We have seen that, if the neutron multiplication factor of a reactor assembly keff < 1, the chain reaction cannot be sustained. However, if a source of neutrons is introduced inside the multiplying medium, the initial neutron number is multiplied by a factor which can be very large. Since neutrons are associated with an equivalent number of fissions, a large energy could be produced with a subcritical system, provided a neutron source is available.

If N0 is the number of primary neutrons following, for example, the interaction of a proton with a target surrounded by a multiplying medium, chacterized by a multiplication factor k[27] the total number of created neutrons, after multiplication, is

N0

(1 — k ■

The number of secondary neutrons (produced after at least one multi­plication) is

kNp

(1 — k) •