The reaction rate density between species A and B, having number densities n A, B, is given by: r = n A n B k where k is the reaction rate constant of each single elementary binary reaction composing the nuclear fusion process: k = ⟨ ⟨ σ σ ( v ) v ⟩ ⟩ here, σ (v) is the cross-section at relative velocity v, and averaging is performed over all velocities.