks of graphite would be even more favorable from the point of view of a chain reaction than the system of plane uranium layers which was initially considered. The arrangement Szilard had in mind he called a lattice. (A geodesic dome would represent such a lattice arrangement schematically if it were a complete sphere and if all its interior volume were filled like its surface with evenly spaced points.) His calculations indicated somewhat larger volumes of material than had Fermi s perhaps 50 tons of carbon and 5 tons of uranium. The entire experiment, he thought, would cost about 35,000. 

The rods and tubes (about 1695 in number) will be disposed in a square lattice arrangement with their axes all horizontal and parallel to the axis of the graphite cylinder. The square lattice spacing will be about 83/8" center to center horizontally and vertically, with the rods and tubes grouped symmetrically about the center of the pile and located within the geometrical boundaries of an enclosing cylinder. 

It is known that a self-sustaining chain reaction can be obtained in devices known as neutronic reactors utilizing natural uranium, as a result of slow neutron fission of the content of the natural uranium. In such reactors, discrete bodies of natural uranium of high purity are disposed, usually in the form of a lattice arrangement of spheres or rods, in a neutron moderator such as graphite, baryllium or heavy water of high purity, surrounded by a neutron reflector. Neutron absorption in the U s content of the natural uranium during the reaction leads to the production of the transuranic isotope 94, known as plutonium (symbol Pu), which is fissionable in much the same manner as 94 3 or Pu 3 is formed in... 

Figure 25.1. Spatial arrangement of carbon atoms in the diamond lattice (left) and graphite lattice (right).

The continuous reduction in size of a solid finally leads to a situation where the original solid state properties can be only partially observed or may be even completely lost, as these properties are exclusively the result of the cooperation between an infinite number of building blocks. Further reduction of size finally leads to typical molecular behavior. On the other hand, even here are structural relations to the bulk occasionally detectable. For instance, the arrangements of the sp hybridized carbon atoms in cyclohexane or in adamantane can easily be traced back to the diamond lattice, whereas benzene or phenanthrene represent derivatives of the graphite lattice. However, neither cyclohexane, benzene, nor phenanthrene have chemical properties which are comparable with those of the carbon modifications they originate from. The existence of the above mentioned Q, C]o or Ci4 units is otUy made possible by the saturation of the free valencies by hydrogen atoms. Comparable well known examples for other elements are numerous, for instance the elements boron, silicon, and phosphorous. Figure 1-1 illustrates some of the relations between elementary and molecular structures. 

Figure 1.3 Carbon crystallizing in two different modifications - as graphite and as diamond -having different lattice arrangements.

The results of the PCTR measurements are given In Table 2.6.2. Actually, two lattice arrangements were studied, one vas as exact a copy of the actual lattice as possible, the other vas a "condensed lattice In that the large steam vent and othbr voids In the stack vere removed, and the lattice spacing coapressed to maintain the same graphite-to-uranlum ratio. 

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