ATOMIC ENERGY — OLIPHANT 229 



by a process involving the radioactive emission of two negative elec- 

 trons, into a new chemical element, with 94 protons in the nucleus, 

 which has been called plutonium. This substance, which does not 

 exist in the natural state, is a metal that undergoes slowly a radioactive 

 change into U"*, but this change is so slow, requiring 10,000 years or 

 more to be half completed, that for all practical purposes plutonium is 

 a normal metal. It undergoes fission when it absorbs a neutron of 

 any energy and hence can replace U^^^ as a nuclear fuel. 



SLOW NEUTRON REACTORS 



The capture of neutrons by the much more plentiful U^^^ prevents the 

 establishment of a chain process with natural uranium. There is, how- 

 ever, an ingenious way out of this difficulty, which enables reacting 

 systems to be built using natural uranium. TJ-^^ does not capture neu- 

 trons that are moving with less than a certain minimum velocity. 

 Neutrons can be slowed down by passage through materials like pure 

 carbon or heavy water, which do not absorb neutrons at all readily. 

 The particles then make collisions with atoms of carbon or of heavy 

 hydrogen, handing over to the struck atom at each collision a part of 

 their energy. By placing rods of uranium in a geometrical pattern, 

 which can be calculated, in a mass of pure graphite, it is possible to 

 arrange that fission neutrons escaping from the relatively thin rods 

 without appreciable absorption wander around in the gi'aphite for a 

 considerable time, making many collisions and losing most of their 

 velocity. When they do again wander into a uranium rod they are, in 

 general, moving too slowly to be absorbed by the U^^^, and hence ignore 

 it. However, they are very readily captured by the rarer U^^^ atoms, 

 giving rise to fission. 



In order that a chain reaction may be produced, a slow neutron re- 

 actor of this type must also be above a certain critical size, where the 

 number of neutrons lost from the outer surface is less than half the 

 number generated within the pile by the chain reaction. The critical 

 size is large and even a small slow neutron reactor will contain many 

 tons of uranium and hundreds or thousands of tons of carbon as pure 

 graphite. The many neutrons escaping from the outer surface are, to 

 some extent, reflected back by a thick layer of graphite placed around 

 the reactor, while the remainder, together with the accompanying 

 X-rays, which are so harmful to living matter, must be absorbed in the 

 walls of a massive concrete enclosure. The energy set free in the fission 

 process in the uranium rods appears as heat, which can be removed 

 by passing gas or water over them. Here again it is necessary to coat 

 the uranium with a protective layer of corrosion-resistant metal that 

 does not absorb neutrons readily, aluminum being employed at present 

 in the absence of a better material. 



