before the reaction has got well under way; could produce the most serious vitiating influences on the energy release factor for this particular type of weapon. Even if the fusion component could be satisfactorily initiated, the problem of securing extensive "bum-up" of the nuclear fuel must pose the most complicated series of problems for the scientists and design engineers.

5. High Energy Fission
     One economically attractive possibility has been referred to by some research investigators and resides in the fact that the plentiful uranium isotope, 92U238, is fissionable by very fast neutrons. The isotope 92U235 is fissile to neutrons of much lower energy which it generates by its own disruption, and this material is widely publicized as being the probable main ingredient of some of the nominal bomb assemblies. This latter isotope, however, exists naturally only in small quantities and is obtained by an expensive diffusion process involving gaseous uranium hexafluoride.
     If, in a given weapon assembly, a subsidiary nuclear system could be made to function in such a manner that it yielded huge quantities of very fast neutrons (i.e. much faster neutrons than exist in nominal fission weapons), then this might permit the propagation of a fission reaction in the cheaper U238 isotope. Such weapons would naturally be characterized by the generation of huge quantities of radioactive fission products and would exhibit the general characteristics of mammoth weapons based on a fissile nuclear process.

Summary and conclusion
     This chapter contains a brief outline based on published data of the theoretical systems which might be employed to produce high-yield nuclear devices. Whether any such system or any alternative unique invention has been employed in the development of British, American or Russian weapons can only be left to conjecture. One thing is transparently obvious. The problems involved are of most intricate and complex character. Only superlative progress in scientific achievement and in engineering design can offer any real hope of ultimate success in a project such as this which probes the innermost secrets of Nature and the Universe.