232 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



can be captured by uranium of mass 238 to produce an atom of plu- 

 tonium, thus replacing the atom of fuel that has been used. The 

 third neutron can be absorbed by a second U^*^ atom giving another 

 plutonium atom. Thus a reactor operating in this way should pro- 

 duce more nuclear fuel than is burnt and is called a "breeding" reactor. 

 The breeding process should make it possible to utilize the whole of 

 the uranium, the plentiful U^^* as well as the scarce U^^^. If the excess 

 neutrons are absorbed in thorium instead of U^^, a new fissile material 

 which can replace U^^^ or plutonium is produced by a process very 

 similar to that which produces plutonium. We can represent this 

 process by the nuclear equation 



232 1 233* / 233* / 233 



Th + n > Th ^ > Pa ^ > U 



90 90 91 92 



The thorium nucleus absorbs a neutron producing a radioactive 

 isotope of thorium of mass 233. This emits a negative electron, or 

 ^-particle, transforming into a radioactive form of protoactinium, 

 which emits a further /^-particle to give an isotope of uranium of mass 

 233. U^^^ is a fissile substance which can be used as a fuel in nuclear 

 reactors. Thus, in time, it should be possible to change over from 

 uranium to the more plentiful thorium as fuel for the production of 

 nuclear power. 



The design of a successful breeding reactor depends upon the elimi- 

 nation from the reactor of materials that capture an appreciable frac- 

 tion of the neutrons without contributing to the production of power 

 or of fresh fissile material. There are reasons why this may prove to 

 be more practicable with the fast neutron reactors than with those 

 using slow neutrons, partly because the smaller mass of fissile material 

 in the reactor can be prepared in a state of higher purity, partly 

 because there is no moderator, but principally because the essential 

 materials of mechanical construction and the cooling fluid which can 

 be used in a fast neutron reactor are not so restricted in properties. 

 The most important of the factors to which answers have yet to be 

 found is the extent to which the materials in the reactor retain their 

 physical properties of strength, etc., when the atoms of which they 

 consist are continually stirred up and knocked out of place by collisions 

 with fast neutrons. 



EXPERIMENTAL PROGRAM IN U. S. A. 



The Atomic Energy Commission in the United States has announced 

 the construction of two types of experimental breeding reactor and a 

 materials-testing reactor. The first of these is designed to test the 

 practical feasibility of breeding with fast neutrons and to investigate 



