224 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 3 



Uranium, which heralded the discovery and use of naturally occur- 

 ring radioactivity, reentered the scene to make an even greater con- 

 tribution in the production of man-made radioactivity or radio- 

 isotopes. 



NUCLEAR REACTOR 



A few facts concerning the Oak Ridge reactor, the production unit 

 for most of the radioisotopes made in the United States today, may 

 be of interest. 



As one first sees the Oak Ridge reactor (fig. 2; pi. 1) it appears to 

 be a concrete structure 47 feet long, 38 feet high, and 32 feet deep. 

 The concrete, however, is a 7-foot thick shield built around the reactor 

 to protect operating personnel. The reactive portion of the reactor 

 is a 24-foot cube built of stacks of graphite blocks through which pass 

 some 1,200 channels containing uranium metal as fuel. 



GRAPHITE MOOtfcATCX 



BOSON STta CONTO. ROD' 



«MOVlNG AlUMINUM TUBES 

 CONTAINING RADOI50T0«S 



HOTKTlVt UADSHaO; 



flUST FLOOR 



Hats FOR AOMNUM TUBES 



NUCLEAR REACTOR 



URANIUM "PILE" 



Figure 2. — This schematic sketch of the reactor is designed to show the two principal 

 ways in which radioisotopes are produced. The three most important functional parts 

 of the reactor are the uranium slugs, the graphite moderator, and the boron steel control 

 rods. When a fissionable uranium 235 atom in one of the slugs is hit by a neutron, it 

 fissions or splits. In the fission process, 1 to 3 more neutrons are produced which, when 

 slowed down by the graphite moderator, are available for splitting more uranium 235 

 atoms. The multiplication of this process many many times leads to the chain reaction. 

 Boron has a greater affinity for neutrons than does uranium, and therefore when the boron 

 steel control rods are inserted into the reactor, they "soak up" a sufficient number of 

 neutrons to slow down the chain reaction or stop it, depending on how far they are inserted 

 into the reactor. 



