ATOMIC ENERGY — OLIPHANT 231 



them are being attacked vigorously in both America and Great Britain, 

 though, in the present state of international tension, they must take 

 a position subordinate to the development and manufacture of atomic 

 weapons. 



CONTROL OF NUCLEAR REACTORS 



If a nuclear reactor is operating at a constant power level the number 

 of fissions taking place each second must be constant. In the systems 

 we have considered so far the number of fissions taking place in 1 

 second must increase rapidly. We have indicated that a fast neutron 

 reaction can be controlled by moving the component parts so that more 

 or less of the neutrons escape from the surface. This control is ren- 

 dered easier because some of the neutrons set free escape a fraction 

 cf a second after the fission process itself. This means that if the 

 system is just larger than the critical size, the multiplication proceeds 

 rather slowly. The component parts of a slow neutron reactor are 

 too large and bulky to be moved in this way, so the system is controlled 

 by pushing into the interior of the reactor rods of cadmium or of boron, 

 which have the property of absorbing neutrons readily, so robbing the 

 chain process of the number of neutrons required to keep it going. 

 There is no difficulty about controlling completely the rate of energy 

 release in a reactor, so that the chance of the process running away and 

 giving rise to an explosion is so small as to be entirely ruled out. Very 

 elaborate precautions are taken to prevent the escape of radioactive 

 materials or of harmful radiations, and workers in an atomic-energy 

 plant and those who live in the neighborhood are not subjected to any 

 abnormal risks. 



"BREEDER" REACTORS 



The picture of the production of atomic energy for industrial pur- 

 poses that we have drawn so far is not encouraging. The fast neutron 

 reactors require rather concentrated fissile material as atomic fuel, 

 and this is very difficult and expensive to produce. The slow neutron 

 reactors utilize only a very small fraction of the rarer isotope of ura- 

 nium and produce about the same quantity of plutonium. Much more 

 than this must be achieved if atomic energy is to compete successfully 

 with coal as a source of power. Fortunately the way out is clear, 

 though it has not yet been achieved in practice, and the solution brings 

 with it the possibility of using thorium, as well as uranium, as a 

 nuclear fuel. 



On the average a fission process releases several neutrons, say three. 

 If it is assumed that there are in the reactor no impurities or materials 

 of construction (other that uranium) that absorb neutrons, the three 

 fission neutrons can be utilized in the following way. At constant 

 power output one of these neutrons must produce a fresh fission by 

 absorption into the nucleus of another fissile atom. A second neutron 



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