Fast Effect in Lattice Reactors

An argument is made for defining the fast fission factor ϵ as the number of neutrons making first collision with moderator per neutron arising from thermal fission. Formulas for the fast effect in U²³⁸ lattices are presented; the (productive) absorption associated with fast effect is also formulated. The implications of this point of view with regard to measurements of resonance escape probability and resonance integrals are discussed.