ABSTRACT
Niels Bohr and John Wheeler determined from theory that 92U235 was much more likely to undergo ssion than was 92U238. Their prediction was soon conrmed by experiments. Naturally occurring uranium is about 99.3% 92U238, and only 0.7% ssionable 92U235. Consequently, natural uranium, even if produced in very high purity, offers essentially no chance for a sustained nuclear chain reaction, because 993 atoms out of every 1000 would be the 92U238 isotope, which absorbs neutrons without undergoing ssion. To sustain a nuclear chain reaction based on uranium ssion, the uranium fuel has to be prepared such that the ssionable isotope is present in greater concentration than in natural uranium. The process for doing this is called uranium enrichment (shown conceptually in Figure 24.1). It requires effecting a separation of isotopes, which is not an easy task. This is because isotopes are, after all, the same chemical element, and therefore can’t be separated by processes that rely on differences in chemical behavior. The only distinction is the slight difference in mass number-a difference of about 1.3% in the case of the two uranium isotopes. Therefore, separation processes must rely upon small differences in those physical properties that depend on differences in atomic or molecular weights.
