[1] Macroscopic cross sections

The cross section used in the neutron transport equation, £, is called the macroscopic cross section and its unit is (cm-1). The macroscopic cross section of nuclear reaction x is the sum of the product of atomic number density Ni and microscopic cross section alx of a constituent nuclide i of the material at a position ~. It is expressed as Eq. (2.8).

їЛг, E, 0=5>*(r, t)ai(E) (2.8)

і

The atomic number density Nl can be given by

(2.9)

where pl is the density occupied by i in the material, Ml is the atomic mass, and Na is Avogadro’s number. Variation of atomic number density with position and time should be considered in power reactors, reflecting fuel burnup or coolant void fraction.