228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



in a mass of U^^^ large enough to absorb the neutrons from a fission 

 process taking place at the center, a chain process can build up, and 

 since the neutrons are moving with high velocity, the number of fission 

 processes taking place multiplies with great rapidity. A mass of U"^ 

 in which this chain process will just take place is said to be of critical 

 size, and the greater the extent to which the mass exceeds the critical 

 size, the more rapid is the multiplication, so that if the critical size is 

 exceeded appreciably an atomic explosion takes place. The critical 

 mass for U^^^ is officially stated to be between 2 and 400 pounds, and 

 since it is a very dense substance this corresponds with a sphere of 

 metal less than 12 inches in diameter. Since the neutrons released 

 with high velocity in the fission process are absorbed directly, a nuclear 

 chain reaction of this type is called a fast fission process. 



THE FAST NEUTRON REACTOR 



If the mass of uranium 235 consists of two pieces, either of which 

 alone is smaller than the critical mass, and these are brought together 

 slowly and cautiously, it is possible to find a position where the system 

 is so little above the critical size that the energy release builds up very 

 slowly. The two pieces of uranium metal will then be heated and we 

 can arrange an automatic mechanism that pulls them apart if they 

 get too hot or brings them together if they cool off, so maintaining 

 them in a red-hot condition. The heat given off can be transferred 

 to water or to gas and can be converted into useful power in a heat 

 engine. This is the simplest type of nuclear reactor that can give 

 useful power. However, there are a number of difficulties. 



Uranium 235 can be separated from natural uranium, of which it 

 forms 1 part in 140, only by very elaborate and expensive physical proc- 

 esses. Uranium metal is very active chemically and must be protected 

 from attack by the cooling water or gas. Very large numbers of neu- 

 trons escape from the outer surface of the uranium, together with a 

 quantity of radiation, akin to X-rays. These must be absorbed in thick 

 shields of concrete to prevent lethal danger to living things in the 

 neighborhood. The amount of heat that can be abstracted from pieces 

 of metal with so small a surface is small, and in practice it is necessary 

 to increase the cooling area greatly and use a larger mass of fissionable 

 material. The problems of control must be solved in such a way as to 

 eliminate all risk of explosion. The charge of U"^^ must be removed 

 periodically and treated chemically to remove the accumulated fission 

 products and to add fresh U-^^ and this involves serious problems of 

 engineering and of handling the highly radioactive material. 



PLUTONIUM 



The principal isotope of uranium, U^^^, behaves in a different way 

 when it absorbs a neutron. Instead of midergoing fission it changes, 



