NTTCLEAB POWER FOR PEACEFUL PURPOSES — SMYTH 191 



produce neutrons in sufficient quantity to cause other nuclear fissions 

 in the vicinity and to set up a self-propagating nuclear chain reaction. 

 Actually the number of neutrons produced by a single fission is not 

 very large. On the average, for every neutron used up in producing 

 a fission, about 2i/^ new neutrons are released, a net gain of 1^ neutrons 

 per fission. At first sight, this would appear plenty to produce a 

 multiplication of fissions. Unfortunately, from the point of view 

 of neutron economy, all the neutrons produced in a single fission are 

 not absorbed in uranium 235 to produce additional fissions. 



There are, in fact, four things that can happen to the neutrons that 

 are produced in the fission process. First of all, since neutrons are 

 extremely penetrating, they may simply escape to the outside environ- 

 ment. A second way in which they disappear is by capture by ura- 

 nium 238 without causing fission. A third possibility is that they 

 may be captured by impurities in the uranimn or by the structural 

 materials that have to be introduced for cooling or other purposes. 

 The fourth possible process that can occur is, of course, the capture 

 of neutrons by uranium 235 resulting in fission. If the fourth proc- 

 ess produces more neutrons than are lost by the first three processes, 

 the chain reaction occurs. Otherwise, it does not. Evidently, in a 

 given arrangement the first three processes may have such a high 

 probability that the extra neutrons created by fission will bo insuffi- 

 cient to keep the reaction going. 



One obvious way to reduce the probability of the escape of neutrons 

 is to increase the amount of uranium present. The more uranium 

 there is, the more likely it is that the neutrons will be absorbed in it 

 and cause fission rather than escape. This leads, of course, to the 

 concept of critical mass, which is familiar to many and which I will 

 not discuss any further. 



The second process we need to minimize is the capture of neutrons 

 by uranium without producing fission. There are several things that 

 can be done to minimize this process. Two of them depend on the 

 great effect which the speed of the neutrons has on the probability of 

 their absorption in uranium 238. This probability is reduced by using 

 a slowing-down material, called a moderator, and arranging the ura- 

 nium in a lattice. Another way to reduce nonfission capture by 

 uranium is to eliminate part or all of the uranium 238 isotope, since it 

 contributes very little to the fission process and does absorb many 

 neutrons. Of course, in the Hanford reactors, this was not desirable 

 because one of the objectives of the Hanford reactors was to produce 

 plutonium by absorption of neutrons in uranium 238. 



To reduce the third process, the nonfission capture of neutrons by 

 impurities or structural materials, requires that the uranium itself be 

 very highly purified in the first place and that structural materials be 



