'DOE-HDBK-1019/1-93NUCLEAR CROSS SECTIONS AND NEUTRON FLUXReactor Theory (Neutron Characteristics)NP-02Page 6Rev. 0IntroductionFission neutrons are born with an average energy of about 2 MeV. These fast neutrons interactwith the reactor core materials in various absorption and scattering reactions. Collisions thatresult in scattering are useful in slowing neutrons to thermal energies. Thermal neutrons maybe absorbed by fissile nuclei to produce more fissions or be absorbed in fertile material forconversion to fissionable fuel. Absorption of neutrons in structural components, coolant, andother non-fuel material results in the removal of neutrons without fulfilling any useful purpose.To safely and efficiently operate a nuclear reactor it is necessary to predict the probability thata particular absorption or scattering reaction will occur. Once these probabilities are known, ifthe availability of neutrons can be determined, then the rate at which these nuclear reactions takeplace can be predicted.AtomDensityOne important property of a material is the atom density. The atom density is the number ofatoms of a given type per unit volume of the material. To calculate the atom density of asubstance use Equation (2-1).(2-1)where:N= atom density (atoms/cm )^{3}'= density (g/cm )^{3}N= Avogadro's number (6.022 x 10 atoms/mole)A23M = gram atomic weight