192 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



used which have a low capacity for the absorption of neutrons. This 

 last consideration puts many restrictions in the path of the designer of 

 a nuclear chain reactor. 



THE EFFECTS OF NUCLEAR RADIATION 



The effects of nuclear radiation have several aspects that the de- 

 signer needs to keep in mind. Perhaps the most important one tech- 

 nically is the fact that the constant bombardment of structural ma- 

 terials and of uranimn itself causes changes in their properties. A 

 piece of uranium, a piece of steel or aluminum in a nuclear reactor is 

 continually bombarded by neutrons, by ganmia radiation, and to some 

 extent by other nuclear radiations. The result of such bombardment 

 may be a change of shape, an embrittlement, a change in thermal 

 conductivity, or of almost any other property of the material. The 

 rate of corrosion of a material is affected by the presence of nuclear 

 radiation. 



Nuclear radiation is dangerous to health. Consequently, the whole 

 reactor structure must be surrounded by a shield which will not be 

 penetrated by the neutrons and other radiation. Radiation is present 

 not only while the reactor is running, but induces a lasting radioactiv- 

 ity in the materials of the reactor. In particular, fuel elements in the 

 reactor become highly radioactive, and when they are unloaded for 

 chemical processing, they have to be handled by remote control. It is 

 unsafe for any personnel to handle them directly. Similarly, main- 

 tenance must be held to an absolute minimmn, and actual direct access 

 of the operators to the heart of the reactor must be avoided. 



HEAT TRANSFER OR REMOVAL 



The principal interest in establishing a nuclear reaction is because 

 the fission processes release such enormous amounts of energy, millions 

 of times the amount of energy released in chemical reactions in corre- 

 sponding amounts of material. To be sure, the Hanford reactors 

 were not designed for the purpose of producing energy but for the 

 purpose of producing plutonium. Nevertheless, the production of 

 large amounts of energy is inescapably associated with the fission 

 process, and, therefore, the designers of the Hanford reactors had to 

 provide some means of removing that energy. It was a simpler prob- 

 lem for tliem than for the designers of a reactor intended to produce 

 energy. The Hanford designers had merely to get rid of the energy 

 in some way. 



The designers of a power reactor must extract the energy in a form 

 which can be put to use. Nevertheless, many of the problems are the 

 same. They differ from ordinary heat transfer problems for reasons 

 that have already been suggested, namely, that the choice of materials 

 is limited by neutron economy, that corrosion effects may be enhanced 



