The electrons enter insignificantly into those considerations affecting energy release. It is changes occurring in the central nuclei of atoms which yield the vast stores of natural energy upon whose release the function of a nuclear device depends. Let us now consider the conditions necessary to
The relatively large plutonium atom,
94Pu239, has 94 protons in its core of 239 nucleons:
Modern scientific investigations (Hahn and Strassman, 1939), have revealed that if large nuclei of this nature are subjected to bombardment by uncharged neutrons, they are liable to disrupt into two fragments of variable size. This phenomenon was called "fission" by Dr.
Lise Meitner and Professor Otto Frisch and they also pointed out that major energy releases were involved in the disruption accompanied by the release of intensely radioactive products. Symbolically the reaction is usually written in the form.
> Fission Product A +
Fission Product B
+3 onl +200
It is readily calculable on the basis of a 200 MeVe
nergy release per unit fission that the20 kiloton energy of a nominal nuclear weapon involves the fissioning of all the atoms in approximately one kilogram of plutonium, and we have already seen that one gram of
this material substance must consequently suffer annihilation into energy. The efficiency
of the fission reaction is probably
not high and it is obvious that a large number of kilogramsof fissile metal must be used in the original weapon.
The preceding equation is indicative of a remarkable phenomenon which in the period 1939-1945 set the scientific world agog with excitement. It will be observed that although one neutron is consumed in the fission of a single plutonium atom, an estimated number of three neutrons is simultaneously produced. A surplus of neutrons is thus available to fission more and more plutonium atoms and form a violent chain reaction which could run rapidly through a massive lump of plutonium, disrupting many billions of millions of atoms and yielding enormous energy. This is an over-simplified picture because neutrons can escape from the
system or be lost in various ways and hence the reaction chains can be broken.Moreover, once energy is developed in a large lump of plutonium, the mass would tend to blow itself apart and so stop the fission reaction. The essential requirement for successful fission weapons is the sudden creation of an assembly of fissionable material under such conditions that more neutrons are born within the material at any particular time than are lost uselessly in different ways. These criteria lead to the concept of what is called the "critical size", i.e. that minimum size where this neutron production requirement is just achieved. It might be inferred that fission weapon designers would endeavour to assemble very rapidly certain separated (sub-critical) quantities of plutonium or uranium to give a state in excess of critical, Le. supercritical. This might be done by some form of explosive assembly system, but it would not be possible to maintain such a structure in equilibrium for many microseconds because of the disruptive effects of the vast energies unleashed once such a state of