ATOMIC ENERGY AS A HUMAN ASSET—COMPTON 169 
of producing new elements by transmutation to become of increasing 
importance. Plutonium is a concentrated source of available energy 
and will be a valuable material for peaceful purposes as well as for 
building weapons. 
Other artificial radioactive elements, especially radioactive ones, 
will also find use in medicine, in industry, and in many branches of 
science. It is yet too early to see clearly how important these uses 
may become. 
We have not yet built an atomic power plant that is generating elec- 
trical power. This is merely because we have been engaged in winning 
a war and there has been no serious shortage of electric power. If 
there were sufficient demand for a demonstration, a reasonably efficient 
plant using super-heated steam for driving a turbine could be put into 
operation within a year. Before, however, such plants can be made 
economical competitors with existing practice, a number of years’ 
development will be required. 
Whiie there are several other possibilities, the most obvious method 
of producing power from atomic fission is to heat a cooling agent 
such as air or steam or liquid metal in the chain reactor unit and 
pass this heated coolant through a heat exchanger which heats the 
steam for driving a turbine. Beyond the heat exchanger of such a 
plant everything would be done according to standard practice. Up 
to the heat exchanger all the design requires new features, among 
them protection against the extreme radioactivity of everything, in- 
cluding the coolant, that has been exposed to the neutrons. 
The chain reacting unit itself can assume many forms. The one 
essential is that it shall contain a fissionable substance such as 
uranium, either in its natural state or, if a small unit is desired, en- 
riched with additional U-235 or plutonium. H. D. Smyth, in his 
official report, has described in some detail how this active material 
can be combined with a moderator such as carbon or beryllium or 
heavy water so as to bring about the chain reaction. 
The large atomic power plants now used for producing plutonium 
have in them many tons of natural uranium and graphite. By using 
uranium containing more than the usual fraction of U-235, chain re- 
acting units have been built that are of much smaller size. 
There is, however, a lower limit to the size and weight of an atomic 
power plant that is imposed by the massive shield needed to prevent 
the neutrons and other dangerous radiations from getting out. Next 
to cosmic rays, these radiations are the most penetrating that we know 
and, for a plant designed to deliver for example no more than 100 
horsepower, are enormously more intense than the rays from a large 
supply of radium or an X-ray tube. To stop them, a shield equivalent 
in weight to at least 2 or 8 feet of solid steel is needed. There are 
basic laws of physics that make it appear very unlikely that a lighter 
