k® is less than unity in Section 4.3, largely because the inelastic collisions of the fast neutrons in the natural uranium fuel lead to the subsequent loss of the neutrons in the U-238 resonance capture peaks. The fast fission cross-section values of U-238 and U-235 are much the same and too low to counter the neutron losses.

It follows that if the proportion of U-238 in the fuel is reduced, thus increasing the proportion of U-235, the effect is to replace a high neutron cap­ture material (U-238) by another material (U-235) which has much the same fast fission cross-section but less neutron capture characteristics. This is called fuel enrichment.

As U-235 has no energy threshold for fission (in fact of increases with decreasing neutron energy) the net result of fuel enrichment is that the neutron produc­tion from fission increases and neutron absorption decreases. Calculation shows that U-235 enrichment of at least 20% is necessary to give k® > 1 for fast neutrons. This is the basis of ‘fast reactors’, the title fast reactor being derived from the fact that the neutrons inducing fission are largely energetic fast neutrons.